The Theory and Practice of Model Aeroplaning Part 5

D. Use vaseline on the cogs to make them run as easily as possible.

[Ill.u.s.tration: FIG. 17.--GEARED RUBBER MOTOR.

Designed and constructed by the writer. For description of the model, etc., see Appendix.]

E. The material of the containing framework must be of maximum strength and minimum lightness. Construct it of minimum size, box shaped, use the thinnest tin (really tinned sheet-iron) procurable, and lighten by drilling holes, not too large, all over it. Do not use aluminium or magnalium. Steel, could it be procured thin enough, would be better still.

F. Use steel pianoforte wire for the spindles, and hooks for the rubber strands, using as thin wire as will stand the strain.

Unless these directions are carefully carried out no advantage will be gained--the writer speaks from experience. The requisite number of rubber strands to give the best result must be determined by experiment.

-- 18. One advantage in using such a motor as this is that the two equal strands untwisting in opposite directions have a decided steadying effect on the model, similar almost to the case in which two propellers are used.

The "best" model flights that the writer has achieved have been obtained with a motor of this description.[18]

In the case of twin screws two such gearings can be used, and the rubber split up into four strands. The containing framework in this case can be simply light pieces of tubing let into the wooden framework, or very light iron pieces fastened thereto.

Do not attempt to split up the rubber into more than two strands to each propeller.

SECTION II.--OTHER FORMS OF MOTORS.

-- 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 1 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 5 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 55 in. diameter, of aluminium plate 02 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 f.l.a.n.g.e, 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 54 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 pa.s.sage 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 pa.s.sing of the air through a reducing valve, whereby a constant pressure, say, of 90 to 150, according to circ.u.mstances, 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 320F., 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 pa.s.s 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 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 1 H.P., total weight (including moving parts) 26 oz. The cylinders, two in number, had each a diameter of 1 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 "aelopile," 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 1, 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 1 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.

[Ill.u.s.tration: FIG. 18.--MR. STANGER'S MODEL IN FULL FLIGHT.]

[Ill.u.s.tration: FIG. 19.--MR. STANGER'S PETROL-DRIVEN MODEL AEROPLANE.

(_Ill.u.s.trations 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_.