§ 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.