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.
