PART IV

ENGINE DEVELOPMENT

I

THE VERTICAL TYPE

The balloon was but a year old when the brothers Robert, in 1784,

attempted propulsion of an aerial vehicle by hand-power, and succeeded,

to a certain extent, since they were able to make progress when there

was only a slight wind to counteract their work. But, as may be easily

understood, the manual power provided gave but a very slow speed, and

in any wind at all the would-be airship became an uncontrolled balloon.

Henson and Stringfellow, with their light steam engines, were first

to attempt conquest of the problem of mechanical propulsion in the

air; their work in this direction is so fully linked up with their

constructed models that it has been outlined in the section dealing

with the development of the aeroplane (_ante_, page 57). But, very

shortly after these two began, there came into the field a Monsieur

Henri Giffard, who first achieved success in the propulsion by

mechanical means of dirigible balloons, for his was the first airship

to fly against the wind. He employed a small steam-engine developing

about 3 horse-power and weighing 350 lbs. with boiler, fitting the

whole in a car suspended from the gas-bag of his dirigible. The

propeller which this engine worked was 11 feet in diameter, and the

inventor, who made several flights, obtained a speed of 6 miles an

hour against a slight wind. The power was not sufficient to render the

invention practicable, as the dirigible could only be used in calm

weather, but Giffard was sufficiently encouraged by his results to get

out plans for immense dirigibles, which through lack of funds he was

unable to construct. When, later, his invention of the steam-injector

gave him the means he desired, he became blind, and in 1882 died,

having built but the one famous dirigible.

This appears to have been the only instance of a steam engine being

fitted to a dirigible; the inherent disadvantage of this form of motive

power is that a boiler to generate the steam must be carried, and this,

together with the weight of water and fuel, renders the steam engine

uneconomical in relation to the lift either of plane or gas-bag. Again,

even if the weight could be brought down to a reasonable amount, the

attention required by steam plant renders it undesirable as a motive

power for aircraft when compared with the internal combustion engine.

Maxim, in _Artificial and Natural Flight_, details the engine which

he constructed for use with his giant experimental flying machine,

and his description is worthy of reproduction since it is that of the

only steam engine besides Giffard’s, and apart from those used for the

propulsion of models, designed for driving an aeroplane. ‘In 1889,’

Maxim says, ‘I had my attention drawn to some very thin, strong, and

comparatively cheap tubes which were being made in France, and it

was only after I had seen these tubes that I seriously considered

the question of making a flying machine. I obtained a large quantity

of them and found that they were very light, that they would stand

enormously high pressures, and generate a very large quantity of

steam. Upon going into a mathematical calculation of the whole subject,

I found that it would be possible to make a machine on the aeroplane

system, driven by a steam engine, which would be sufficiently strong

to lift itself into the air. I first made drawings of a steam engine,

and a pair of these engines was afterwards made. These engines are

constructed, for the most part, of a very high grade of cast steel, the

cylinders being only 3/32 of an inch thick, the crank shafts hollow,

and every part as strong and light as possible. They are compound,

each having a high-pressure piston with an area of 20 square inches,

a low-pressure piston of 50.26 square inches, and a common stroke of

1 foot. When first finished they were found to weigh 300 lbs. each;

but after putting on the oil cups, felting, painting, and making some

slight alterations, the weight was brought up to 320 lbs. each, or a

total of 640 lbs. for the two engines, which have since developed 362

horse-power with a steam pressure of 320 lbs. per square inch.’

The result is remarkable, being less than 2 lbs. weight per

horse-power, especially when one considers the state of development

to which the steam engine had attained at the time these experiments

were made. The fining down of the internal combustion engine, which

has done so much to solve the problems of power in relation to weight

for use with aircraft, had not then been begun, and Maxim had nothing

to guide him, so far as work on the part of his predecessors was

concerned, save the experimental engines of Stringfellow, which, being

constructed on so small a scale in comparison with his own, afforded

little guidance. Concerning the factor of power, he says: ‘When first

designing this engine, I did not know how much power I might require

from it. I thought that in some cases it might be necessary to allow

the high-pressure steam to enter the low-pressure cylinder direct, but

as this would involve a considerable loss, I constructed a species of

injector. This injector may be so adjusted that when the steam in the

boiler rises above a certain predetermined point, say 300 lbs., to

the square inch, it opens a valve and escapes past the high-pressure

cylinder instead of blowing off at the safety valve. In escaping

through this valve, a fall of about 200 lbs. pressure per square inch

is made to do work on the surrounding steam and drive it forward in

the pipe, producing a pressure on the low-pressure piston considerably

higher than the back-pressure on the high-pressure piston. In this way

a portion of the work which would otherwise be lost is utilised, and it

is possible, with an unlimited supply of steam, to cause the engines to

develop an enormous amount of power.’

* * * * *

With regard to boilers, Maxim writes,--

‘The first boiler which I made was constructed something on the

Herreshof principle, but instead of having one simple pipe in one

very long coil, I used a series of very small and light pipes,

connected in such a manner that there was a rapid circulation

through the whole--the tubes increasing in size and number as the

steam was generated. I intended that there should be a pressure of

about 100 lbs. more on the feed-water end of the series than on the

steam end, and I believed that this difference in pressure would

be sufficient to ensure a direct and positive circulation through

every tube in the series. The first boiler was exceedingly light,

but the workmanship, as far as putting the tubes together was

concerned, was very bad, and it was found impossible to so adjust

the supply of water as to make dry steam without overheating and

destroying the tubes.

‘Before making another boiler I obtained a quantity of copper

tubes, about 8 feet long, ⅜ inch external diameter, and 1/50 of an

inch thick. I subjected about 100 of these tubes to an internal

pressure of 1 ton per square inch of cold kerosene oil, and as

none of them leaked I did not test any more, but commenced my

experiments by placing some of them in a white-hot petroleum fire.

I found that I could evaporate as much as 26½ lbs. of water per

square foot of heating surface per hour, and that with a forced

circulation, although the quantity of water passing was very small

but positive, there was no danger of overheating. I conducted many

experiments with a pressure of over 400 lbs. per square inch,

but none of the tubes failed. I then mounted a single tube in a

white-hot furnace, also with a water circulation, and found that

it only burst under steam at a pressure of 1,650 lbs. per square

inch. A large boiler, having about 800 square feet of heating

surface, including the feed-water heater, was then constructed.

This boiler is about 4½ feet wide at the bottom, 8 feet long and

6 feet high. It weighs, with the casing, the dome, and the smoke

stack and connections, a little less than 1,000 lbs. The water

first passes through a system of small tubes--¼ inch in diameter

and 1/60 inch thick--which were placed at the top of the boiler

and immediately over the large tubes.... This feed-water heater is

found to be very effective. It utilises the heat of the products

of combustion after they have passed through the boiler proper and

greatly reduces their temperature, while the feed-water enters the

boiler at a temperature of about 250 F. A forced circulation is

maintained in the boiler, the feed-water entering through a spring

valve, the spring valve being adjusted in such a manner that the

pressure on the water is always 30 lbs. per square inch in excess

of the boiler pressure. This fall of 30 lbs. in pressure acts upon

the surrounding hot water which has already passed through the

tubes, and drives it down through a vertical outside tube, thus

ensuring a positive and rapid circulation through all the tubes.

This apparatus is found to act extremely well.’

Thus Maxim, who with this engine as power for his large aeroplane

achieved free flight once, as a matter of experiment, though for what

distance or time the machine was actually off the ground is matter for

debate, since it only got free by tearing up the rails which were to

have held it down in the experiment. Here, however, was a steam engine

which was practicable for use in the air, obviously, and only the rapid

success of the internal combustion engine prevented the steam-producing

type from being developed toward perfection.

The first designers of internal combustion engines, knowing nothing

of the petrol of these days, constructed their examples with a view

to using gas as fuel. As far back as 1872 Herr Paul Haenlein obtained

a speed of about 10 miles an hour with a balloon propelled by an

internal combustion engine, of which the fuel was gas obtained from

the balloon itself. The engine in this case was of the Lenoir type,

developing some 6 horse-power, and, obviously, Haenlein’s flights were

purely experimental and of short duration, since he used the gas that

sustained him and decreased the lifting power of his balloon with every

stroke of the piston of his engine. No further progress appears to have

been made with the gas-consuming type of internal combustion engine for

work with aircraft; this type has the disadvantage of requiring either

a gas-producer or a large storage capacity for the gas, either of which

makes the total weight of the power plant much greater than that of

a petrol engine. The latter type also requires less attention when

working, and the fuel is more convenient both for carrying and in the

matter of carburation.

The first airship propelled by the present-day type of internal

combustion engine was constructed by Baumgarten and Wolfert in 1879

at Leipzig, the engine being made by Daimler with a view to working

on benzine--petrol as a fuel had not then come to its own. The

construction of this engine is interesting since it was one of the

first of Daimler’s make, and it was the development brought about

by the experimental series of which this engine was one that led to

the success of the motor-car in very few years, incidentally leading

to that fining down of the internal combustion engine which has

facilitated the development of the aeroplane with such remarkable

rapidity. Owing to the faulty construction of the airship no useful

information was obtained from Daimler’s pioneer installation, as the

vessel got out of control immediately after it was first launched for

flight, and was wrecked. Subsequent attempts at mechanically-propelled

flight by Wolfert ended, in 1897, in the balloon being set on fire by

an explosion of benzine vapour, resulting in the death of both the

aeronauts.

Daimler, from 1882 onward, devoted his attention to the perfecting of

the small, high-speed petrol engine for motor-car work, and owing to

his efforts, together with those of other pioneer engine-builders, the

motor-car was made a success. In a few years the weight of this type

of engine was reduced from near on a hundred pounds per horse-power to

less than a tenth of that weight, but considerable further improvement

had to be made before an engine suitable for use with aircraft was

evolved.

The increase in power of the engines fitted to airships has made

steady progress from the outset; Haenlein’s engine developed about

6 horse-power; the Santos-Dumont airship of 1898 was propelled by a

motor of 4 horse-power; in 1902 the Lebaudy airship was fitted with an

engine of 40 horse-power, while, in 1910, the Lebaudy brothers fitted

an engine of nearly 300 horse-power to the airship they were then

constructing--1,400 horse-power was common in the airships of the War

period, and the later British rigids developed yet more.

Before passing on to consideration of the petrol-driven type of engine,

it is necessary to accord brief mention to the dirigible constructed

in 1884 by Gaston and Albert Tissandier, who at Grenelle, France,

achieved a directed flight in a wind of 8 miles an hour, obtaining

their power for the propeller from 1⅓ horse-power Siemens electric

motor, which weighed 121 lbs. and took its current from a bichromate

battery weighing 496 lbs. A two-bladed propeller, 9 feet in diameter,

was used, and the horse-power output was estimated to have run up to

1½ as the dirigible successfully described a semicircle in a wind of 8

miles an hour, subsequently making headway transversely to a wind of 7

miles an hour. The dirigible with which this motor was used was of the

conventional pointed-end type, with a length of 92 feet, diameter of

30 feet, and capacity of 37.440 cubic feet of gas. Commandant Renard,

of the French army balloon corps, followed up Tissandier’s attempt in

the next year--1885--making a trip from Chalais-Meudon to Paris and

returning to the point of departure quite successfully. In this case

the motive power was derived from an electric plant of the type used

by the Tissandiers, weighing altogether 1,174 lbs., and developing

9 horse-power. A speed of 14 miles an hour was attained with this

dirigible, which had a length of 165 feet, diameter of 27 feet, and

capacity of 65,836 cubic feet of gas.

Reverting to the petrol-fed type again, it is to be noted that

Santos-Dumont was practically the first to develop the use of the

ordinary automobile engine for air work--his work is of such importance

that it has been considered best to treat of it as one whole, and

details of the power plants are included in the account of his

experiments. Coming to the Lebaudy brothers and their work, their

engine of 1902 was a 40 horse-power Daimler, four-cylindered; it was

virtually a large edition of the Daimler car engine, the arrangement of

the various details being on the lines usually adopted for the standard

Daimler type of that period. The cylinders were fully water-jacketed,

and no special attempt toward securing lightness for air-work appears

to have been made.

The fining down of detail that brought weight to such limits as would

fit the engine for work with heavier-than-air craft appears to have

waited for the brothers Wright. Toward the end of 1903 they fitted

to their first practicable flying machine the engine which made the

historic first aeroplane flight; this engine developed 30 horse-power,

and weighed only about 7 lbs. per horse-power developed, its design and

workmanship being far ahead of any previous design in this respect,

with the exception of the remarkable engine, designed by Manly,

installed in Langley’s ill-fated aeroplane--or ‘aerodrome,’ as he

preferred to call it--tried in 1903.

The light weight of the Wright brothers’ engine did not necessitate

a high number of revolutions per minute to get the requisite power;

the speed was only 1,300 revolutions per minute, which, with a piston

stroke of 3.94 inches, was quite moderate. Four cylinders were used,

the cylinder diameter being 4.42 inches; the engine was of the

vertical type, arranged to drive two propellers at a rate of about 350

revolutions per minute, gearing being accomplished by means of chain

drive from crank-shaft end to propeller spindle.

The methods adopted by the Wrights for obtaining a light-weight engine

were of considerable interest, in view of the fact that the honour

of first achieving flight by means of the driven plane belongs to

them--unless Ader actually flew as he claimed. The cylinders of this

first Wright engine were separate castings of steel, and only the

barrels were jacketed, this being done by fixing loose, thin aluminium

covers round the outside of each cylinder. The combustion head and

valve pockets were cast together with the cylinder barrel, and were not

water cooled. The inlet valves were of the automatic type, arranged on

the tops of the cylinders, while the exhaust valves were also overhead,

operated by rockers and push-rods. The pistons and piston rings were

of the ordinary type, made of cast-iron, and the connecting rods were

circular in form, with a hole drilled down the middle of each to reduce

the weight.

Necessity for increasing power and ever lighter weight in relation to

the power produced has led to the evolution of a number of different

designs of internal combustion engines. It was quickly realised that

increasing the number of cylinders on an engine was a better way of

getting more power than that of increasing the cylinder diameter, as

the greater number of cylinders gives better torque--even turning

effect--as well as keeping down the weight--this latter because the

bigger cylinders must be more stoutly constructed than the small sizes;

this fact has led to the construction of engines having as many as

eighteen cylinders, arranged in three parallel rows in order to keep

the length of crank-shaft within reasonable limits. The aero engine

of to-day may, roughly, be divided into four classes: these are the

V type, in which two rows of cylinders are set parallel at a certain

angle to each other; the radial type, which consists of cylinders

arranged radially and remaining stationary while the crankshaft

revolves; the rotary, where the cylinders are disposed round a common

centre and revolve round a stationary shaft, and the vertical type,

of four or six cylinders--seldom more than this--arranged in one row.

A modification of the V type is the eighteen-cylindered engine--the

Sunbeam is one of the best examples--in which three rows of cylinders

are set parallel to each other, working on a common crankshaft. The

development of these four types started with that of the vertical--the

simplest of all; the V, radial, and rotary types came after the

vertical, in the order given.

The evolution of the motor-car led to the adoption of the vertical

type of internal combustion engine in preference to any other, and

it followed naturally that vertical engines should be first used for

aeroplane propulsion, as by taking an engine that had been developed to

some extent, and adapting it to its new work, the problem of mechanical

flight was rendered easier than if a totally new type had had to be

evolved. It was quickly realised--by the Wrights, in fact--that the

minimum of weight per horse-power was the prime requirement for the

successful development of heavier-than-air machines, and at the same

time it was equally apparent that the utmost reliability had to be

obtained from the engine, while a third requisite was economy, in order

to reduce the weight of petrol necessary for flight.

[Illustration: Two Cylinder Daimler Engine, 1897.]

Daimler, working steadily toward the improvement of the internal

combustion engine, had made considerable progress by the end of

last century. His two-cylinder engine of 1897 was approaching to

the present-day type, except as regards the method of ignition; the

cylinders had 3.55 inch diameter, with a 4.75 inch piston stroke, and

the engine was rated at 4.5 brake horse-power, though it probably

developed more than this in actual running at its rated speed of 800

revolutions per minute. Power was limited by the inlet and exhaust

passages, which, compared with present-day practice, were very small.

The heavy castings of which the engine was made up are accounted for

by the necessity for considering foundry practice of the time, for in

1897 castings were far below the present-day standard. The crank-case

of this two-cylinder vertical Daimler engine was the only part made of

aluminium, and even with this no attempt was made to attain lightness,

for a circular flange was cast at the bottom to form a stand for

the engine during machining and erection. The general design can be

followed from the sectional views, and these will show, too, that

ignition was by means of a hot tube on the cylinder head, which had

to be heated with a blow-lamp before starting the engine. With all

its well known and hated troubles, at that time tube ignition had an

advantage over the magneto, and the coil and accumulator system, in

reliability; sparking plugs, too, were not so reliable then as they are

now. Daimler fitted a very simple type of carburettor to this engine,

consisting only of a float with a single jet placed in the air passage.

It may be said that this twin-cylindered vertical was the first of the

series from which has been evolved the Mercedes-Daimler car and airship

engines, built in sizes up to and even beyond 240 horse-power.

In 1901 the development of the petrol engine was still so slight that

it did not admit of the construction, by any European maker, of an

engine weighing less than 12 lbs. per horse-power. Manly, working

at the instance of Professor Langley, produced a five-cylindered

radial type engine, in which both the design and workmanship showed a

remarkable advance in construction. At 950 revolutions per minute it

developed 52.4 horse-power, weighing only 2.4 pounds per horse-power;

it was a very remarkable achievement in engine design, considering the

power developed in relation to the total weight, and it was, too, an

interruption in the development of the vertical type which showed that

there were other equally great possibilities in design.

In England, the first vertical aero engine of note was that designed

by Green, the cylinder dimensions being 4.15 inch diameter by 4.75

stroke--a fairly complete idea of this engine can be obtained from

the accompanying diagrams. At a speed of 1,160 revolutions per minute

it developed 35 brake horse-power, and by accelerating up to 1,220

revolutions per minute a maximum of 40 brake horse-power could be

obtained--the first-mentioned was the rated working speed of the engine

for continuous runs. A flywheel, weighing 23.5 lbs., was fitted to the

engine, and this, together with the ignition system, brought the weight

up to 188 lbs., giving 5.4 lbs. per horse-power. In comparison with the

engine fitted to the Wrights’ aeroplane a greater power was obtained

from approximately the same cylinder volume, and an appreciable

saving in weight had also been effected. The illustration shows the

arrangement of the vertical valves at the top of the cylinder and the

overhead cam shaft, while the position of the carburettor and inlet

pipes can be also seen. The water jackets were formed by thin copper

casings, each cylinder being separate and having its independent jacket

rigidly fastened to the cylinder at the top only, thus allowing for

free expansion of the casing; the joint at the bottom end was formed by

sliding the jacket over a rubber ring. Each cylinder was bolted to the

crank-case and set out of line with the crankshaft, so that the crank

has passed over the upper dead centre by the time that the piston is at

the top of its stroke when receiving the full force of fuel explosion.

The advantage of this _desaxe_ setting is that the pressure in the

cylinder acts on the crank-pin with a more effective leverage during

that part of the stroke when that pressure is highest, and in addition

the side pressure of the piston on the cylinder wall, due to the

thrust of the connecting rod, is reduced. Possibly the charging of the

cylinder is also more complete by this arrangement, owing to the slower

movement of the piston at the bottom of its stroke allowing time for an

increased charge of mixture to enter the cylinder.

A 60 horse-power engine was also made, having four vertical cylinders,

each with a diameter of 5.5 inches and stroke of 5.75 inches,

developing its rated power at 1,100 revolutions per minute. By

accelerating up to 1,200 revolutions per minute 70 brake horse-power

could be obtained, and a maximum of 80 brake horse-power was actually

attained with the type. The flywheel, fitted as with the original 35

horse-power engine, weighed 37 lbs.; with this and with the ignition

system the total weight of the engine was only 250 lbs., or 4.2 lbs.

per horse-power at the normal rating. In this design, however, low

weight in relation to power was not the ruling factor, for Green gave

more attention to reliability and economy of fuel consumption, which

latter was approximately 0.6 pint of petrol per brake horse-power per

hour. Both the oil for lubricating the bearings and the water for

cooling the cylinders were circulated by pumps, and all parts of the

valve gear, etc., were completely enclosed for protection from dust.

[Illustration: Green Vertical Engine 35 b. h. p.]

A later development of the Green engine was a six-cylindered vertical,

cylinder dimensions being 5.5 inch diameter by 6 inch stroke,

developing 120 brake horse-power when running at 1,250 revolutions

per minute. The total weight of the engine with ignition system was

440 lbs., or 3.66 lbs. per horse-power. One of these engines was used

on the machine which, in 1909, won the prize of £ 1,000 for the first

circular mile flight, and it may be noted, too, that S. F. Cody, making

the circuit of England in 1911, used a four-cylinder Green engine.

Again, it was a Green engine that in 1914 won the £5,000 prize offered

for the best aero engine in the Naval and Military aeroplane engine

competition.

Manufacture of the Green engines, in the period of the War, had

standardised to the production of three types. Two of these were

six-cylinder models, giving respectively 100 and 150 brake horse-power,

and the third was a twelve-cylindered model rated at 275 brake

horse-power.

In 1910 J. S. Critchley compiled a list showing the types of engine

then being manufactured; twenty-two out of a total of seventy-six were

of the four-cylindered vertical type, and in addition to these there

were two six-cylindered verticals. The sizes of the four-cylinder types

ranged from 26 up to 118 brake horse-power; fourteen of them developed

less than 50 horse-power, and only two developed over 100 horse-power.

It became apparent, even in the early stages of heavier-than-air

flying, that four-cylinder engines did not produce the even torque

that was required for the rotation of the power shaft, even though

a flywheel was fitted to the engine. With this type of engine the

breakage of air-screws was of frequent occurrence, and an engine having

a more regular rotation was sought, both for this and to avoid the

excessive vibration often experienced with the four-cylinder type.

Another point that forced itself on engine builders was that the

increased power which was becoming necessary for the propulsion of

aircraft made an increase in the number of cylinders essential, in

order to obtain a light engine. An instance of the weight reduction

obtainable in using six cylinders instead of four is shown in

Critchley’s list, for one of the four-cylinder engines developed 118.5

brake horse-power and weighed 1,100 lbs., whereas a six-cylinder

engine by the same manufacturer developed 117.5 brake horse-power with

a weight of 880 lbs., the respective cylinder dimensions being 7.48

diameter by 9.06 stroke for the four-cylinder engine, and 6.1 diameter

by 7.28 stroke for the six-cylinder type.

A list of aeroplane engines, prepared in 1912 by Graham Clark, showed

that, out of the total number of 112 engines then being manufactured,

forty-two were of the vertical type, and of this number twenty-four

had four-cylinders while sixteen were six-cylindered. The German

aeroplane engine trials were held a year later, and sixty-six engines

entered the competition, fourteen of these being made with air-cooled

cylinders. All of the ten engines that were chosen for the final trials

were of the water-cooled type, and the first place was won by a Benz

four-cylinder vertical engine which developed 102 brake horse-power at

1,288 revolutions per minute. The cylinder dimensions of this engine

were 5.1 inch diameter by 7.1 inch stroke, and the weight of the engine

worked out at 3.4 lbs. per brake horse-power. During the trials the

full-load petrol consumption was 0.53 pint per horse-power per hour,

and the amount of lubricating oil used was 0.0385 pint per brake

horse-power per hour. In general construction this Benz engine was

somewhat similar to the Green engine already described; the overhead

valves, fitted in the tops of the cylinders, were similarly arranged,

as was the cam-shaft; two springs were fitted to each of the valves to

guard against the possibility of the engine being put out of action

by breakage of one of the springs, and ignition was obtained by two

high-tension magnetos giving simultaneous sparks in each cylinder

by means of two sparking plugs--this dual ignition reduced the

possibility of ignition troubles. The cylinder jackets were made of

welded sheet steel so fitted around the cylinder that the head was also

water-cooled, and the jackets were corrugated in the middle to admit

of independent expansion. Even the lubrication system was duplicated,

two sets of pumps being used, one to circulate the main supply of

lubricating oil, and the other to give a continuous supply of fresh oil

to the bearings, so that if the supply from one pump failed the other

could still maintain effective lubrication.

Development of the early Daimler type brought about the four-cylinder

vertical Mercedes-Daimler engine of 85 horse-power, with cylinders

of 5.5 diameter with 5.9 inch stroke, the cylinders being cast in

two pairs. The overhead arrangement of valves was adopted, and in

later designs push-rods were eliminated, the overhead cam-shaft being

adopted in their place. By 1914 the four-cylinder Mercedes-Daimler

had been partially displaced from favour by a six-cylindered model,

made in two sizes; the first of these gave a nominal brake horse-power

of 80, having cylinders of 4.1 inches diameter by 5.5 inches stroke;

the second type developed 100 horse-power with cylinders 4.7 inches

in diameter and 5.5 inches stroke, both types being run at 1,200

revolutions per minute. The cylinders of both these types were cast in

pairs, and, instead of the water jackets forming part of the casting,

as in the design of the original four-cylinder Mercedes-Daimler engine,

they were made of steel welded to flanges on the cylinders. Steel

pistons, fitted with cast-iron rings, were used, and the overhead

arrangement of valves and cam-shaft was adopted. About 0.55 pint per

brake horse-power per hour was the usual fuel consumption necessary to

full load running, and the engine was also economical as regards the

consumption of lubricating oil, the lubricating system being ‘forced’

for all parts, including the cam-shaft. The shape of these engines was

very well suited for work with aircraft, being narrow enough to admit

of a stream-line form being obtained, while all the accessories could

be so mounted as to produce little or no wind resistance, and very

little obstruction to the pilot’s view.

The eight-cylinder Mercedes-Daimler engine, used for airship propulsion

during the War, developed 240 brake horse-power at 1,100 revolutions

per minute; the cylinder dimensions were 6.88 diameter by 6.5

stroke--one of the instances in which the short stroke in relation to

bore was very noticeable.

Other instances of successful vertical design--the types already

detailed are fully sufficient to give particulars of the type

generally--are the Panhard, Chenu, Maybach, N.A.G., Argus, Mulag, and

the well-known Austro-Daimler, which by 1917 was being copied in every

combatant country. There are also the later Wright engines, and in

America the Wisconsin six-cylinder vertical, weighing well under 4 lbs.

per horse-power, is evidence of the progress made with this first type

of aero engine to develop.

II

THE VEE TYPE

An offshoot from the vertical type, doubling the power of this with

only a very slight--if any--increase in the length of crankshaft, the

Vee or diagonal type of aero engine leaped to success through the

insistent demand for greater power. Although the design came after that

of the vertical engine, by 1910, according to Critchley’s list of aero

engines, there were more Vee type engines being made than any other

type, twenty-five sizes being given in the list, with an average rating

of 57·4 brake horse-power.

The arrangement of the cylinders in Vee form over the crankshaft,

enabling the pistons of each pair of opposite cylinders to act upon the

same crank pin, permits of a very short, compact engine being built,

and also permits of reduction of the weight per horse-power, comparing

this with that of the vertical type of engine, with one row of

cylinders. Further, at the introduction of this type of engine it was

seen that crankshaft vibration, an evil of the early vertical engines,

was practically eliminated, as was the want of longitudinal stiffness

that characterised the higher-powered vertical engines.

Of the Vee type engines shown in Critchley’s list in 1910, nineteen

different sizes were constructed with eight cylinders, and with

horse-powers ranging from thirty to just over the hundred; the

lightest of these weighed 2·9 lbs. per horse-power--a considerable

advance in design on the average vertical engine, in this respect of

weight per horse-power. There were also two sixteen-cylinder engines of

Vee design, the larger of which developed 134 horse-power with a weight

of only 2 lbs. per brake horse-power. Subsequent developments have

indicated that this type, with the further development from it of the

double-Vee, or engine with three rows of cylinders, is likely to become

the standard design of aero engine where high powers are required.

The construction permits of placing every part so that it is easy of

access, and the form of the engine implies very little head resistance,

while it can be placed on the machine--supposing that machine to be of

the single-engine type--in such a way that the view of the pilot is

very little obstructed while in flight.

An even torque, or great uniformity of rotation, is transmitted to

the air-screw by these engines, while the design also permits of

such good balance of the engine itself that vibration is practically

eliminated. The angle between the two rows of cylinders is varied

according to the number of cylinders, in order to give working impulses

at equal angles of rotation and thus provide even torque; this angle is

determined by dividing the number of degrees in a circle by the number

of cylinders in either row of the engine. In an eight-cylindered Vee

type engine, the angle between the cylinders is 90 degrees; if it is a

twelve-cylindered engine, the angle drops to 60 degrees.

One of the earliest of the British-built Vee type engines was an

eight-cylinder 50 horse-power by the Wolseley Company, constructed

in 1908 with a cylinder bore of 3·75 inches and stroke of 5 inches,

running at a normal speed of 1,350 revolutions per minute. With this

engine, a gearing was introduced to enable the propeller to run at a

lower speed than that of the engine, the slight loss of efficiency

caused by the friction of the gearing being compensated by the slower

speed of the air-screw, which had higher efficiency than would have

been the case if it had been run at the engine speed. The ratio of the

gearing--that is, the speed of the air-screw relatively to that of the

engine, could be chosen so as to suit exactly the requirements of the

air-screw, and the gearing itself, on this engine, was accomplished on

the half-speed shaft actuating the valves.

Very soon after this first design had been tried out, a second Vee type

engine was produced which, at 1,200 revolutions per minute, developed

60 horse-power; the size of this engine was practically identical with

that of its forerunner, the only exception being an increase of half an

inch in the cylinder stroke--a very long stroke of piston in relation

to the bore of the cylinder. In the first of these two engines, which

was designed for airship propulsion, the weight had been about 8 lbs.

per brake horse-power, no special attempt appearing to have been made

to fine down for extreme lightness; in this 60 horse-power design, the

weight was reduced to 6·1 lbs. per horse-power, counting the latter

as normally rated; the engine actually gave a maximum of 75 brake

horse-power, reducing the ratio of weight to power very considerably

below the figure given.

[Illustration: Sikh, 12-cylinder magneto, end view.]

[Illustration: Sikh, 12-cylinder, side view.]

The accompanying diagram illustrates a later Wolseley model, end

elevation, the eight-cylindered 120 horse-power Vee type aero engine

of the early war period. With this engine, each crank pin has two

connecting rods bearing on it, these being placed side by side and

connected to the pistons of opposite cylinders, and the two cylinders

of the pair are staggered by an amount equal to the width of the

connecting rod-bearing, to afford accommodation for the rods. The

crankshaft was a nickel chrome steel forging, machined hollow, with

four crank pins set at 180 degrees to each other, and carried in three

bearings lined with anti-friction metal. The connecting rods were made

of tubular nickel chrome steel, and the pistons of drawn steel, each

being fitted with four piston rings. Of these the two rings nearest

to the piston head were of the ordinary cast-iron type, while the

others were of phosphor bronze, so arranged as to take the side thrust

of the piston. The cylinders were of steel, arranged in two groups

or rows of four, the angular distance between them being 90 degrees.

In the space above the crankshaft, between the cylinder rows, was

placed the valve-operating mechanism, together with the carburettor

and ignition system, thus rendering this a very compact and accessible

engine. The combustion heads of the cylinders were made of cast-iron,

screwed into the steel cylinder barrels; the water-jacket was of spun

aluminium, with one end fitting over the combustion head and the other

free to slide on the cylinder; the water-joint at the lower end was

made tight by a Dermatine ring carried between small flanges formed

on the cylinder barrel. Overhead valves were adopted, and in order

to make these as large as possible the combustion chamber was made

slightly larger in diameter than the cylinder, and the valves set at an

angle. Dual ignition was fitted in each cylinder, coil and accumulator

being used for starting and as a reserve in case of failure of the

high-tension magneto system fitted for normal running. There was a

double set of lubricating pumps, ensuring continuity of the oil supply

to all the bearings of the engine.

[Illustration: End View of Wolseley 120 horse-power Vee-type Engine.]

The feature most noteworthy in connection with the running of this

type of engine was its flexibility; the normal output of power was

obtained with 1,150 revolutions per minute of the crankshaft, but,

by accelerating up to 1,400 revolutions, a maximum of 147 brake

horse-power could be obtained. The weight was about 5 lbs. per

horse-power, the cylinder dimensions being 5 inches bore by 7 inches

stroke. Economy in running was obtained, the fuel consumption being

0·58 pint per brake horse-power per hour at full load, with an

expenditure of about 0·075 pint lubricating oil per brake horse-power

per hour.

Another Wolseley Vee type that was standardised was a 90 horse-power

eight-cylinder engine running at 1,800 revolutions per minute, with a

reducing gear introduced by fitting the air-screw on the half-speed

shaft. First made semi-cooled--the exhaust valve was left air-cooled,

and then entirely water-jacketed--this engine demonstrated the

advantage of full water cooling, for under the latter condition the

same power was developed with cylinders a quarter of an inch less in

diameter than in the semi-cooled pattern; at the same time the weight

was brought down to 4½ lbs. per horse-power.

A different but equally efficient type of Vee design was the Dorman

engine, of which an end elevation is shown; this developed 80 brake

horse-power at a speed of 1,300 revolutions per minute, with a cylinder

bore of 5 inches; each cylinder was made in cast-iron in one piece with

the combustion chamber, the barrel only being water-jacketed. Auxiliary

exhaust ports were adopted, the holes through the cylinder wall being

uncovered by the piston at the bottom of its stroke--the piston, 4·75

inches in length, was longer than its stroke, so that these ports were

covered when it was at the top of the cylinder. The exhaust discharged

through the ports into a belt surrounding the cylinder, the belts on

the cylinders being connected so that the exhaust gases were taken

through a single pipe. The air was drawn through the crank case, before

reaching the carburettor, this having the effect of cooling the oil

in the crank case as well as warming the air and thus assisting in

vaporising the petrol for each charge of the cylinders. The inlet and

exhaust valves were of the overhead type, as may be gathered from the

diagram, and in spite of cast-iron cylinders being employed a light

design was obtained, the total weight with radiator, piping, and water

being only 5·5 lbs. per horse-power.

[Illustration: Dorman 80 horse-power Vee-type Engine.]

Here was the antithesis of the Wolseley type in the matter of bore in

relation to stroke; from about 1907 up to the beginning of the war, and

even later, there was controversy as to which type--that in which the

bore exceeded the stroke, or _vice versa_--gave greater efficiency.

The short-stroke enthusiasts pointed to the high piston speed of the

long-stroke type, while those who favoured the latter design contended

that full power could not be obtained from each explosion in the

short-stroke type of cylinder. It is now generally conceded that the

long-stroke engine yields higher efficiency, and in addition to this,

so far as car engines are concerned, the method of rating horse-power

in relation to bore without taking stroke into account has given the

long-stroke engine an advantage, actual horse-power with a long stroke

engine being in excess of the nominal rating. This may have had some

influence on aero engine design, but, however this may have been, the

long-stroke engine has gradually come to favour, and its rival has

taken second place.

For some time pride of place among British Vee type engines was held

by the Sunbeam Company, which, owing to the genius of Louis Coatalen,

together with the very high standard of construction maintained by

the firm, achieved records and fame in the middle and later periods

of the war. Their 225 horse-power twelve-cylinder engine ran at a

normal speed of 2,000 revolutions per minute; the air-screw was driven

through gearing at half this speed, its shaft being separate from the

timing gear and carried in ball-bearings on the nose-piece of the

engine. The cylinders were of cast-iron, entirely water-cooled; a thin

casing formed the water-jacket, and a very light design was obtained,

the weight being only 3·2 lbs. per horse-power. The first engine of

Sunbeam design had eight cylinders and developed 150 horse-power at

2,000 revolutions per minute; the final type of Vee design produced

during the war was twelve-cylindered, and yielded 310 horse-power with

cylinders 4·3 inches bore by 6·4 inches stroke. Evidence in favour of

the long-stroke engine is afforded in this type as regards economy of

working; under full load, working at 2,000 revolutions per minute, the

consumption was 0·55 pints of fuel per brake horse-power per hour,

which seems to indicate that the long stroke permitted of full use

being made of the power resulting from each explosion, in spite of the

high rate of speed of the piston.

Developing from the Vee type, the eighteen-cylinder 475 brake

horse-power engine, designed during the war, represented for a time

the limit of power obtainable from a single plant. It was water-cooled

throughout, and the ignition to each cylinder was duplicated; this

engine proved fully efficient, and economical in fuel consumption. It

was largely used for seaplane work, where reliability was fully as

necessary as high power.

The abnormal needs of the war period brought many British firms into

the ranks of Vee-type engine-builders, and, apart from those mentioned,

the most notable types produced are the Rolls-Royce and the Napier.

The first mentioned of these firms, previous to 1914, had concentrated

entirely on car engines, and their very high standard of production in

this department of internal combustion engine work led, once they took

up the making of aero engines, to extreme efficiency both of design

and workmanship. The first experimental aero engine, of what became

known as the ‘Eagle’ type, was of Vee design--it was completed in March

of 1915--and was so successful that it was standardised for quantity

production. How far the original was from the perfection subsequently

ascertained is shown by the steady increase in developed horse-power

of the type; originally designed to develop 200 horse-power, it was

developed and improved before its first practical trial in October

of 1915, when it developed 255 horse-power on a brake test. Research

and experiment produced still further improvements, for, without any

enlargement of the dimensions, or radical alteration in design, the

power of the engine was brought up to 266 horse-power by March of

1916, the rate of revolutions of 1,800 per minute being maintained

throughout. July, 1916, gave 284 horse-power; by the end of the year

this had been increased to 322 horse-power; by September of 1917 the

increase was to 350 horse-power, and by February of 1918 the ‘Eagle’

type of engine was rated at 360 horse-power, at which standard it

stayed. But there is no more remarkable development in engine design

than this, a 75 per cent increase of power in the same engine in a

period of less than three years.

To meet the demand for a smaller type of engine for use on training

machines, the Rolls-Royce firm produced the ‘Hawk’ Vee-type engine of

100 horse-power, and, intermediately between this and the ‘Eagle,’

the ‘Falcon’ engine came to being with an original rated horse-power

of 205 at 1,800 revolutions per minute, in April of 1916. Here was

another case of growth of power in the same engine through research,

almost similar to that of the ‘Eagle’ type, for by July of 1918 the

‘Falcon’ was developing 285 horse-power with no radical alteration of

design. Finally, in response to the constant demand for increase of

power in a single plant, the Rolls-Royce company designed and produced

the ‘Condor’ type of engine, which yielded 600 horse-power on its first

test in August of 1918. The cessation of hostilities and consequent

falling off in the demand for extremely high-powered plants prevented

the ‘Condor’ being developed to its limit, as had been the ‘Falcon’ and

‘Eagle’ types.

The ‘Eagle’ engine was fitted to the two Handley-Page aeroplanes which

made flights from England to India--it was virtually standard on the

Handley-Page bombers of the later War period, though to a certain

extent the American ‘Liberty’ engine was also used. Its chief record,

however, is that of being the type fitted to the Vickers-Vimy aeroplane

which made the first Atlantic flight, covering the distance of 1,880

miles at a speed averaging 117 miles an hour.

The Napier Company specialised on one type of engine from the outset,

a power plant which became known as the ‘Lion’ engine, giving 450

horse-power with twelve cylinders arranged in three rows of four each.

Considering the engine as ‘dry,’ or without fuel and accessories, an

abnormally light weight per horse-power--only 1·89 lbs.--was attained

when running at the normal rate of revolution. The cylinders and

water-jackets are of steel, and there is fitted a detachable aluminium

cylinder head containing inlet and exhaust valves and valve actuating

mechanism; pistons are of aluminium alloy, and there are two inlet and

two exhaust valves to each cylinder, the whole of the valve mechanism

being enclosed in an oil-tight aluminium case. Connecting rods and

crankshaft are of steel, the latter being machined from a solid steel

forging and carried in five roller bearings and one plain bearing at

the forward end. The front end of the crank-case encloses reduction

gear for the propeller shaft, together with the shaft and bearings.

There are two suction and one pressure type oil pumps driven through

gears at half-engine speed, and two 12 spark magnetos, giving 2 sparks

in each cylinder.

The cylinders are set with the central row vertical, and the two side

rows at angles of 60 degrees each; cylinder bore is 5½ inches, and

stroke 5⅛ inches; the normal rate of revolution is 1,350 per minute,

and the reducing gear gives one revolution of the propeller shaft to

1·52 revolutions of crankshaft. Fuel consumption is 0·48 lbs. of

fuel per brake horse-power hour at full load, and oil consumption is

0·020 lbs. per brake horse-power hour. The dry weight of the engine,

complete with propeller boss, carburettors, and induction pipes, is 850

lbs., and the gross weight in running order, with fuel and oil for six

hours working, is 2,671 lbs., exclusive of cooling water.

[Illustration: Napier ‘Lion.’]

[Illustration: Napier ‘Lion.’]

To this engine belongs an altitude record of 30,500 feet, made at

Martlesham, near Ipswich, on January 2nd, 1919, by Captain Lang,

R.A.F., the climb being accomplished in 66 minutes 15 seconds. Previous

to this, the altitude record was held by an Italian pilot, who made

25,800 feet in an hour and 57 minutes in 1916. Lang’s climb was

stopped through the pressure of air, at the altitude he reached, being

insufficient for driving the small propellers on the machine which

worked the petrol and oil pumps, or he might have made the height said

to have been attained by Major Schroeder on February 27th, 1920, at

Dayton, Ohio. Schroeder is said to have reached an altitude of 36,020

feet on a Napier biplane, and, owing to failure of the oxygen supply,

to have lost consciousness, fallen five miles, righted his machine when

2,000 feet in the air, and alighted successfully. Major Schroeder is an

American.

Turning back a little, and considering other than British design of Vee

and double-Vee or ‘Broad arrow’ type of engine, the Renault firm from

the earliest days devoted considerable attention to the development

of this type, their air-cooled engines having been notable examples

from the earliest days of heavier-than-air machines. In 1910 they were

making three sizes of eight-cylindered Vee-type engines, and by 1915

they had increased to the manufacture of five sizes, ranging from

25 to 100 brake horse-power, the largest of the five sizes having

twelve cylinders but still retaining the air-cooled principle. The

De Dion firm, also, made Vee-type engines in 1914, being represented

by an 80 horse-power eight-cylindered engine, air-cooled, and a 150

horse-power, also of eight cylinders, water-cooled, running at a

normal rate of 1,600 revolutions per minute. Another notable example

of French construction was the Panhard and Levassor 100 horse-power

eight-cylinder Vee engine, developing its rated power at 1,500

revolutions per minute, and having the--for that time--low weight of

4·4 lbs. per horse-power.

American Vee design has followed the British fairly closely; the

Curtiss Company produced originally a 75 horse-power eight-cylinder Vee

type running at 1,200 revolutions per minute, supplementing this with

a 170 horse-power engine running at 1,600 revolutions per minute, and

later with a twelve-cylinder model Vee type, developing 300 horse-power

at 1,500 revolutions per minute, with cylinder bore of 5 inches and

stroke of 7 inches. An exceptional type of American design was the Kemp

Vee engine of 80 horse-power, in which the cylinders were cooled by a

current of air obtained from a fan at the forward end of the engine.

With cylinders of 4·25 inches bore and 4·75 inches stroke, the rated

power was developed at 1,150 revolutions per minute, and with the

engine complete the weight was only 4·75 lbs. per horse-power.

III

THE RADIAL TYPE

The very first successful design of internal combustion aero engine

made was that of Charles Manly, who built a five-cylinder radial engine

in 1901 for use with Langley’s ‘aerodrome,’ as the latter inventor

decided to call what has since become known as the aeroplane. Manly

made a number of experiments, and finally decided on radial design, in

which the cylinders are so rayed round a central crank-pin that the

pistons act successively upon it; by this arrangement a very short and

compact engine is obtained, with a minimum of weight, and a regular

crankshaft rotation and perfect balance of inertia forces.

When Manly designed his radial engine, high-speed internal combustion

engines were in their infancy, and the difficulties in construction

can be partly realised when the lack of manufacturing methods for

this high-class engine work, and the lack of experimental data on the

various materials, are taken into account. During its tests, Manly’s

engine developed 52·4 brake horse-power at a speed of 950 revolutions

per minute, with the remarkably low weight of only 2·4 lbs. per

horse-power; this latter was increased to 3·6 lbs. when the engine was

completed by the addition of ignition system, radiator, petrol tank,

and all accessories, together with the cooling water for the cylinders.

[Illustration: Cross Section, Manly’s 5 Cylinder Radial Engine.]

In Manly’s engine the cylinders were of steel, machined outside and

inside to 1/16 of an inch thickness; on the side of the cylinder, at

the top end, the valve chamber was brazed, being machined from a solid

forging. The casing which formed the water-jacket was of sheet steel,

1/50 of an inch in thickness, and this also was brazed on the cylinder

and to the valve chamber. Automatic inlet valves were fitted, and

the exhaust valves were operated by a cam which had two points, 180

degrees apart; the cam was rotated in the opposite direction to the

engine at one-quarter engine speed. Ignition was obtained by using

a one-spark coil and vibrator for all cylinders, with a distributor

to select the right cylinder for each spark--this was before the days

of the high-tension magneto and the almost perfect ignition systems

that makers now employ. The scheme of ignition for this engine was

originated by Manly himself, and he also designed the sparking plugs

fitted in the tops of the cylinders. Through fear of trouble resulting

if the steel pistons worked on the steel cylinders, cast-iron liners

were introduced in the latter, 1/16 of an inch thick.

The connecting rods of this engine were of virtually the same type

as is employed on nearly all modern radial engines. The rod for one

cylinder had a bearing along the whole of the crank pin, and its end

enclosed the pin; the other four rods had bearings upon the end of the

first rod, and did not touch the crank pin. The accompanying diagram

shows this construction, together with the means employed for securing

the ends of the four rods--the collars were placed in position after

the rods had been put on. The bearings of these rods did not receive

any of the rubbing effect due to the rotation of the crank pin, the

rubbing on them being only that of the small angular displacement

of the rods during each revolution; thus there was no difficulty

experienced with the lubrication.

Another early example of the radial type of engine was the French

Anzani, of which type one was fitted to the machine with which Bleriot

first crossed the English Channel--this was of 25 horse-power. The

earliest Anzani engines were of the three-cylinder fan type, one

cylinder being vertical, and the other two placed at an angle of 72

degrees on each side, as the possibility of over-lubrication of the

bottom cylinders was feared if a regular radial construction were

adopted. In order to overcome the unequal balance of this type, balance

weights were fitted inside the crank case.

The final development of this three-cylinder radial was the ‘Y’ type of

engine, in which the cylinders were regularly disposed at 120 degrees

apart; the bore was 4·1, stroke 4·7 inches, and the power developed was

30 brake horse-power at 1,300 revolutions per minute.

Critchley’s list of aero engines being constructed in 1910 shows twelve

of the radial type, with powers of between 14 and 100 horse-power, and

with from three to ten cylinders--this last is probably the greatest

number of cylinders that can be successfully arranged in circular form.

Of the twelve types of 1910, only two were water-cooled, and it is to

be noted that these two ran at the slowest speeds and had the lowest

weight per horse-power of any.

The Anzani radial was considerably developed, special attention

being paid to this type by its makers, and by 1914 the Anzani list

comprised seven different sizes of air-cooled radials. Of these the

largest had twenty cylinders, developing 200 brake horse-power--it

was virtually a double radial--and the smallest was the original 30

horse-power three-cylinder design. A six-cylinder model was formed by

a combination of two groups of three cylinders each, acting upon a

double-throw crankshaft; the two crank pins were set at 180 degrees to

each other, and the cylinder groups were staggered by an amount equal

to the distance between the centres of the crank pins. Ten-cylinder

radial engines are made with two groups of five cylinders acting upon

two crank pins set at 180 degrees to each other; the largest Anzani

‘ten’ developed 125 horse-power at 1,200 revolutions per minute, the

ten cylinders being each 4·5 inches in bore with stroke of 5·9 inches,

and the weight of the engine being 3·7 lbs. per horse-power. In the 200

horse-power Anzani radial the cylinders are arranged in four groups

of five each, acting on two crank pins. The bore of the cylinders

in this engine is the same as in the three-cylinder, but the stroke

is increased to 5·5 inches. The rated power is developed at 1,300

revolutions per minute, and the engine complete weighs 3·4 lbs. per

horse-power.

With this 200 horse-power Anzani, a petrol consumption of as low as

0·49 lbs. of fuel per brake horse-power per hour has been obtained,

but the consumption of lubricating oil is compensatingly high, being

up to one-fifth of the fuel used. The cylinders are set _desaxé_ with

the crank shaft, and are of cast-iron, provided with radiating ribs

for air-cooling; they are attached to the crank case by long bolts

passing through bosses at the top of the cylinders, and connected to

other bolts at right angles through the crank case. The tops of the

cylinders are formed flat, and seats for the inlet and exhaust valves

are formed on them. The pistons are cast-iron, fitted with ordinary

cast-iron spring rings. An aluminium crank case is used, being made

in two halves connected together by bolts, which latter also attach

the engine to the frame of the machine. The crankshaft is of nickel

steel, made hollow, and mounted on ball-bearings in such a manner that

practically a combination of ball and plain bearings is obtained; the

central web of the shaft is bent to bring the centres of the crank pins

as close together as possible, leaving only room for the connecting

rods, and the pins are 180 degrees apart. Nickel steel valves of the

cone-seated, poppet type are fitted, the inlet valves being automatic,

and those for the exhaust cam-operated by means of push-rods. With an

engine having such a number of cylinders a very uniform rotation of the

crankshaft is obtained, and in actual running there are always five of

the cylinders giving impulses to the crankshaft at the same time.

An interesting type of pioneer radial engine was the Farcot, in which

the cylinders were arranged in a horizontal plane, with a vertical

crankshaft which operated the air-screw through bevel gearing. This

was an eight-cylinder engine, developing 64 horse-power at 1,200

revolutions per minute. The R.E.P. type, in the early days, was a ‘fan’

engine, but the designer, M. Robert Pelterie, turned from this design

to a seven-cylinder radial, which at 1,100 revolutions per minute gave

95 horse-power. Several makers entered into radial engine development

in the years immediately preceding the War, and in 1914 there were

some twenty-two different sizes and types, ranging from 30 to 600

horse-power, being made, according to report; the actual construction

of the latter size at this time, however, is doubtful.

Probably the best example of radial construction up to the outbreak

of War was the Salmson (Canton-Unne) water-cooled, of which in 1914

six sizes were listed as available. Of these the smallest was a

seven-cylinder 90 horse-power engine, and the largest, rated at 600

horse-power, had eighteen cylinders. These engines, during the War,

were made under licence by the Dudbridge Ironworks in Great Britain.

[Illustration: Section of 200 h.p. Salmson Radial Engine.]

The accompanying diagram shows the construction of the cylinders

in the 200 horse-power size, showing the method of cooling, and the

arrangement of the connecting rods. A patent planetary gear, also shown

in the diagram, gives exactly the same stroke to all the pistons. The

complete engine has fourteen cylinders, of forged steel machined all

over, and so secured to the crank case that any one can be removed

without parting the crank case. The water-jackets are of spun copper,

brazed on to the cylinder, and corrugated so as to admit of free

expansion; the water is circulated by means of a centrifugal pump.

The pistons are of cast-iron, each fitted with three rings, and the

connecting rods are of high-grade steel, machined all over and fitted

with bushes of phosphor bronze; these rods are connected to a central

collar, carried on the crank pin by two ball-bearings. The crankshaft

has a single throw, and is made in two parts to allow the cage for

carrying the big end-pins of the connecting rods to be placed in

position.

The casing is in two parts, on one of which the brackets for fixing the

engine are carried, while the other part carries the valve-gear. Bolts

secure the two parts together. The mechanically-operated steel valves

on the cylinders are each fitted with double springs, and the valves

are operated by rods and levers. Two Zenith carburettors are fitted

on the rear half of the crank case, and short induction pipes are led

to each cylinder; each of the carburettors is heated by the exhaust

gases. Ignition is by two high-tension magnetos, and a compressed air

self-starting arrangement is provided. Two oil pumps are fitted for

lubricating purposes, one of which forces oil to the crankshaft and

connecting-rod bearings, while the second forces oil to the valve

gear, the cylinders being so arranged that the oil which flows along

the walls cannot flood the lower cylinders. This engine operates upon

a six-stroke cycle, a rather rare arrangement for internal combustion

engines of the electrical ignition type; this is done in order to

obtain equal angular intervals for the working impulses imparted to the

rotating crankshaft, as the cylinders are arranged in groups of seven,

and all act upon the one crankshaft. The angle, therefore, between the

impulses is 77-1/7 degrees. A diagram is inset giving a side view of

the engine, in order to show the grouping of the cylinders.

[Illustration: Salmson 200 h.p. Radial Engine, Side View.]

The 600 horse-power Salmson engine was designed with a view to fitting

to airships, and was in reality two nine-cylindered engines, with a

gear-box connecting them; double air-screws were fitted, and these

were so arranged that either or both of them might be driven by either

or both engines; in addition to this, the two engines were complete

and separate engines as regards carburation and ignition, etc., so

that they could be run independently of each other. The cylinders were

exceptionally ‘long stroke,’ being 5·9 inches bore to 8·27 inches

stroke, and the rated power was developed at 1,200 revolutions per

minute, the weight of the complete engine being only 4·1 lbs. per

horse-power at the normal rating.

A type of engine specially devised for airship propulsion is that in

which the cylinders are arranged horizontally instead of vertically,

the main advantages of this form being the reduction of head resistance

and less obstruction to the view of the pilot. A casing, mounted on

the top of the engine, supports the air-screw, which is driven through

bevel gearing from the upper end of the crankshaft. With this type of

engine a better rate of air-screw efficiency is obtained by gearing the

screw down to half the rate of revolution of the engine, this giving a

more even torque. The petrol consumption of the type is very low, being

only 0·48 lbs. per horse-power per hour, and equal economy is claimed

as regards lubricating oil, a consumption of as little as 0·04 lbs. per

horse-power per hour being claimed.

Certain American radial engines were made previous to 1914, the

principal being the Albatross six-cylinder engines of 50 and 100

horse-powers. Of these the smaller size was air-cooled, with

cylinders of 4·5 inches bore and 5 inches stroke, developing the

rated power at 1,230 revolutions per minute, with a weight of about

5 lbs. per horse-power. The 100 horse-power size had cylinders of

5·5 inches bore, developing its rated power at 1,230 revolutions per

minute, and weighing only 2·75 lbs. per horse-power. This engine was

markedly similar to the six-cylindered Anzani, having all the valves

mechanically operated, and with auxiliary exhaust ports at the bottoms

of the cylinders, overrun by long pistons. These Albatross engines

had their cylinders arranged in two groups of three, with each group

of three pistons operating on one of two crank pins, each 180 degrees

apart.

The radial type of engine, thanks to Charles Manly, had the honour of

being first in the field as regards aero work. Its many advantages,

among which may be specially noted the very short crankshaft as

compared with vertical, Vee, or ‘broad arrow’ type of engine, and

consequent greater rigidity, ensure it consideration by designers of

to-day, and render it certain that the type will endure. Enthusiasts

claim that the ‘broad arrow’ type, or Vee with a third row of cylinders

inset between the original two, is just as much a development from the

radial engine as from the vertical and resulting Vee; however this may

be, there is a place for the radial type in air-work for as long as the

internal combustion engine remains as a power plant.

IV

THE ROTARY TYPE

M. Laurent Seguin, the inventor of the Gnome rotary aero engine,

provided as great a stimulus to aviation as any that was given anterior

to the war period, and brought about a great advance in mechanical

flight, since these well-made engines gave a high-power output for

their weight, and were extremely smooth in running. In the rotary

design the crankshaft of the engine is stationary, and the cylinders,

crank case, and all their adherent parts rotate; the working is thus

exactly opposite in principle to that of the radial type of aero

engine, and the advantage of the rotary lies in the considerable

flywheel effect produced by the revolving cylinders, with consequent

evenness of torque. Another advantage is that air-cooling, adopted in

all the Gnome engines, is rendered much more effective by the rotation

of the cylinders, though there is a tendency to distortion through the

leading side of each cylinder being more efficiently cooled than the

opposite side; advocates of other types are prone to claim that the air

resistance to the revolving cylinders absorbs some 10 per cent of the

power developed by the rotary engine, but that has not prevented the

rotary from attaining to great popularity as a prime mover.

There were, in the list of aero engines compiled in 1910, five rotary

engines included, all air-cooled. Three of these were Gnome engines,

and two of the make known as ‘International.’ They ranged from 21·5

to 123 horse-power, the latter being rated at only 1·8 lbs. weight

per brake horse-power, and having fourteen cylinders, 4·33 inches in

diameter by 4·7 inches stroke. By 1914 forty-three different sizes and

types of rotary engine were being constructed, and in 1913 five rotary

type engines were entered for the series of aeroplane engine trials

held in Germany. Minor defects ruled out four of these, and only the

German Bayerischer Motoren Flugzeugwerke completed the seven-hour test

prescribed for competing engines. Its large fuel consumption barred

this engine from the final trials, the consumption being some 0·95

pints per horse-power per hour. The consumption of lubricating oil,

also was excessive, standing at 0·123 pint per horse-power per hour.

The engine gave 37·5 effective horse-power during its trial, and the

loss due to air resistance was 4·6 horse-power, about 11 per cent.

The accompanying drawing shows the construction of the engine, in

which the seven cylinders are arranged radially on the crank case; the

method of connecting the pistons to the crank pins can be seen. The

mixture is drawn through the crank chamber, and to enter the cylinder

it passes through the two automatic valves in the crown of the piston;

the exhaust valves are situated in the tops of the cylinders, and are

actuated by cams and push-rods. Cooling of the cylinder is assisted by

the radial rings, and the diameter of these rings is increased round

the hottest part of the cylinder. When long flights are undertaken

the advantage of the light weight of this engine is more than

counterbalanced by its high fuel and lubricating oil consumption, but

there are other makes which are much better than this seven-cylinder

German in respect of this.

[Illustration: Bayerischer 7 Cylinder Rotary Engine, 1913.]

Rotation of the cylinders in engines of this type is produced by the

side pressure of the pistons on the cylinder walls, and in order to

prevent this pressure from becoming abnormally large it is necessary to

keep the weight of the piston as low as possible, as the pressure is

produced by the tangential acceleration and retardation of the piston.

On the upward stroke the circumferential velocity of the piston is

rapidly increased, which causes it to exert a considerable tangential

pressure on the side of the cylinder, and on the return stroke there

is a corresponding retarding effect due to the reduction of the

circumferential velocity of the piston. These side pressures cause an

appreciable increase in the temperatures of the cylinders and pistons,

which makes it necessary to keep the power rating of the engines fairly

low.

Seguin designed his first Gnome rotary as a 34 horse-power engine when

run at a speed of 1,300 revolutions per minute. It had five cylinders,

and the weight was 3·9 lbs. per horse-power. A seven-cylinder model

soon displaced this first engine, and this latter, with a total weight

of 165 lbs., gave 61·5 horse-power. The cylinders were machined out of

solid nickel chrome-steel ingots, and the machining was carried out

so that the cylinder walls were under ⅙ of an inch in thickness. The

pistons were cast-iron, fitted each with two rings, and the automatic

inlet valve to the cylinder was placed in the crown of the piston. The

connecting rods, of ‘H’ section, were of nickel chrome-steel, and the

large end of one rod, known as the ‘master-rod’ embraced the crank pin;

on the end of this rod six hollow steel pins were carried, and to these

the remaining six connecting-rods were attached. The crankshaft of the

engine was made of nickel chrome-steel, and was in two parts connected

together at the crank pin; these two parts, after the master-rod had

been placed in position and the other connecting rods had been attached

to it, were firmly secured. The steel crank case was made in five

parts, the two central ones holding the cylinders in place, and on one

side another of the five castings formed a cam-box, to the outside of

which was secured the extension to which the air-screw was attached.

On the other side of the crank case another casting carried the

thrust-box, and the whole crank case, with its cylinders and gear, was

carried on the fixed crank shaft by means of four ball-bearings, one of

which also took the axial thrust of the air-screw.

For these engines, castor oil is the lubricant usually adopted, and it

is pumped to the crankshaft by means of a gear-driven oil pump; from

this shaft the other parts of the engine are lubricated by means of

centrifugal force, and in actual practice sufficient unburnt oil passes

through the cylinders to lubricate the exhaust valve, which partly

accounts for the high rate of consumption of lubricating oil. A very

simple carburettor of the floatless, single-spray type was used, and

the mixture was passed along the hollow crankshaft to the interior of

the crank case, thence through the automatic inlet valves in the tops

of the pistons to the combustion chambers of the cylinders. Ignition

was by means of a high-tension magneto specially geared to give the

correct timing, and the working impulses occurred at equal angular

intervals of 102·85 degrees. The ignition was timed so that the firing

spark occurred when the cylinder was 26 degrees before the position in

which the piston was at the outer end of its stroke, and this timing

gave a maximum pressure in the cylinder just after the piston had

passed this position.

By 1913, eight different sizes of the Gnome engine were being

constructed, ranging from 45 to 180 brake horse-power; four of these

were single-crank engines, one having nine and the other three having

seven cylinders. The remaining four were constructed with two cranks;

three of them had fourteen cylinders apiece, ranged in groups of seven,

acting on the cranks, and the one other had eighteen cylinders ranged

in two groups of nine, acting on its two cranks. Cylinders of the

two-crank engines are so arranged (in the fourteen-cylinder type) that

fourteen equal angular impulses occur during each cycle; these engines

are supported on bearings on both sides of the engine, the air-screw

being placed outside the front support. In the eighteen-cylinder model

the impulses occur at each 40 degrees of angular rotation of the

cylinders, securing an extremely even rotation of the air-screw.

In 1913 the Gnome Monosoupape engine was introduced, a model in which

the inlet valve to the cylinder was omitted, while the piston was of

the ordinary cast-iron type. A single exhaust valve in the cylinder

head was operated in a manner similar to that on the previous Gnome

engines, and the fact of this being the only valve on the cylinder

gave the engine its name. Each cylinder contained ports at the bottom

which communicated with the crank chamber, and were overrun by the

piston when this was approaching the bottom end of its stroke. During

the working cycle of the engine the exhaust valve was opened early to

allow the exhaust gases to escape from the cylinder, so that by the

time the piston overran the ports at the bottom the pressure within

the cylinder was approximately equal to that in the crank case, and

practically no flow of gas took place in either direction through the

ports. The exhaust valve remained open as usual during the succeeding

up-stroke of the piston, and the valve was held open until the piston

had returned through about one-third of its downward stroke, thus

permitting fresh air to enter the cylinder. The exhaust valve then

closed, and the downward motion of the piston, continuing, caused a

partial vacuum inside the cylinder; when the piston overran the ports,

the rich mixture from the crank case immediately entered. The cylinder

was then full of the mixture, and the next upward stroke of the piston

compressed the charge; upon ignition the working cycle was repeated.

The speed variation of this engine was obtained by varying the extent

and duration of the opening of the exhaust valves, and was controlled

by the pilot by hand-operated levers acting on the valve tappet

rollers. The weight per horse-power of these engines was slightly less

than that of the two-valve type, while the lubrication of the gudgeon

pin and piston showed an improvement, so that a lower lubricating oil

consumption was obtained. The 100 horse-power Gnome Monosoupape was

built with nine cylinders, each 4·33 inches bore by 5·9 inches stroke,

and it developed its rated power at 1,200 revolutions per minute.

[Illustration: Clerget 115 h.p. Rotary Aero Engine, Side Elevation.]

An engine of the rotary type, almost as well known as the Gnome, is

the Clerget, in which both cylinders and crank case are made of steel,

the former having the usual radial fins for cooling. In this type the

inlet and exhaust valves are both located in the cylinder head, and

mechanically operated by push-rods and rockers. Pipes are carried from

the crank case to the inlet valve casings to convey the mixture to the

cylinders, a carburettor of the central needle type being used. The

carburetted mixture is taken into the crank case chamber in a manner

similar to that of the Gnome engine. Pistons of aluminium alloy, with

three cast-iron rings, are fitted, the top ring being of the obturator

type. The large end of one of the nine connecting rods embraces the

crank pin and the pressure is taken on two ball-bearings housed in the

end of the rod. This carries eight pins, to which the other rods are

attached, and the main rod being rigid between the crank pin and piston

pin determines the position of the pistons. Hollow connecting-rods

are used, and the lubricating oil for the piston pins passes from the

crankshaft through the centres of the rods. Inlet and exhaust valves

can be set quite independently of one another--a useful point, since

the correct timing of the opening of these valves is of importance. The

inlet valve opens 4 degrees from top centre and closes after the bottom

dead centre of the piston; the exhaust valve opens 68 degrees before

the bottom centre and closes 4 degrees after the top dead centre of

the piston. The magnetos are set to give the spark in the cylinder at

25 degrees before the end of the compression stroke--two high-tension

magnetos are used; if desired, the second one can be adjusted to give

a later spark for assisting the starting of the engine. The lubricating

oil pump is of the valveless two-plunger type, so geared that it runs

at seven revolutions to 100 revolutions of the engine; by counting

the pulsations the speed of the engine can be quickly calculated by

multiplying the pulsations by 100 and dividing by seven. In the 115

horse-power nine-cylinder Clerget the cylinders are 4·7 bore with a

6·3 inches stroke, and the rated power of the engine is obtained at

1,200 revolutions per minute. The petrol consumption is 0·75 pint per

horse-power per hour.

A third rotary aero engine, equally well known with the foregoing

two, is the Le Rhone, made in four different sizes with power outputs

of from 50 to 160 horse-power; the two smaller sizes are single

crank engines with seven and nine cylinders respectively, and the

larger sizes are of double-crank design, being merely the two smaller

sizes doubled--fourteen and eighteen-cylinder engines. The inlet and

exhaust valves are located in the cylinder head, and both valves are

mechanically operated by one push-rod and rocker, radial pipes from

crank case to inlet valve casing taking the mixture to the cylinders.

The exhaust valves are placed on the leading, or air-screw side, of

the engine, in order to get the fullest possible cooling effect. The

rated power of each type of engine is obtained at 1,200 revolutions

per minute, and for all four sizes the cylinder bore is 4·13 inches,

with a 5·5 inches piston stroke. Thin cast-iron liners are shrunk

into the steel cylinders in order to reduce the amount of piston

friction. Although the Le Rhone engines are constructed practically

throughout of steel, the weight is only 2·9 lbs. per horse-power in the

eighteen-cylinder type.

[Illustration: Gyro-Duplex Rotary Engine, Cross Section.]

American enterprise in the construction of the rotary type is perhaps

best illustrated in the ‘Gyro’ engine; this was first constructed with

inlet valves in the heads of the pistons, after the Gnome pattern, the

exhaust valves being in the heads of the cylinders. The inlet valve in

the crown of each piston was mechanically operated in a very ingenious

manner by the oscillation of the connecting-rod. The Gyro-Duplex engine

superseded this original design, and a small cross-section illustration

of this is appended. It is constructed in seven and nine-cylinder

sizes, with a power range of from 50 to 100 horse-power; with the

largest size the low weight of 2·5 lbs. per horse-power is reached. The

design is of considerable interest to the internal combustion engineer,

for it embodies a piston valve for controlling auxiliary exhaust

ports, which also acts as the inlet valve to the cylinder. The piston

uncovers the auxiliary ports when it reaches the bottom of its stroke,

and at the end of the power stroke the piston is in such a position

that the exhaust can escape over the top of it. The exhaust valve in

the cylinder head is then opened by means of the push-rod and rocker,

and is held open until the piston has completed its upward stroke and

returned through more than half its subsequent return stroke. When the

exhaust valve closes, the cylinder has a charge of fresh air, drawn in

through the exhaust valve, and the further motion of the piston causes

a partial vacuum; by the time the piston reaches bottom dead centre the

piston-valve has moved up to give communication between the cylinder

and the crank case, therefore the mixture is drawn into the cylinder.

Both the piston valve and exhaust valve are operated by cams formed on

the one casting, which rotates at seven-eighths engine speed for the

seven-cylinder type, and nine-tenths engine speed for the nine-cylinder

engines. Each of these cams has four or five points respectively, to

suit the number of cylinders.

The steel cylinders are machined from solid forgings and provided with

webs for air-cooling as shown. Cast-iron pistons are used, and are

connected to the crankshaft in the same manner as with the Gnome and Le

Rhone engines. Petrol is sprayed into the crank case by a small geared

pump and the mixture is taken from there to the piston valves by radial

pipes. Two separate pumps are used for lubrication, one forcing oil to

the crank-pin bearing and the other spraying the cylinders.

Among other designs of rotary aero engines the E.J.C. is noteworthy,

in that the cylinders and crank case of this engine rotate in opposite

directions, and two air-screws are used, one being attached to the end

of the crankshaft, and the other to the crank case. Another interesting

type is the Burlat rotary, in which both the cylinders and crankshaft

rotate in the same direction, the rotation of the crankshaft being

twice that of the cylinders as regards speed. This engine is arranged

to work on the four-stroke cycle with the crankshaft making four, and

the cylinders two, revolutions per cycle.

It would appear that the rotary type of engine is capable of but little

more improvement--save for such devices as these of the last two

engines mentioned, there is little that Laurent Seguin has not already

done in the Gnome type. The limitation of the rotary lies in its

high fuel and lubricating oil consumption, which renders it unsuited

for long-distance aero work; it was, in the war period, an admirable

engine for such short runs as might be involved in patrol work ‘over

the lines,’ and for similar purposes, but the water-cooled Vee or

even vertical, with its much lower fuel consumption, was and is to be

preferred for distance work. The rotary air-cooled type has its uses,

and for them it will probably remain among the range of current types

for some time to come. Experience of matters aeronautical is sufficient

to show, however, that prophecy in any direction is most unsafe.

V

THE HORIZONTALLY-OPPOSED ENGINE

Among the first internal combustion engines to be taken into use with

aircraft were those of the horizontally-opposed four-stroke cycle

type, and, in every case in which these engines were used, their

excellent balance and extremely even torque rendered them ideal--until

the tremendous increase in power requirements rendered the type too

long and bulky for placing in the fuselage of an aeroplane. As power

increased, there came a tendency toward placing cylinders radially

round a central crankshaft, and, as in the case of the early Anzani, it

may be said that the radial engine grew out of the horizontal opposed

piston type. There were, in 1910--that is, in the early days of small

power units, ten different sizes of the horizontally opposed engine

listed for manufacture, but increase in power requirements practically

ruled out the type for air work.

The Darracq firm were the leading makers of these engines in 1910;

their smallest size was a 24 horse-power engine, with two cylinders

each of 5·1 inches bore by 4·7 inches stroke. This engine developed

its rated power at 1,500 revolutions per minute, and worked out at

a weight of 5 lbs. per horse-power. With these engines the cranks

are so placed that two regular impulses are given to the crankshaft

for each cycle of working, an arrangement which permits of very even

balancing of the inertia forces of the engine. The Darracq firm also

made a four-cylindered horizontal opposed piston engine, in which two

revolutions were given to the crankshaft per revolution, at equal

angular intervals.

The Dutheil-Chambers was another engine of this type, and had the

distinction of being the second largest constructed. At 1,000

revolutions per minute it developed 97 horse-power; its four cylinders

were each of 4·93 inches bore by 11·8 inches stroke--an abnormally long

stroke in comparison with the bore. The weight--which owing to the

build of the engine and its length of stroke was bound to be rather

high, actually amounted to 8·2 lbs. per horse-power. Water cooling

was adopted, and the engine was, like the Darracq four-cylinder type,

so arranged as to give two impulses per revolution at equal angular

intervals of crankshaft rotation.

One of the first engines of this type to be constructed in England was

the Alvaston, a water-cooled model which was made in 20, 30, and 50

brake horse-power sizes, the largest being a four-cylinder engine. All

three sizes were constructed to run at 1,200 revolutions per minute.

In this make the cylinders were secured to the crank case by means of

four long tie bolts passing through bridge pieces arranged across the

cylinder heads, thus relieving the cylinder walls of all longitudinal

explosion stresses. These bridge pieces were formed from chrome

vanadium steel and milled to an ‘H’ section, and the bearings for the

valve-tappet were forged solid with them. Special attention was given

to the machining of the interiors of the cylinders and the combustion

heads, with the result that the exceptionally high compression of 95

lbs. per square inch was obtained, giving a very flexible engine. The

cylinder heads were completely water-jacketed, and copper water-jackets

were also fitted round the cylinders. The mechanically operated valves

were actuated by specially shaped cams, and were so arranged that only

two cams were required for the set of eight valves. The inlet valves at

both ends of the engine were connected by a single feed-pipe to which

the carburettor was attached, the induction piping being arranged above

the engine in an easily accessible position. Auxiliary air ports were

provided in the cylinder walls so that the pistons overran them at the

end of their stroke. A single vertical shaft running in ball-bearings

operated the valves and water circulating pump, being driven by spiral

gearing from the crankshaft at half speed. In addition to the excellent

balance obtained with this engine, the makers claimed with justice that

the number of working parts was reduced to an absolute minimum.

In the two-cylinder Darracq, the steel cylinders were machined from

solid, and auxiliary exhaust ports, overrun by the piston at the inner

end of its stroke, were provided in the cylinder walls, consisting of

a circular row of drilled holes--this arrangement was subsequently

adopted on some of the Darracq racing car engines. The water jackets

were of copper, soldered to the cylinder walls; both the inlet and

exhaust valves were located in the cylinder heads, being operated by

rockers and push-rods actuated by cams on the half-time shaft driven

from one end of the crankshaft. Ignition was by means of a high-tension

magneto, and long induction pipes connected the ends of the cylinders

to the carburettor, the latter being placed underneath the engine.

Lubrication was effected by spraying oil into the crank case by means

of a pump, and a second pump circulated the cooling water.

Another good example of this type of engine was the Eole, which had

eight opposed pistons, each pair of which was actuated by a common

combustion chamber at the centre of the engine, two crankshafts being

placed at the outer ends of the engine. This reversal of the ordinary

arrangement had two advantages; it simplified induction, and further

obviated the need for cylinder heads, since the explosion drove at two

piston heads instead of at one piston head and the top of the cylinder;

against this, however, the engine had to be constructed strongly enough

to withstand the longitudinal stresses due to the explosions, as the

cranks are placed on the outer ends and the cylinders and crank-cases

take the full force of each explosion. Each crankshaft drove a separate

air-screw.

This pattern of engine was taken up by the Dutheil-Chambers firm in

the pioneer days of aircraft, when the firm in question produced

seven different sizes of horizontal engines. The Demoiselle monoplane

used by Santos-Dumont in 1909 was fitted with a two-cylinder,

horizontally-opposed Dutheil-Chambers engine, which developed 25 brake

horse-power at a speed of 1,100 revolutions per minute, the cylinders

being of 5 inches bore by 5·1 inches stroke, and the total weight

of the engine being some 120 lbs. The crankshafts of these engines

were usually fitted with steel flywheels in order to give a very even

torque, the wheels being specially constructed with wire spokes. In

all the Dutheil-Chambers engines water cooling was adopted, and the

cylinders were attached to the crank cases by means of long bolts

passing through the combustion heads.

For their earliest machines, the Clement-Bayard firm constructed

horizontal engines of the opposed piston type. The best known of these

was the 30 horse-power size, which had cylinders of 4·7 inches diameter

by 5·1 inches stroke, and gave its rated power at 1,200 revolutions

per minute. In this engine the steel cylinders were secured to the

crank case by flanges, and radiating ribs were formed around the barrel

to assist the air-cooling. Inlet and exhaust valves were actuated by

push-rods and rockers actuated from the second motion shaft mounted

above the crank case; this shaft also drove the high-tension magneto

with which the engine was fitted. A ring of holes drilled round each

cylinder constituted auxiliary ports which the piston uncovered at the

inner end of its stroke, and these were of considerable assistance not

only in expelling exhaust gases, but also in moderating the temperature

of the cylinder and of the main exhaust valve fitted in the cylinder

head. A water-cooled Clement-Bayard horizontal engine was also made,

and in this the auxiliary exhaust ports were not embodied; except

in this particular, the engine was very similar to the water-cooled

Darracq.

The American Ashmusen horizontal engine, developing 100 horse-power,

is probably the largest example of this type constructed. It was made

with six cylinders arranged on each side of a common crank case, with

long bolts passing through the cylinder heads to assist in holding them

down. The induction piping and valve-operating gear were arranged below

the engine, and the half-speed shaft carried the air-screw.

Messrs Palons and Beuse, Germans, constructed a light-weight,

air-cooled, horizontally-opposed engine, two-cylindered. In this the

cast-iron cylinders were made very thin, and were secured to the

crank case by bolts passing through lugs cast on the outer ends of

the cylinders; the crankshaft was made hollow, and holes were drilled

through the webs of the connecting-rods in order to reduce the weight.

The valves were fitted to the cylinder heads, the inlet valves being of

the automatic type, while the exhaust valves were mechanically operated

from the cam-shaft by means of rockers and push-rods. Two carburettors

were fitted, to reduce the induction piping to a minimum; one was

attached to each combustion chamber, and ignition was by the normal

high-tension magneto driven from the half-time shaft.

There was also a Nieuport two-cylinder air-cooled horizontal engine,

developing 35 horse-power when running at 1,300 revolutions per minute,

and being built at a weight of 5 lbs. per horse-power. The cylinders

were of 5·3 inches diameter by 5·9 inches stroke; the engine followed

the lines of the Darracq and Dutheil-Chambers pretty closely, and thus

calls for no special description.

The French Kolb-Danvin engine of the horizontal type, first constructed

in 1905, was probably the first two-stroke cycle engine designed

to be applied to the propulsion of aircraft; it never got beyond

the experimental stage, although its trials gave very good results.

Stepped pistons were adopted, and the charging pump at one end was

used to scavenge the power cylinder at the other ends of the engine,

the transfer ports being formed in the main casting. The openings of

these ports were controlled at both ends by the pistons, and the

location of the ports appears to have made it necessary to take the

exhaust from the bottom of one cylinder and from the top of the other.

The carburetted mixture was drawn into the scavenging cylinders, and

the usual deflectors were cast on the piston heads to assist in the

scavenging and to prevent the fresh gas from passing out of the exhaust

ports.

VI

THE TWO-STROKE CYCLE ENGINE

Although it has been little used for aircraft propulsion, the

possibilities of the two-stroke cycle engine render some study of

it desirable in this brief review of the various types of internal

combustion engine applicable both to aeroplanes and airships.

Theoretically the two-stroke cycle engine--or as it is more commonly

termed, the ‘two-stroke,’ is the ideal power producer; the doubling

of impulses per revolution of the crankshaft should render it of

very much more even torque than the four-stroke cycle types, while,

theoretically, there should be a considerable saving of fuel, owing

to the doubling of the number of power strokes per total of piston

strokes. In practice, however, the inefficient scavenging of virtually

every two-stroke cycle engine produced nullifies or more than nullifies

its advantages over the four-stroke cycle engine; in many types, too,

there is a waste of fuel gases through the exhaust ports, and much has

yet to be done in the way of experiment and resulting design before

the two-stroke cycle engine can be regarded as equally reliable,

economical, and powerful with its elder brother.

The first commercially successful engine operating on the two-stroke

cycle was invented by Mr Dugald Clerk, who in 1881 proved the design

feasible. As is more or less generally understood, the exhaust gases

of this engine are discharged from the cylinder during the time that

the piston is passing the inner dead centre, and the compression,

combustion, and expansion of the charge take place in similar manner

to that of the four-stroke cycle engine. The exhaust period is usually

controlled by the piston overrunning ports in the cylinder at the end

of its working stroke, these ports communicating direct with the outer

air--the complication of an exhaust valve is thus obviated; immediately

after the escape of the exhaust gases, charging of the cylinder occurs,

and the fresh gas may be introduced either through a valve in the

cylinder head or through ports situated diametrically opposite to the

exhaust ports. The continuation of the outward stroke of the piston,

after the exhaust ports have been closed, compresses the charge into

the combustion chamber of the cylinder, and the ignition of the mixture

produces a recurrence of the working stroke.

Thus, theoretically, is obtained the maximum of energy with the minimum

of expenditure; in practice, however, the scavenging of the power

cylinder, a matter of great importance in all internal combustion

engines, is often imperfect, owing to the opening of the exhaust ports

being of relatively short duration; clearing the exhaust gases out

of the cylinder is not fully accomplished, and these gases mix with

the fresh charge and detract from its efficiency. Similarly, owing to

the shorter space of time allowed, the charging of the cylinder with

the fresh mixture is not so efficient as in the four-stroke cycle

type; the fresh charge is usually compressed slightly in a separate

chamber--crank case, independent cylinder, or charging pump, and is

delivered to the working cylinder during the beginning of the return

stroke of the piston, while in engines working on the four-stroke

cycle principle a complete stroke is devoted to the expulsion of the

waste gases of the exhaust, and another full stroke to recharging the

cylinder with fresh explosive mixture.

Theoretically the two-stroke and the four-stroke cycle engines possess

exactly the same thermal efficiency, but actually this is modified by a

series of practical conditions which to some extent tend to neutralise

the very strong case in favour of the two-stroke cycle engine. The

specific capacity of the engine operating on the two-stroke principle

is theoretically twice that of one operating on the four-stroke cycle,

and consequently, for equal power, the former should require only about

half the cylinder volume of the latter; and, owing to the greater

superficial area of the smaller cylinder, relatively, the latter should

be far more easily cooled than the larger four-stroke cycle cylinder;

thus it should be possible to get higher compression pressures,

which in turn should result in great economy of working. Also the

obtaining of a working impulse in the cylinder for each revolution

of the crankshaft should give a great advantage in regularity of

rotation--which it undoubtedly does--and the elimination of the

operating gear for the valves, inlet and exhaust, should give greater

simplicity of design.

In spite of all these theoretical--and some practical--advantages the

four-stroke cycle engine was universally adopted for aircraft work;

owing to the practical equality of the two principles of operation,

so far as thermal efficiency and friction losses are concerned,

there is no doubt that the simplicity of design (in theory) and

high power output to weight ratio (also in theory) ought to have

given the ‘two-stroke’ a place on the aeroplane. But this engine has

to be developed so as to overcome its inherent drawbacks; better

scavenging methods have yet to be devised--for this is the principal

drawback--before the two-stroke can come to its own as a prime mover

for aircraft.

Mr Dugald Clerk’s original two-stroke cycle engine is indicated

roughly, as regards principle, by the accompanying diagram, from which

it will be seen that the elimination of the ordinary inlet and exhaust

valves of the four-stroke type is more than compensated by a separate

cylinder which, having a piston worked from the connecting-rod of the

power cylinder, was used to charging, drawing the mixture from the

carburettor past the valve in the top of the charging cylinder, and

then forcing it through the connecting pipe into the power cylinder.

The inlet valves both on the charging and the power cylinders are

automatic; when the power piston is near the bottom of its stroke the

piston in the charging cylinder is compressing the carburetted air,

so that as soon as the pressure within the power cylinder is relieved

by the exit of the burnt gases through the exhaust ports the pressure

in the charging cylinder causes the valve in the head of the power

cylinder to open, and fresh mixture flows into the cylinder, replacing

the exhaust gases. After the piston has again covered the exhaust ports

the mixture begins to be compressed, thus automatically closing the

inlet valve. Ignition occurs near the end of the compression stroke,

and the working stroke immediately follows, thus giving an impulse to

the crankshaft on every down stroke of the piston. If the scavenging

of the cylinder were complete, and the cylinder were to receive a

full charge of fresh mixture for every stroke, the same mean effective

pressure as is obtained with four-stroke cycle engines ought to be

realised, and at an equal speed of rotation this engine should give

twice the power obtainable from a four-stroke cycle engine of equal

dimensions. This result was not achieved, and, with the improvements

in construction brought about by experiment up to 1912, the output was

found to be only about fifty per cent more than that of a four-stroke

cycle engine of the same size, so that, when the charging cylinder is

included, this engine has a greater weight per horse-power, while the

lowest rate of fuel consumption recorded was 0.68 lb. per horse-power

per hour.

[Illustration: Dugald Clerk’s Two-stroke Cycle Engine.]

In 1891 Mr Day invented a two-stroke cycle engine which used the crank

case as a scavenging chamber, and a very large number of these engines

have been built for industrial purposes. The charge of carburetted air

is drawn through a non-return valve into the crank chamber during the

upstroke of the piston, and compressed to about 4 lbs. pressure per

square inch on the down stroke. When the piston approaches the bottom

end of its stroke the upper edge first overruns an exhaust port, and

almost immediately after uncovers an inlet port on the opposite side of

the cylinder and in communication with the crank chamber; the entering

charge, being under pressure, assists in expelling the exhaust gases

from the cylinder. On the next upstroke the charge is compressed into

the combustion space of the cylinder, a further charge simultaneously

entering the crank case to be compressed after the ignition for the

working stroke. To prevent the incoming charge escaping through the

exhaust ports of the cylinder a deflector is formed on the top of the

piston, causing the fresh gas to travel in an upward direction, thus

avoiding as far as possible escape of the mixture to the atmosphere.

From experiments conducted in 1910 by Professor Watson and Mr Fleming

it was found that the proportion of fresh gases which escaped unburnt

through the exhaust ports diminished with increase of speed; at 600

revolutions per minute about 36 per cent of the fresh charge was lost;

at 1,200 revolutions per minute this was reduced to 20 per cent, and at

1,500 revolutions it was still farther reduced to 6 per cent.

So much for the early designs. With regard to engines of this type

specially constructed for use with aircraft, three designs call for

special mention. Messrs A. Gobe and H. Diard, Parisian engineers,

produced an eight-cylindered two-stroke cycle engine of rotary design,

the cylinders being co-axial. Each pair of opposite pistons was secured

together by a rigid connecting rod, connected to a pin on a rotating

crankshaft which was mounted eccentrically to the axis of rotation

of the cylinders. The crankshaft carried a pinion gearing with an

internally toothed wheel on the transmission shaft which carried the

air-screw. The combustible mixture, emanating from a common supply

pipe, was led through conduits to the front ends of the cylinders,

in which the charges were compressed before being transferred to the

working spaces through ports in tubular extensions carried by the

pistons. These extensions had also exhaust ports, registering with

ports in the cylinder which communicated with the outer air, and the

extensions slid over depending cylinder heads attached to the crank

case by long studs. The pump charge was compressed in one end of each

cylinder, and the pump spaces each delivered into their corresponding

adjacent combustion spaces. The charges entered the pump spaces during

the suction period through passages which communicated with a central

stationary supply passage at one end of the crank case, communication

being cut off when the inlet orifice to the passage passed out of

register with the port in the stationary member. The exhaust ports at

the outer end of the combustion space opened just before and closed a

little later than the air ports, and the incoming charge assisted in

expelling the exhaust gases in a manner similar to that of the earlier

types of two-stroke cycle engine. The accompanying rough diagram

assists in showing the working of this engine.

[Illustration: The Gobe and Diard Co-axial Two-stroke Engine.]

Exhibited in the Paris Aero Exhibition of 1912, the Laviator two-stroke

cycle engine, six-cylindered, could be operated either as a radial or

as a rotary engine, all its pistons acting on a single crank. Cylinder

dimensions of this engine were 3.94 inches bore by 5.12 inches stroke,

and a power output of 50 horse-power was obtained when working at

a rate of 1,200 revolutions per minute. Used as a radial engine, it

developed 65 horse-power at the same rate of revolution, and, as

the total weight was about 198 lbs., the weight of about 3 lbs. per

horse-power was attained in radial use. Stepped pistons were employed,

the annular space between the smaller or power piston and the walls

of the larger cylinder being used as a charging pump for the power

cylinder situated 120 degrees in rear of it. The charging cylinders

were connected by short pipes to ports in the crank case which

communicated with the hollow crankshaft through which the fresh gas was

supplied, and once in each revolution each port in the case registered

with the port in the hollow shaft. The mixture which then entered the

charging cylinder was transferred to the corresponding working cylinder

when the piston of that cylinder had reached the end of its power

stroke, and immediately before this the exhaust ports diametrically

opposite the inlet ports were uncovered; scavenging was thus assisted

in the usual way. The very desirable feature of being entirely

valveless was accomplished with this engine, which is also noteworthy

for exceedingly compact design.

The Lamplough six-cylinder two-stroke cycle rotary, shown at the Aero

Exhibition at Olympia in 1911, had several innovations, including a

charging pump of rotary blower type. With the six cylinders, six power

impulses at regular intervals were given on each rotation; otherwise,

the cycle of operations was carried out much as in other two-stroke

cycle engines. The pump supplied the mixture under slight pressure

to an inlet port in each cylinder, which was opened at the same time

as the exhaust port, the period of opening being controlled by the

piston. The rotary blower sucked the mixture from the carburettor and

delivered it to a passage communicating with the inlet ports in the

cylinder walls. A mechanically-operated exhaust valve was placed in the

centre of each cylinder head, and towards the end of the working stroke

this valve opened, allowing part of the burnt gases to escape to the

atmosphere; the remainder was pushed out by the fresh mixture going

in through the ports at the bottom end of the cylinder. In practice,

one or other of the cylinders was always taking fresh mixture while

working, therefore the delivery from the pump was continuous and the

mixture had not to be stored under pressure.

The piston of this engine was long enough to keep the ports covered

when it was at the top of the stroke, and a bottom ring was provided

to prevent the mixture from entering the crank case. In addition to

preventing leakage, this ring no doubt prevented an excess of oil

working up the piston into the cylinder. As the cylinder fired with

every revolution, the valve gear was of the simplest construction, a

fixed cam lifting each valve as the cylinder came into position. The

spring of the exhaust valve was not placed round the stem in the usual

way, but at the end of a short lever, away from the heat of the exhaust

gases. The cylinders were of cast steel, the crank case of aluminium,

and ball-bearings were fitted to the crankshaft, crank pins, and the

rotary blower pump. Ignition was by means of a high-tension magneto of

the two-spark pattern, and with a total weight of 300 lbs. the maximum

output was 102 brake horse-power, giving a weight of just under 3 lbs.

per horse-power.

One of the most successful of the two-stroke cycle engines was that

designed by Mr G. F. Mort and constructed by the New Engine Company.

With four cylinders of 3.69 inches bore by 4.5 inches stroke, and

running at 1,250 revolutions per minute, this engine developed 50

brake horse-power; the total weight of the engine was 155 lbs., thus

giving a weight of 3.1 lbs. per horse-power. A scavenging pump of the

rotary type was employed, driven by means of gearing from the engine

crankshaft, and in order to reduce weight to a minimum the vanes were

of aluminium. This engine was tried on a biplane, and gave very

satisfactory results.

American design yields two apparently successful two-stroke cycle

aero engines. A rotary called the Fredericson engine was said to give

an output of 70 brake horse-power with five cylinders 4·5 inches

diameter by 4·75 inches stroke, running at 1,000 revolutions per

minute. Another, the Roberts two-stroke cycle engine, yielded 100

brake horse-power from six cylinders of the stepped piston design;

two carburettors, each supplying three cylinders, were fitted

to this engine. Ignition was by means of the usual high-tension

magneto, gear-driven from the crankshaft, and the engine, which was

water-cooled, was of compact design.

It may thus be seen that the two-stroke cycle type got as far as actual

experiment in air work, and that with considerable success. So far,

however, the greater reliability of the four-stroke cycle has rendered

it practically the only aircraft engine, and the two-stroke has yet

some way to travel before it becomes a formidable competitor, in spite

of its admitted theoretical and questioned practical advantages.

VII

ENGINES OF THE WAR PERIOD

The principal engines of British, French, and American design used in

the war period and since are briefly described under the four distinct

types of aero engine; such notable examples as the Rolls-Royce,

Sunbeam, and Napier engines have been given special mention, as they

embodied--and still embody--all that is best in aero engine practice.

So far, however, little has been said about the development of German

aero engine design, apart from the early Daimler and other pioneer

makes.

At the outbreak of hostilities in 1914, thanks to subsidies to

contractors and prizes to aircraft pilots, the German aeroplane

industry was in a comparatively flourishing condition. There were about

twenty-two establishments making different types of heavier-than-air

machines, monoplane and biplane, engined for the most part with the

four-cylinder Argus or the six-cylinder Mercedes vertical type engines,

each of these being of 100 horse-power--it was not till war brought

increasing demands on aircraft that the limit of power began to rise.

Contemporary with the Argus and Mercedes were the Austro-Daimler,

Benz, and N.A.G., in vertical design, while as far as rotary types

were concerned there were two, the Oberursel and the Stahlhertz; of

these the former was by far the most promising, and it came to virtual

monopoly of the rotary-engined ‘plane as soon as the war demand began.

It was practically a copy of the famous Gnome rotary, and thus deserves

little description.

Germany, from the outbreak of war, practically, concentrated on the

development of the Mercedes engine; and it is noteworthy that, with one

exception, increase of power corresponding with the increased demand

for power was attained without increasing the number of cylinders. The

various models ranged between 75 and 260 horse-power, the latter being

the most recent production of this type. The exception to the rule

was the eight-cylinder 240 horse-power, which was replaced by the 260

horse-power six-cylinder model, the latter being more reliable and but

very slightly heavier. Of the other engines, the 120 horse-power Argus

and the 160 and 225 horse-power Benz were the most used, the Oberursel

being very largely discarded after the Fokker monoplane had had its

day, and the N.A.G. and Austro-Daimler also falling to comparative

disuse. It may be said that the development of the Mercedes engine

contributed very largely to such success as was achieved in the war

period by German aircraft, and, in developing the engine, the builders

were careful to make alterations in such a way as to effect the least

possible change in the design of aeroplane to which they were to be

fitted. Thus the engine base of the 175 horse-power model coincided

precisely with that of the 150 horse-power model, and the 200 and 240

horse-power models retained the same base dimensions. It was estimated,

in 1918, that well over eighty per cent of German aircraft was engined

with the Mercedes type.

In design and construction, there was nothing abnormal about the

Mercedes engine, the keynote throughout being extreme reliability and

such simplification of design as would permit of mass production in

different factories. Even before the war, the long list of records set

up by this engine formed practical application of the wisdom of this

policy; Bohn’s flight of 24 hours 10 minutes, accomplished on July 10th

and 11th, 1914, is an instance of this--the flight was accomplished on

an Albatross biplane with a 75 horse-power Mercedes engine. The radial

type, instanced in other countries by the Salmson and Anzani makes, was

not developed in Germany; two radial engines were made in that country

before the war, but the Germans seemed to lose faith in the type under

war conditions, or it may have been that insistence on standardisation

ruled out all but the proved examples of engine.

Details of one of the middle sizes of Mercedes motor, the 176

horse-power type, apply very generally to the whole range; this size

was in use up to and beyond the conclusion of hostilities, and it may

still be regarded as characteristic of modern (1920) German practice.

The engine is of the fixed vertical type, has six cylinders in line,

not off-set, and is water-cooled. The cam shaft is carried in a special

bronze casing, seated on the immediate top of the cylinders, and a

vertical shaft is interposed between crankshaft and camshaft, the

latter being driven by bevel gearing.

On this vertical connecting-shaft the water pump is located, serving

to steady the motion of the shaft. Extending immediately below the

camshaft is another vertical shaft, driven by bevel gears from the

crankshaft, and terminating in a worm which drives the multiple piston

oil pumps.

The cylinders are made from steel forgings, as are the valve chamber

elbows, which are machined all over and welded together. A jacket of

light steel is welded over the valve elbows and attached to a flange on

the cylinders, forming a water-cooling space with a section of about

7/16 of an inch. The cylinder bore is 5·5 inches, and the stroke 6·29

inches. The cylinders are attached to the crank case by means of dogs

and long through bolts, which have shoulders near their lower ends and

are bolted to the lower half of the crank chamber. A very light and

rigid structure is thus obtained, and the method of construction won

the flattery of imitation by makers of other nationality.

The cooling system for the cylinders is extremely efficient. After

leaving the water pump, the water enters the top of the front cylinders

and passes successively through each of the six cylinders of the row;

short tubes, welded to the tops of the cylinders, serve as connecting

links in the system. The Panhard car engines for years were fitted with

a similar cooling system, and the White and Poppe lorry engines were

also similarly fitted; the system gives excellent cooling effect where

it is most needed, round the valve chambers and the cylinder heads.

The pistons are built up from two pieces; a dropped forged steel

piston head, from which depend the piston pin bosses, is combined

with a cast-iron skirt, into which the steel head is screwed. Four

rings are fitted, three at the upper and one at the lower end of the

piston skirt, and two lubricating oil grooves are cut in the skirt,

in addition to the ring grooves. Two small rivets retain the steel

head on the piston skirt after it has been screwed into position, and

it is also welded at two points. The coefficient of friction between

the cast-iron and steel is considerably less than that which would

exist between two steel parts, and there is less tendency for the

skirt to score the cylinder walls than would be the case if all steel

were used--so noticeable is this that many makers, after giving steel

pistons a trial, discarded them in favour of cast-iron; the Gnome is an

example of this, being originally fitted with a steel piston carrying a

brass ring, discarded in favour of a cast-iron piston with a percentage

of steel in the metal mixture. In the Le Rhone engine the difficulty is

overcome by a cast-iron liner to the cylinders.

The piston pin of the Mercedes is of chrome nickel steel, and is

retained in the piston by means of a set screw and cotter pin. The

connecting rods, of I section, are very short and rigid, carrying

floating bronze bushes which fit the piston pins at the small end, and

carrying an oil tube on each for conveying oil from the crank pin to

the piston pin.

The crankshaft is of chrome nickel steel, carried on seven bearings.

Holes are drilled through each of the crank pins and main bearings,

for half the diameter of the shaft, and these are plugged with pressed

brass studs. Small holes, drilled through the crank cheeks, serve

to convey lubricant from the main bearings to the crank pins. The

propeller thrust is taken by a simple ball thrust bearing at the

propeller end of the crankshaft, this thrust bearing being seated in

a steel retainer which is clamped between the two halves of the crank

case. At the forward end of the crankshaft there is mounted a master

bevel gear on six splines; this bevel floats on the splines against

a ball thrust bearing, and, in turn, the thrust is taken by the crank

case cover. A stuffing box prevents the loss of lubricant out of the

front end of the crank chamber, and an oil thrower ring serves a

similar purpose at the propeller end of the crank chamber.

With a motor speed of 1,450 r.p.m., the vertical shaft at the forward

end of the motor turns at 2,175 r.p.m., this being the speed of the two

magnetos and the water pump. The lower vertical shaft bevel gear and

the magneto driving gear are made integral with the vertical driving

shaft, which is carried in plain bearings in an aluminium housing. This

housing is clamped to the upper half of the crank case by means of

three studs. The cam-shaft carries eighteen cams, these being the inlet

and exhaust cams, and a set of half compression cams which are formed

with the exhaust cams and are put into action when required by means of

a lever at the forward end of the cam-shaft. The cam-shaft is hollow,

and serves as a channel for the conveyance of lubricating oil to each

of the camshaft bearings. At the forward end of this shaft there is

also mounted an air pump for maintaining pressure on the fuel supply

tank, and a bevel gear tachometer drive.

Lubrication of the engine is carried out by a full pressure system.

The oil is pumped through a single manifold, with seven branches to

the crankshaft main bearings, and then in turn through the hollow

crankshaft to the connecting-rod big ends and thence through small

tubes, already noted, to the small end bearings. The oil pump has four

pistons and two double valves driven from a single eccentric shaft on

which are mounted four eccentrics. The pump is continuously submerged

in oil; in order to avoid great variations in pressure in the oil lines

there is a piston operated pressure regulator, cut in between the

pump and the oil lines. The two small pistons of the pump take fresh

oil from a tank located in the fuselage of the machine; one of these

delivers oil to the cam shaft, and one delivers to the crankshaft;

this fresh oil mixes with the used oil, returns to the base, and back

to the main large oil pump cylinders. By means of these small pump

pistons a constant quantity of oil is kept in the motor, and the oil

is continually being freshened by means of the new oil coming in. All

the oil pipes are very securely fastened to the lower half of the crank

case, and some cooling of the oil is effected by air passing through

channels cast in the crank case on its way to the carburettor.

A light steel manifold serves to connect the exhaust ports of the

cylinders to the main exhaust pipe, which is inclined about 25 degrees

from vertical and is arranged to give on to the atmosphere just over

the top of the upper wing of the aeroplane.

As regards carburation, an automatic air valve surrounds the throat of

the carburettor, maintaining normal composition of mixture. A small jet

is fitted for starting and running without load. The channels cast in

the crank chamber, already alluded to in connection with oil-cooling,

serve to warm the air before it reaches the carburettor, of which the

body is water-jacketed.

Ignition of the engine is by means of two Bosch Z H 6 magnetos, driven

at a speed of 2,175 revolutions per minute when the engine is running

at its normal speed of 1,450 revolutions. The maximum advance of spark

is 12 mm., or 32 degrees before the top dead centre, and the firing

order of the cylinders is 1, 5, 3, 6, 2, 4.

The radiator fitted to this engine, together with the water-jackets,

has a capacity of 25 litres of water, it is rectangular in shape, and

is normally tilted at an angle of 30 degrees from vertical. Its weight

is 26 kg., and it offers but slight head resistance in flight.

The radial type of engine, neglected altogether in Germany, was brought

to a very high state of prefection at the end of the War period by

British makers. Two makes, the Cosmos Engineering Company’s ‘Jupiter’

and ‘Lucifer,’ and the A.B.C. ‘Wasp II’ and ‘Dragon Fly 1A’ require

special mention for their light weight and reliability on trials.

The Cosmos ‘Jupiter’ was--for it is no longer being made--a 450

horse-power nine-cylinder radial engine, air-cooled, with the cylinders

set in one single row; it was made both geared to reduce the propeller

revolutions relatively to the crankshaft revolutions, and ungeared;

the normal power of the geared type was 450 horse-power, and the total

weight of the engine, including carburettors, magnetos, etc., was only

757 lbs.; the engine speed was 1,850 revolutions per minute, and the

propeller revolutions were reduced by the gearing to 1,200. Fitted to a

‘Bristol Badger’ aeroplane, the total weight was 2,800 lbs, including

pilot, passenger, two machine-guns, and full military load; at 7,000

feet the registered speed, with corrections for density, was 137 miles

per hour; in climbing, the first 2,000 feet was accomplished in 1

minute 4 seconds; 4,000 feet was reached in 2 minutes 10 seconds; 6,000

feet was reached in 3 minutes 33 seconds, and 7,000 feet in 4 minutes

15 seconds. It was intended to modify the plane design and fit a new

propeller, in order to attain even better results, but, if trials were

made with these modifications, the results are not obtainable.

The Cosmos ‘Lucifer’ was a three-cylinder radial type engine of

100 horse-power, inverted Y design, made on the simplest possible

principles with a view to quantity production and extreme reliability.

The rated 100 horse-power was attained at 1,600 revolutions per minute,

and the cylinder dimensions were 5·75 bore by 6·25 inches stroke.

The cylinders were of aluminium and steel mixture, with aluminium

heads; overhead valves, operated by push-rods on the front side of

the cylinders, were fitted, and a simple reducing gear ran them at

half engine speed. The crank case was a circular aluminium casting,

the engine being attached to the fuselage of the aeroplane by a

circular flange situated at the back of the case; propeller shaft and

crankshaft were integral. Dual ignition was provided, the generator

and distributors being driven off the back end of the engine and the

distributors being easily accessible. Lubrication was by means of two

pumps, one scavenging and one suction, oil being fed under pressure

from the crankshaft. A single carburettor fed all three cylinders, the

branch pipe from the carburettor to the circular ring being provided

with an exhaust heater. The total weight of the engine, ‘all on,’ was

280 lbs.

[Illustration: ‘Dragonfly’ 1 A.]

[Illustration: ‘Dragonfly’ piston assembly.]

[Illustration: ‘Dragonfly’ cylinder.]

The A.B.C. ‘Wasp II,’ made by Walton Motors, Limited, is a

seven-cylinder radial, air-cooled engine, the cylinders having a bore

of 4·75 inches and stroke 6·25 inches. The normal brake horse-power

at 1,650 revolutions is 160, and the maximum 200 at a speed of

1,850 revolutions per minute. Lubrication is by means of two rotary

pumps, one feeding through the hollow crankshaft to the crank pin,

giving centrifugal feed to big end and thence splash oiling, and

one feeding to the nose of the engine, dropping on to the cams and

forming a permanent sump for the gears on the bottom of the engine

nose. Two carburettors are fitted, and two two-spark magnetos, running

at one and three-quarters engine speed. The total weight of this

engine is 350 lbs., or 1·75 lbs. per horse-power. Oil consumption at

1,850 revolutions is ·03 pints per horse-power per hour, and petrol

consumption is ·56 pints per horse-power per hour. The engine thus

shows as very economical in consumption, as well as very light in

weight.

The A.B.C. ‘Dragon Fly 1A’ is a nine-cylinder radial engine having

one overhead inlet and two overhead exhaust valves per cylinder. The

cylinder dimensions are 5·5 inches bore by 6·5 inches stroke, and

the normal rate of speed, 1,650 revolutions per minute, gives 340

horse-power. The oiling is by means of two pumps, the system being

practically identical with that of the ‘Wasp II.’ Oil consumption is

·021 pints per brake horse-power per hour, and petrol consumption ·56

pints--the same as that of the ‘Wasp II.’ The weight of the complete

engine, including propeller boss, is 600 lbs., or 1·765 lbs. per

horse-power.

These A.B.C. radials have proved highly satisfactory on tests, and

their extreme simplicity of design and reliability commend them as

engineering products and at the same time demonstrate the value,

for aero work, of the air-cooled radial design--when this latter

is accompanied by sound workmanship. These and the Cosmos engines

represent the minimum of weight per horse-power yet attained, together

with a practicable degree of reliability, in radial and probably any

aero engine design.

APPENDIX A

GENERAL MENSIER’s REPORT ON THE TRIALS OF CLEMENT ADER’S ‘AVION.’

PARIS, _October 21, 1897_.

_Report on the trials of M. Clement Ader’s aviation apparatus._

M. Ader having notified the Minister of War by letter, July 21, 1897,

that the Apparatus of Aviation which he had agreed to build under the

conditions set forth in the convention of July 24th, 1894, was ready,

and therefore requesting that trials be undertaken before a Committee

appointed for this purpose as per the decision of August 4th, the

Committee was appointed as follows:--

Division General Mensier, Chairman; Division General Delambre,

Inspector General of the Permanent Works of Coast Defence, Member of

the Technical Committee of the Engineering Corps; Colonel Laussedat,

Director of the Conservatoire des Arts et Métiers; Sarrau, Member of

the Institute, Professor of Mechanical Engineering at the Polytechnic

School; Leaute, Member of the Institute, Professor of Mechanical

Engineering at the Polytechnique School.

Colonel Laussedat gave notice at once that his health and work as

Director of the Conservatoire des Arts et Métiers did not permit him

to be a member of the Committee; the Minister therefore accepted his

resignation on September 24th, and decided not to replace him.

Later on, however, on the request of the Chairman of the Committee,

the Minister appointed a new member, General Grillon, commanding the

Engineer Corps of the Military Government of Paris.

To carry on the trials which were to take place at the camp of Satory,

the Minister ordered the Governor of the Military Forces of Paris to

requisition from the Engineer Corps, on the request of the Chairman of

the Committee, the men necessary to prepare the grounds at Satory.

After an inspection made on the 16th an aerodrome was chosen. M. Ader’s

idea was to have it of circular shape with a width of 40 metres and an

average diameter of 450 metres. The preliminary work, laying out the

grounds, interior and exterior circumference, etc., was finished at the

end of August; the work of smoothing off the grounds began September

1st with forty-five men and two rollers, and was finished on the day of

the first tests, October 12th.

The first meeting of the Committee was held August 18th in M. Ader’s

workshop; the object being to demonstrate the machine to the Committee

and give all the information possible on the tests that were to be

held. After a careful examination and after having heard all the

explanations by the inventor which were deemed useful and necessary,

the Committee decided that the apparatus seemed to be built with a

perfect understanding of the purpose to be fulfilled as far as one

could judge from a study of the apparatus at rest; they therefore

authorised M. Ader to take the machine apart and carry it to the camp

at Satory so as to proceed with the trials.

By letter of August 19th the Chairman made report to the Minister of

the findings of the Committee.

The work on the grounds having taken longer than was anticipated,

the Chairman took advantage of this delay to call the Committee

together for a second meeting, during which M. Ader was to run the two

propulsive screws situated at the forward end of the apparatus.

The meeting was held October 2nd. It gave the Committee an opportunity

to appreciate the motive power in all its details; firebox, boiler,

engine, under perfect control, absolute condensation, automatic fuel

and feed of the liquid to be vaporised, automatic lubrication and

scavenging; everything, in a word, seemed well designed and executed.

The weights in comparison with the power of the engine realised

a considerable advance over anything made to date, since the two

engines weighed together realised 42 kg., the firebox and boiler 60

kg., the condenser 15 kg., or a total of 117 kg. for approximately 40

horse-power or a little less than 3 kg. per horse-power.

One of the members summed up the general opinion by saying: ‘Whatever

may be the result from an aviation point of view, a result which could

not be foreseen for the moment, it was nevertheless proven that from

a mechanical point of view M. Ader’s apparatus was of the greatest

interest and real ingeniosity. He expressed a hope that in any case the

machine would not be lost to science.’

The second experiment in the workshop was made in the presence of the

Chairman, the purpose being to demonstrate that the wings, having a

spread of 17 metres, were sufficiently strong to support the weight

of the apparatus. With this object in view, 14 sliding supports were

placed under each one of these, representing imperfectly the manner

in which the wings would support the machine in the air; by gradually

raising the supports with the slides, the wheels on which the machine

rested were lifted from the ground. It was evident at that time

that the members composing the skeleton of the wings supported the

apparatus, and it was quite evident that when the wings were supported

by the air on every point of their surface, the stress would be better

equalised than when resting on a few supports, and therefore the

resistance to breakage would be considerably greater.

After this last test, the work on the ground being practically

finished, the machine was transported to Satory, assembled and again

made ready for trial.

At first M. Ader was to manœuvre the machine on the ground at a

moderate speed, then increase this until it was possible to judge

whether there was a tendency for the machine to rise; and it was only

after M. Ader had acquired sufficient practice that a meeting of the

Committee was to be called to be present at the first part of the

trials; namely, volutions of the apparatus on the ground.

The first test took place on Tuesday, October 12th, in the presence of

the Chairman of the Committee. It had rained a good deal during the

night and the clay track would have offered considerable resistance to

the rolling of the machine; furthermore, a moderate wind was blowing

from the south-west, too strong during the early part of the afternoon

to allow of any trials.

Toward sunset, however, the wind having weakened, M. Ader decided to

make his first trial; the machine was taken out of its hangar, the

wings were mounted and steam raised. M. Ader in his seat had, on each

side of him, one man to the right and one to the left, whose duty was

to rectify the direction of the apparatus in the event that the action

of the rear wheel as a rudder would not be sufficient to hold the

machine in a straight course.

At 5.25 p.m. the machine was started, at first slowly and then at an

increased speed; after 250 or 300 metres, the two men who were being

dragged by the apparatus were exhausted and forced to fall flat on

the ground in order to allow the wings to pass over them, and the

trip around the track was completed, a total of 1,400 metres, without

incident, at a fair speed, which could be estimated to be from 300

to 400 metres per minute. Notwithstanding M. Ader’s inexperience,

this being the first time that he had run his apparatus, he followed

approximately the chalk line which marked the centre of the track and

he stopped at the exact point from which he started.

The marks of the wheels on the ground, which was rather soft, did not

show up very much, and it was clear that a part of the weight of the

apparatus had been supported by the wings, though the speed was only

about one-third of what the machine could do had M. Ader used all its

motive power; he was running at a pressure of from 3 to 4 atmospheres,

when he could have used 10 to 12.

This first trial, so fortunately accomplished, was of great importance;

it was the first time that a comparatively heavy vehicle (nearly 400

kg., including the weight of the operator, fuel, and water) had been

set in motion by a tractive apparatus, using the air solely as a

propelling medium. The favourable report turned in by the Committee

after the meeting of October 2nd was found justified by the results

demonstrated on the grounds, and the first problem of aviation, namely,

the creation of efficient motive power, could be considered as solved,

since the propulsion of the apparatus in the air would be a great deal

easier than the traction on the ground, provided that the second part

of the problem, the sustaining of the machine in the air, would be

realised.

The next day, Wednesday the 13th, no further trials were made on

account of the rain and wind.

On Thursday the 14th the Chairman requested that General Grillon, who

had just been appointed a member of the Committee, accompany him so as

to have a second witness.

The weather was fine, but a fairly strong, gusty wind was blowing from

the south. M. Ader explained to the two members of the Committee the

danger of these gusts, since at two points of the circumference the

wind would strike him sideways. The wind was blowing in the direction

A B, the apparatus starting from C, and running in the direction shown

by the arrow. The first dangerous spot would be at B. The apparatus

had been kept in readiness in the event of the wind dying down. Toward

sunset the wind seemed to die down, as it had done on the evening of

the 12th. M. Ader hesitated, which, unfortunately, further events only

justified, but decided to make a new trial.

At the start, which took place at 5.15 p.m., the apparatus, having

the wind in the rear, seemed to run at a fairly regular speed; it

was, nevertheless, easy to note from the marks of the wheels on the

ground that the rear part of the apparatus had been lifted and that

the rear wheel, being the rudder, had not been in constant contact

with the ground. When the machine came to the neighbourhood of B, the

two members of the Committee saw the machine swerve suddenly out of

the track in a semicircle, lean over to the right and finally stop.

They immediately proceeded to the point where the accident had taken

place and endeavoured to find an explanation for the same. The Chairman

finally decided as follows:--

M. Ader was the victim of a gust of wind which he had feared as he

explained before starting out; feeling himself thrown out of his

course, he tried to use the rudder energetically, but at that time the

rear wheel was not in contact with the ground, and therefore did not

perform its function; the canvas rudder, which had as its purpose the

manœuvring of the machine in the air, did not have sufficient action

on the ground. It would have been possible without any doubt to react

by using the propellers at unequal speed, but M. Ader, being still

inexperienced, had not thought of this. Furthermore, he was thrown out

of his course so quickly that he decided, in order to avoid a more

serious accident, to stop both engines. This sudden stop produced the

half-circle already described and the fall of the machine on its side.

The damage to the machine was serious; consisting at first sight of the

rupture of both propellers, the rear left wheel and the bending of the

left wing tip. It will only be possible to determine after the machine

is taken apart whether the engine, and more particularly the organs of

transmission, have been put out of line.

Whatever the damage may be, though comparatively easy to repair, it

will take a certain amount of time, and taking into consideration the

time of year it is evident that the tests will have to be adjourned for

the present.