§ 18A. =Spring Motors.=--This question has already been dealt with
more or less whilst dealing with rubber motors, and the superiority of
the latter over the former pointed out. Rubber has a much greater
superiority over steel or other springs, because in stretch-twisted
rubber far more energy can be stored up weight for weight. One pound
weight of elastic can be made to store up some 320 ft.-lb. of energy,
and steel only some 65 lb. And in addition to this there is the
question of gearing, involving extra weight and friction; that is, if
flat steel springs similar to those used in clockwork mechanism be
made use of, as is generally the case. The only instance in which such
springs are of use is for the purpose of studying the effects of
different distributions of weight on the model, and its effect on the
balance of the machine; but effects such as this can be brought about
without a change of motor.
§ 18B. A more efficient form of spring motor, doing away with gearing
troubles, is to use a long spiral spring (as long as the rubber
strands) made of medium-sized piano wire, similar in principle to
those used in some roller-blinds, but longer and of thinner steel.
The writer has experimented with such, as well as scores of other
forms of spring motors, but none can compare with rubber.
The long spiral form of steel spring is, however, much the best.
§ 18C. =Compressed Air Motors.=--This is a very fascinating form of
motor, on paper, and appears at first sight the ideal form. It is so
easy to write: "Its weight is negligible, and it can be provided free
of cost; all that is necessary is to work a bicycle pump for as many
minutes as the motor is desired to run. This stored-up energy can be
contained in a mere tube, of aluminium or magnalium, forming the
central rib of the machine, and the engine mechanism necessary for
conveying this stored-up energy to the revolving propeller need weigh
only a few ounces." Another writer recommends "a pressure of 300 lb."
§ 18D. A pneumatic drill generally works at about 80 lb. pressure,
and when developing 1 horse-power, uses about 55 cubic ft. of free air
per minute. Now if we apply this to a model aeroplane of average size,
taking a reservoir 3 ft. long by 1½ in. internal diameter, made of
magnalium, say--steel would, of course, be much better--the weight of
which would certainly not be less than 4 oz., we find that at 80 lb.
pressure such a motor would use
55/Horse Power (H.P.)
cub. ft. per minute.
Now 80 lb. is about 5½ atmospheres, and the cubical contents of the
above motor some 63 cub. in. The time during which such a model would
fly depends on the H.P. necessary for flight; but a fair allowance
gives a flight of from 10 to 30 sec. I take 80 lb. pressure as a fair
practical limit.
§ 18E. The pressure in a motor-car tyre runs from 40 to 80 lb.,
usually about 70 lb. Now 260 strokes are required with an ordinary
inflator to obtain so low a pressure as 70 lb., and it is no easy job,
as those who have done it know.
§ 19. Prior to 1893 Mr. Hargraves (of cellular kite fame) studied the
question of compressed-air motors for model flying machines. His motor
was described as a marvel of simplicity and lightness, its cylinder
was made like a common tin can, the cylinder covers cut from sheet tin
and pressed to shape, the piston and junk rings of ebonite.
One of his receivers was 23-3/8 in. long, and 5·5 in. diameter, of
aluminium plate 0·2 in. thick, 3/8 in. by 1/8 in. riveting strips were
insufficient to make tight joints; it weighed 26 oz., and at 80 lb.
water pressure one of the ends blew out, the fracture occurring at the
bend of the flange, and not along the line of rivets. The receiver
which was successful being apparently a tin-iron one; steel tubing was
not to be had at that date in Sydney. With a receiver of this
character, and the engine referred to above, a flight of 343 ft. was
obtained, this flight being the best. (The models constructed by him
were not on the aeroplane, but ornithoptere, or wing-flapping
principle.) The time of flight was 23 _seconds_, with 54½ double
vibrations of the engines. The efficiency of this motor was estimated
to be 29 per cent.
§ 20. By using compressed air, and heating it in its passage to the
cylinder, far greater efficiency can be obtained. Steel cylinders can
be obtained containing air under the enormous pressure of 120
atmospheres.[19] This is practically liquid air. A 20-ft. cylinder
weighs empty 23 lb. The smaller the cylinder the less the
proportionate pressure that it will stand; and supposing a small steel
cylinder, produced of suitable form and weight, and capable of
withstanding with safety a pressure of from 300 to 600 lb. per sq.
in., or from 20 to 40 atmospheres. The most economical way of working
would be to admit the air from the reservoir directly to the motor
cylinders; but this would mean a very great range in the initial
working pressure, entailing not-to-be-thought-of weight in the form of
multi-cylinder compound engines, variable expansion gear, etc.
§ 21. This means relinquishing the advantages of the high initial
pressure, and the passing of the air through a reducing valve, whereby
a constant pressure, say, of 90 to 150, according to circumstances,
could be maintained. By a variation in the ratio of expansion the air
could be worked down to, say, 30 lb.
The initial loss entailed by the use of a reducing valve may be in a
great measure restored by heating the air before using it in the motor
cylinders; by heating it to a temperature of only 320°F., by means of
a suitable burner, the volume of air is increased by one half, the
consumption being reduced in the same proportion; the consumption of
air used in this way being 24 lb. per indicated horse-power per hour.
But this means extra weight in the form of fuel and burners, and what
we gain in one way we lose in another. It is, of course, desirable
that the motor should work at as low a pressure as possible, since as
the store of air is used up the pressure in the reservoir falls, until
it reaches a limit below which it cannot usefully be employed. The air
then remaining is dead and useless, adding only to the weight of the
aeroplane.
§ 22. From calculations made by the writer the _entire_ weight of a
compressed-air model motor plant would be at least _one-third_ the
weight of the aeroplane, and on a small scale probably one-half, and
cannot therefore hold comparison with the _steam engine_ discussed in
the next paragraph. In concluding these remarks on compressed-air
motors, I do not wish to dissuade anyone from trying this form of
motor; but they must not embark on experiments with the idea that
anything useful or anything superior to results obtained with
infinitely less expense by means of rubber can be brought to pass with
a bicycle pump, a bit of magnalium tube, and 60 lb. pressure.
§ 22A. In Tatin's air-compressed motor the reservoir weighed 700
grammes, and had a capacity of 8 litres. It was tested to withstand a
pressure of 20 atmospheres, but was worked only up to seven. The
little engine attached thereto weighed 300 grammes, and developed a
motive power of 2 kilogram-metres per second (_see_ ch. iii.).
§ 23. =Steam-Driven Motors.=--Several successful steam-engined model
aeroplanes have been constructed, the most famous being those of
Professor Langley.
Having constructed over 30 modifications of rubber-driven models, and
experimented with compressed air, carbonic-acid gas, electricity, and
other methods of obtaining energy, he finally settled upon the steam
engine (the petrol motor was not available at that time, 1893). After
many months' work it was found that the weight could not be reduced
below 40 lb., whilst the engine would only develop ½ H.P., and
finally the model was condemned. A second apparatus to be worked by
compressed air was tried, but the power proved insufficient. Then came
another with a carbonic-acid gas engine. Then others with various
applications of electricity and gas, etc., but the steam engine was
found most suitable; yet it seemed to become more and more doubtful
whether it could ever be made sufficiently light, and whether the
desired end could be attained at all. The chief obstacle proved not to
be with the engines, which were made surprisingly light after
sufficient experiment. _The great difficulty was to make a boiler of
almost no weight which would give steam enough._
§ 24. At last a satisfactory boiler and engine were produced.
The engine was of 1 to 1½ H.P., total weight (including moving
parts) 26 oz. The cylinders, two in number, had each a diameter of
1¼ in., and piston stroke 2 in.
The boiler, with its firegrate, weighed a little over 5 lb. It
consisted of a continuous helix of copper tubing, 3/8 in. external
diameter, the diameter of the coil being 3 in. altogether. Through the
centre of this was driven the blast from an "Ælopile," a modification
of the naphtha blow-torch used by plumbers, the flame of which is
about 2000° F.[20] The pressure of steam issuing into the engines
varied from 100 to 150 lb. per sq. in.; 4 lb. weight of water and
about 10 oz. of naphtha could be carried. The boiler evaporated 1 lb.
of water per minute.
The twin propellers, 39 in. in diam., pitch 1¼, revolved from 800
to 1000 a minute. The entire aeroplane was 15 ft. in length, the
aerofoils from tip to tip about 14 ft., and the total weight slightly
less than 30 lb., of which _one-fourth was contained in the
machinery_. Its flight was a little over half a mile in length, and of
1½ minutes' duration. Another model flew for about three-quarters
of a mile, at a rate of about 30 miles an hour.
It will be noted that engine, generator, etc., work out at about 7 lb.
per H.P. Considerable advance has been made in the construction of
light and powerful model steam engines since Langley's time, chiefly
in connexion with model hydroplanes, and a pressure of from 500 to 600
lb. per sq. in. has been employed; the steam turbine has been brought
to a high state of perfection, and it is now possible to make a model
De Laval turbine of considerable power weighing almost next to
nothing,[21] the real trouble, in fact the only one, being the steam
generator. An economization of weight means a waste of steam, of which
models can easily spend their only weight in five minutes.
§ 25. One way to economize without increased weight in the shape of a
condenser is to use spirit (methylated spirit, for instance) for both
fuel and boiler, and cause the exhaust from the engines to be ejected
on to the burning spirit, where it itself serves as fuel. By using
spirit, or some very volatile hydrocarbon, instead of water, we have a
further advantage from the fact that such vaporize at a much lower
temperature than water.
§ 26. When experimenting with an engine of the turbine type we must
use a propeller of small diameter and pitch, owing to the very high
velocity at which such engines run.
Anyone, however, who is not an expert on such matters would do well to
leave such motors alone, as the very highest technical skill, combined
with many preliminary disappointments and trials, are sure to be
encountered before success is attained.
§ 27. And the smaller the model the more difficult the problem--halve
your aeroplane, and your difficulties increase anything from fourfold
to tenfold.
The boiler would in any case be of the flash type of either copper or
steel tubing (the former for safety), with a magnalium container for
the spirit, and a working pressure of from 150 to 200 lb. per sq. in.
Anything less than this would not be worth consideration.
§ 28. Some ten months after Professor Langley's successful model
flights (1896), experiments were made in France at Carquenez, near
Toulon. The total weight of the model aeroplane in this case was 70
lb.; the engine power a little more than 1 H.P. Twin screws were
used--_one in front and one behind_. The maximum velocity obtained was
40 miles per hour; but the length of run only 154 yards, and duration
of flight only a few seconds. This result compares very poorly with
Langley's distance (of best flight), nearly one mile, duration 1 min.
45 sec. The maximum velocity was greater--30 to 40 miles per hour. The
total breadth of this large model was rather more than 6 metres, and
the surface a little more than 8 sq. metres.
§ 29. =Petrol Motors.=--Here it would appear at first thought is the
true solution of the problem of the model aeroplane motor. Such a
motor has solved the problem of aerial locomotion, as the steam engine
solved that of terrestrial and marine travel, both full sized and
model; and if in the case of full sized machines, then why not models.
[Illustration: FIG. 18.--MR. STANGER'S MODEL IN FULL FLIGHT.]
[Illustration: FIG. 19.--MR. STANGER'S PETROL-DRIVEN MODEL AEROPLANE.
(_Illustrations by permission from electros supplied by the "Aero."_)]
§ 30. The exact size of the smallest _working_ model steam engine that
has been made I do not know,[22] but it is or could be surprisingly
small; not so the petrol motor--not one, that is, that would _work_.
The number of petrol motor-driven model aeroplanes that have actually
flown is very small. Personally I only know of one, viz., Mr. D.
Stanger's, exhibited at the aero exhibition at the Agricultural Hall
in 1908.
[Illustration: FIG. 20.--MR. STANGER'S MODEL PETROL ENGINE.]
[Illustration: FIG. 21.--MR. STANGER'S MODEL PETROL ENGINE.]
In Fig. 21 the motor is in position on the aeroplane. Note
small carburettor. In Fig. 20 an idea of the size of engine may
be gathered by comparing it with the ordinary sparking-plug
seen by the side, whilst to the left of this is one of the
special plugs used on this motor. (_Illustrations by permission
from electros supplied by the "Aero."_)
§ 31. The following are the chief particulars of this interesting
machine:--The engine is a four-cylinder one, and weighs (complete with
double carburetter and petrol tank) 5½ lb., and develops 1¼ H.P.
at 1300 revolutions per minute.
[Illustration: FIG. 22.--ONE-CYLINDER PETROL MOTOR.
(_Electro from Messrs. A.W. Gamage's Aviation Catalogue._)]
The propeller, 29 in. in diam. and 36 in. in pitch, gives a static
thrust of about 7 lb. The machine has a spread of 8 ft. 2 in., and is
6 ft. 10 in. in length. Total weight 21 lb. Rises from the ground when
a speed of about 16 miles an hour is attained. A clockwork
arrangement automatically stops the engine. The engine air-cooled. The
cylinder of steel, cast-iron heads, aluminium crank-case, double float
feed carburetter, ignition by single coil and distributor. The
aeroplane being 7 ft. 6 in. long, and having a span 8 ft.
§ 32. =One-cylinder Petrol Motors.=--So far as the writer is aware no
success has as yet attended the use of a single-cylinder petrol motor
on a model aeroplane. Undoubtedly the vibration is excessive; but this
should not be an insuperable difficulty. It is true it is heavier in
proportion than a two-cylinder one, and not so efficient; and so far
has not proved successful. The question of vibration on a model
aeroplane is one of considerable importance. A badly balanced
propeller even will seriously interfere with and often greatly curtail
the length of flight.
§ 33. =Electric Motors.=--No attempt should on any account be made to
use electric motors for model aeroplanes. They are altogether too
heavy, apart even from the accumulator or source of electric energy,
for the power derivable from them. To take an extreme case, and
supposing we use a 2-oz. electric motor capable of driving a propeller
giving a static thrust of 3 oz.,[23] on weighing one of the smallest
size accumulators without case, etc., I find its weight is 4½ oz.
One would, of course, be of no use; at least three would be required,
and they would require practically short circuiting to give sufficient
amperage (running them down, that is, in some 10 to 15 seconds). Total
weight, 1 lb. nearly. Now from a _pound_ weight of rubber one could
obtain a thrust of _pounds_, not ounces. For scale models not intended
for actual flight, of course, electric motors have their uses.
FOOTNOTES:
[12] Also there is no necessity for gearing.
[13] In his latest models the writer uses strands even three times and
not twice as long, viz. fourteen strands 43 in. long.
[14] This refers to 1/16 in. square sectioned rubber.
[15] Of uniform breadth and thickness.
[16] In practice I find not quite so high a proportion as this is
always necessary.
[17] Steel pinion wire is very suitable.
[18] See Appendix.
[19] As high a pressure as 250 atmospheres has been used.
[20] There was a special pump keeping the water circulating rapidly
through the boiler, the intense heat converting some of it into steam
as it flowed. The making of this boiler alone consumed months of work;
the entire machine taking a year to construct, with the best
mechanical help available.
[21] Model Steam Turbines. "Model Engineer" Series, No. 13, price
6_d._
[22] See Introduction, note to § 1.
[23] The voltage, etc., is not stated.