*General Notes*. It is assumed that the reader understands the
principles of the automobile motor and its accessories, for a minute
description of gas-engine principles does not fall within the scope of
this book. If more information is desired on this subject, the reader is
referred to the author’s "Practical Handbook of Gas, Oil and Steam
Engines." Only those features peculiar to aeronautic motors will be
discussed in this chapter.
*Aeronautic Requirements*. The principal requirements of an aeronautic
motor are light weight, low oil and fuel consumption, reliability and
compactness. The outline as viewed from the shaft end is also very
important, for the motor must be mounted in a narrow streamline body.
The compression pressures are much higher than those employed on auto
motors, and the speed is generally lower. With one or two exceptions the
four-stroke cycle has been universally adopted.
Aeronautic service is a severe test for the motor. From the start to the
finish of a flight, the aeroplane motor is on a steady grind, loaded at
least to 75 per cent of its rated power. The foundations are light and
yielding and the air density varies rapidly with changes in the
altitude. As the fuel and oil require an expenditure of power for their
support, the fuel consumption becomes of great importance, especially in
long flights. Because of the heavy normal load the lubricating system
must be as nearly perfect as it is possible to make it.
A motor car runs normally at from 10 to 25 per cent of its rated
horsepower, while the aero motor may develop as high as 75 per cent to
100 per cent for hours at a time. A car engine of 672 cubic inches
displacement is rated at 65 horsepower, while the same size aero engine
has a rating of 154. On the basis of normal output, this ratio is about
7 to 1, and taking the weight of the aero motor as one-half that of the
auto type, the true output ratio becomes 14 to 1. Up to the time of a
complete overhaul (50 hours), and at 100 miles per hour, the average
distance traveled by the aero motor is 5000 miles. The equivalent motor
car mileage is 25,000, and the duration is about 1000 hours. This
suggests the necessity for improved materials of construction. Even on
the present aeronautic motors the fiber stress in the crank-shaft ranges
from 120,000 to 140,000 pounds per square inch against the 80,000-pound
stress used in auto shafts. The crank case of an aeronautic motor must
be particularly rigid to withstand the stresses due to the light
mounting, and this demands a higher grade metal than that ordinarily
used with automobiles. Unlike the car engine, quality comes first and
price is a secondary consideration.
*Cooling Systems*. Both the air and water cooling system is used, the
former for light fast aeroplanes such as speed scouts, and the latter
for the larger and more heavily powered machines. Even in some types of
speed scouts the air-cooled motor has been displaced by the
water-cooled, owing to the fact that the air-cooler cannot be built
satisfactorily for outputs much greater than 110 horsepower. By
increasing the revolutions of the stationary water-cooled type an
increase in power may be had with the same cylinders, but in the case of
the rotary air-cooled type the speed is limited by the centrifugal
forces acting on the cylinders.
[Illustration: A 6-Cylinder Hall-Scott Motor Installed in a Martin
Biplane.]
While the weight of the radiator, water and piping increase the weight
of the water-cooled motor very considerably, the total weight is not
excessive. When the fuel is considered, the total weight is below that
of the rotary when long flights are attempted. The radiator and water
add complication and are a source of danger. The radiators increase the
head resistance and add very considerably to the maintenance cost.
[Illustration: A Motor Installation in a Pusher Type Biplane, Showing
the Motor at the Rear and the Double Radiator Sections Over the Body.]
Each type of cooling has its limitations, and it is hoped that an
improvement in cooling may be had in the near future. This system should
primarily reduce the size and resistance of the power plant, and if
possible the weight, although the latter is a secondary consideration.
At present the cooling system prevents even an approach to the true
streamline form of the body.
*Propeller Speed*. For the best results, the propeller speed should not
exceed 1200 revolutions per minute, and for structural reasons this is
generally limited to 1500 R. P. M. This at once puts a limiting value on
the output of a given size engine unless a gear down arrangement is
used. It should be understood, between certain limits, that the power
output increases roughly as the speed. With direct drive arrangements in
which the propeller is mounted directly on the end of the engine shaft,
the motor revs. are necessarily the propeller revs., and the only way of
increasing the speed is by increasing the length of the stroke or by
gearing down. An increase in stroke adds rapidly to the weight by
increasing the cylinder length, length of connecting rod, length of
crank throws, etc.
*Horsepower Rating*. At present there are many methods of calculating
the horsepower of gasoline engines. Formula applying to auto or boat
motors does not apply to flight conditions, for the aero motor is
essentially a high compression type and has a greater output per unit of
displacement. It is not practical to give the rated horsepower as the
maximum output possible under ideal conditions, for this would give no
idea as to the practical capabilities except by long tedious
calculation. The brake horsepower would give no overload capacity at a
fixed propeller speed, and the conditions are entirely different from
those regulating the rating of auto motors. The latter can be forced up
to the wrecking speed, or many times the normal automobile speed of 30
miles per hour.
As aero engines are generally well kept up, and well tuned at all times,
the rated horsepower may be taken from 15 to 20 per cent below that of
the maximum brake horsepower. In geared-down motors, the gear efficiency
is still to be considered. The question of the quality of the mixture,
and barometric pressure, also enter into the problem whether the power
is rated on the maximum obtained with a rich mixture, or is calculated
from the output at the maximum efficiency. A writer in "Aviation"
suggests that the rated horsepower be taken as 95 per cent of the power
developed at a point midway between the maximum output, and the output
at the greatest efficiency. Barometric pressure to be 30 inches and the
revolutions 1200.
Owing to the great diversity in the bore-stroke ratio, a power formula
must include the bore and stroke. This makes the S.A.E. formula for auto
motors impossible. A formula is proposed by a writer in "Aviation." The
writer has checked this up with the published performance of several
well-known aeronautical motors.
H = B²SNR/12,500 Where B = bore in inches, S = stroke in inches, N =
number of cylinders, R = Crankshaft revolutions per minute, and H =
rated horsepower. This applies only to the four-stroke cycle type.
*Power and Altitude*. The power drops off rapidly with an increase in
altitude unless corrections are made for compression and mixture. With
constant volume, the decreased density causes decreased compression. As
the weight of air taken in per stroke is reduced, this also reduces the
amount of fuel that can be burned per stroke. By holding the compression
constant through adjustment of the clearance or valve motion, a fairly
constant output can be had through a wide range of altitudes.
A compression of 115 pounds per square inch (commonly used) is difficult
to handle with a light construction, but this pressure must be obtained
if the output is to be kept within practical limits. Engines having a
compression ratio of as high as 6 are running satisfactorily at sea
level, this ratio giving a mean effective working pressure of 134 pounds
per square inch. With this ratio the engine cannot be used with full
open throttle at sea level for more than 10 or 15 minutes without
causing damage to the shaft, bearing and valves. At about 10,000 feet
the compression is normal.
At great altitudes carburetion has become a great problem, and as aerial
battles have already taken place at elevations of 20,000 feet, it is
quite possible that future motors will be equipped with some device that
will force a measured fuel charge into the cylinders. The air necessary
for the combustion will also have to be pumped in by some means.
*Weight Per Horsepower*. The weight per horsepower of the engine is a
very loose term since so much depends upon the equipment included in the
weight. As many as 20 items may be considered as being in the doubtful
list, and among these are the radiator water, piping, mounting,
propeller hub, oil in sump, wiring, self-starter, etc. The only true
unit weight is that obtained by taking the plant complete (ready to
run), with the cooling system, gasoline for an hour’s flight, and the
oil. The weight of the bare engine signifies nothing. The weights of the
various items used on well known motors are given in a table under the
chapter "Weight Calculations." While the bare weight of a certain engine
may be very low per brake horsepower, an excessive fuel consumption will
often run the effective weight up and over that of a type in which the
bare weight is far greater. The weight of the engine per horsepower,
including the magneto and carbureter, will run from 2.2 to 5.0 pounds,
according to the type.
[Illustration: Two Examples of Cowls Used Over Rotary Cylinder Motors
(Air Cooled).]
*Fuel Consumption*. The fuel consumption of water-cooled motors varies
from 0.48 to 0.65 pounds per horsepower hour, an average of 0.6 being
safe. The fuel consumption of a rotary air-cooled motor will range from
0.6 to 0.75. The oil consumption varies from 0.18 gallons per horsepower
in the air-cooled type to 0.035 with the water-cooled stationary motor.
*Radiators*. Owing to extremes in the temperature of the air at
different altitudes, the radiating surface should be divided into
sections so that a constant cooling effect can be obtained by varying
the effective surface of the radiator. The temperature can also be
controlled by an automatically regulated by-pass which short circuits a
part of the radiator water at low temperatures. Constant water
temperature has much to do with the efficiency and general operation of
the motor, and there will be only one temperature at which the best
results can be obtained.
[Illustration: Typical Radiators. A) Side or Top Type.]
[Illustration: Typical Radiators. (B) Front Type.]
Hunsaker finds that 0.83 square feet of actual cooling surface per
horsepower is correct at 60 M. P. H., while others give a value of about
100 square foot under similar conditions. The front or projected area
varies with the thickness of the radiator, the thicknesses varying from
2 to 5 inches. The Livingston radiator gives a cooling surface of 50
square inches per square inch of front surface. The total cooling effect
depends upon the speed, the location in regard to the slipstream, and
the position on the body. A radiator maker should always be consulted
when making the final calculations. See Chapter XVI.
*Fuel Tanks and Piping*. The fuel tanks may be of copper, aluminum or
tin-coated steel, and all joints should be welded or riveted. Never
depend upon solder, as such joints soon open through the vibration of
the engine. Gasoline should not come into contact with steel, nor the
zinc used on galvanized iron. Splash plates are provided to keep the
fluid from surging back and forth while in flight. All gas should be
supplied to the engine through a filter or strainer placed in the main
gas line. The valves in the fuel lines should be provided with
stopcocks, so arranged that they can be closed from the pilot’s seat.
In general, the carbureters should be fed by gravity from an overhead
service tank, this tank being supplied from the main reservoir by air
pressure or a gasoline pump. The air can be compressed by a pump on the
engine or by a paddle driven pump operated by the airstream, and as a
rule the latter is preferable, as it can be operated with the aeroplane
gliding and with the engine dead. Air pressure systems are likely to
fail through leaks, while with a good gasoline pump conditions are much
more positive. The gravity service tank should be located so that it
will feed correctly with the aeroplane tilted at least 30 degrees from
the horizontal.
[Illustration: Two Views of the "Monosoupape" Gnome Rotary Cylinder
Motor. This Motor Has 9 Cylinders Arranged Radially Around the
Crankshaft and Develops 100 Horsepower. The Cylinders Are Air Cooled.]
The gasoline piping should be at least 5/16 inch inside diameter, and
should be most securely connected and supported against vibration. To
guard against crystallization at the point of attachment, special
flexible rubber hose is generally used. This must be hose made specially
for this purpose, as ordinary rubber hose is soon dissolved or rotted by
gasoline and oil. Air pockets must be avoided at every point in the fuel
and oil system.
[Illustration: Hall-Scott "Big Six" Aeronautical Motor of the Vertical
Water-cooled Type. 125 Horsepower.]
*Rotating Cylinder Motors*. The first rotating cylinder motor in use was
the American Adams-Farwell, a type that was soon followed by the better
known French "Gnome." Other motors of this type are the Clerget,
LeRhone, Gyro and Obereusel. They are all of the air-cooled type—cooled
partly by the revolution of the cylinders about the crank-shaft, and
partly by the propeller slipstream. While the pistons slide through the
cylinder bore, the rotating cylinder motor is not truly a reciprocating
type, as the pistons do not move back and forth in regard to the crank
shaft. The cylinders revolve about the crank shaft as a center, while
the pistons and connecting rods revolve about the crank pin, the
difference in the pivot point causing relative, but not actual,
reciprocation.
[Illustration: Hall-Scott 4-Cylinder Vertical Water-cooled Motor. 80-90
Horsepower.]
The original Gnome motor drew in the charge through an inlet valve in
the piston head. The gas passed from the mixer, through the hollow
crank-shaft, and then into the crank-case. The exhaust valve was in the
cylinder head. This valve arrangement was not entirely satisfactory, and
the company developed the "Monosoupape" or "Single valve" type. The 100
H. P. Monosoupape Gnome has 9 cylinders, 4.3" x 5.9". The total weight
is 272 pounds and the unit weight is 2.72 pounds per horsepower. It
operates on the four-stroke cycle principle. The gas consumption is 12
gals. per hour, and it uses 2.4 gals. of castor oil. The cylinders and
cooling fins are machined from a solid steel forging, weighing 88
pounds. The finished cylinder weighs 5.5 pounds after machining. The
walls are very thin, probably about 1/16 inch, but they stand up well
under service conditions.
[Illustration: Sturtevant "V" Type 8-Cylinder Water Cooled Aeronautical
Motor. This Motor Is Provided With a Reduction Gear Shown at the Rear of
the Crankcase.]
Assuming the piston to be on the compression stroke, the ignition will
occur from 15° to 20° before the top dead center. Moving down on the
working stroke, and at 85° from top dead center, the exhaust valve
begins to open, and the exhaust continues until the piston returns to
the upper dead center. With the valve still open, pure air now begins to
enter through the exhaust valve and continues to flow until the valve
closes at 65° below the bottom center. Still descending, the piston
forms a partial vacuum in the cylinder, until at 2° before the lower
center the piston opens the ports and a very rich mixture is drawn in
from the crank case. This rich mixture is diluted to the proper density
by the air already in the cylinder, and forms a combustible gas. The
upward movement of the piston on the compression stroke closes the ports
and compression begins. The mixture enters the crank case through a
hollow shaft, with the fuel jets near the crank throws. A timed fuel
pump injects the fuel at the proper intervals.
[Illustration: Dusenberg 4-Cylinder Vertical Water Cooled Motor With
Reduction Gear. Four Valves Are Used Per Cylinder. Note Peculiar Valve
Motion.]
*Curtiss Motors*. The Curtiss motors are of the water-cooled "V" type,
with 6 to 8 cylinders per row. These are probably the best known motors
in America and are the result of years of development, as Curtiss was
the first to manufacture aero motors on a practical scale.
[Illustration: Curtiss Type OX-5 Eight Cylinder Aeroplane Motor]
*Hall-Scott Motors*. These motors are made by one of the pioneer
aeronautical motor builders, and have met with great favor. They are of
the vertical water-cooled type, and with the exception of minor details
and weight are very similar in external appearance to the automobile
motor. Four and 6-cylinder types are built.
*Sturtevant Motors*. These are of the "V" water-cooled type, and are
provided with or without a reducing gear. At least one model is provided
with lined aluminum cylinders.
*Dusenberg Motor*. This is a four-cylinder, water-cooled, vertical motor
with a very peculiar valve motion. The valves are operated by long
levers extending from the camshaft. Two inlet, and two exhaust valves,
are used per cylinder. The motor is generally furnished with a reducing
gear.
*Roberts Motor*. This is a solitary example of the two-stroke cycle
type, and has been used for many years. It is simple and compact, and is
noteworthy for the simplicity of its oiling system. The oil is mixed
with the gasoline, and is fed through the carbureter. This is one of the
many advantages of a two-stroke cycle motor.
*Table of Aeronautical Motors*. The following table will give an idea as
to the general dimensions of American aeronautical motors:
*The Liberty Motor*. The necessity of speed and quantity in the
production of aeronautical motors after the declaration of war caused
the Government to seriously consider the design of a highly standardized
motor. This idea was further developed in a conference with
representatives of the French and British missions on May 28, 1917, and
was then submitted in the form of sketches at a joint meeting of our
allies, the Aircraft Production Board, and the Joint Army and Navy
Technical Board. The speed with which the work was pushed is remarkable,
for on July 3rd, the first model of the eight cylinder type was
delivered to the Bureau of Standards. Work was then concentrated on the
12 cylinder model, and one of the experimental engines passed the 50
hour test August 25, 1917.
It is of the "V" type with the cylinder blocks at an angle of 45 degrees
instead of 60 degrees as in the majority of 12 cylinder "V" motors. This
makes the motor much narrower and more suitable for installation in the
fuselage, and in this respect is similar to the arrangement of the old
Packard aviation motor. It has the additional advantages of
strengthening the crank case. The bore and stroke is 5" x 7" as in the
Hall-Scott models A-5 and A-7. The cylinders combine the leading
features of the German Mercedes, the English Rolls-Royce,
Lorraine-Dietrich, and Isotta-Fraschini. Steel cylinder walls are used
with pressed steel water jackets, the latter being applied by means of a
method developed by the Packard Company. The valve cages are drop
forgings, welded to the cylinder heads.
The camshaft and valve gear are above the cylinder head as in the
Mercedes, but the lubrication of the parts was improved upon by the
Packard Company.
The crankshaft follows standard 12 cylinder practice except as to the
oiling system, the latter following German practice rather closely. The
first system used one pump to keep the crankcase empty delivering the
oil to an outside reservoir. A second pump took the oil from the
reservoir and delivered it to the main crankshaft bearings under
pressure. The overflow from the main bearings traveled out over the face
of the crank throw cheeks to a "Scupper," which collected the excess for
crank pin lubrication. In the present system, a similar general method
is followed except that the pressure oil is not only fed to the main
crankshaft bearings, but also through holes in the crank cheeks to the
crank pins instead of by the former scupper feed.
A special Zenith carburetor is used, that is particularly adapted to the
Liberty motor. A Delco ignition system of special form is installed to
meet the peculiar cylinder block angle of 45 degrees. This ignition is
of the electric generator type and magnetos are not used.
Several American records have been broken by the new motor, and it is
reported to have given very satisfactory service, but full details of
the performance are difficult to obtain owing to the strict censorship
maintained in regard to things aeronautic. The motor is particularly
well adapted to heavy bombing and reconnaissance type machines, or for
heavy duty. It is reported that the use of the motor has been
discontinued on speed scouts, although further developments along this
line may not have been reported.
The following gives the principal characteristics of the Liberty motor,
issued by the National Advisory Committee for Aeronautics.
────────────────────────────────────────────────────────────────────
Year Horse-power Weight Weight Per Gasoline H. P.
(Model) Pounds H. P. Hour
────────────────────────────────────────────────────────────────────
1917 400 801 2.00 0.50
────────────────────────────────────────────────────────────────────
1918 432 808 1.90 0.48
────────────────────────────────────────────────────────────────────
1918 450 825 1.80 0.46
────────────────────────────────────────────────────────────────────
The motors listed are all 12 cylinder models, and the output and unit
weights are based on a crank-shaft speed of 1800 R. P. M. The 5" x 7"
bore and stroke give an output of 37.5 horsepower per cylinder in the
latest model. In 1917, the Liberty motor was 65 per cent more powerful,
and 28 per cent lighter, than the average stock motor in service during
that year.