6 



DISCOVERY 



inside to fit on a threaded shaft. The shaft was re- 

 volved "like the common humming-top" by means 

 of a cord wound round it, when the " helicopter " flew 

 off into the air. This form of toy used to be a common 

 object of the nursery. All Sir George Cayley's writings 

 are extraordinarily interesting even at the present day, 

 and he remains probably the only person in the history 

 of the world who has ever explored the possibilities 

 not only of airships and aeroplanes, but of helicopters 

 and omithopters as well. 



It is impossible in a brief review of the development 

 of the helicopter to mention any except a very few of the 

 long line of experiments in this branch of flying, but 

 no account would be complete without reference to 

 some of the more notable of them. Coming to the 

 twentieth century, one of the outstanding names is 

 that of Louis Brequet, mainly because he has since 

 achieved fame as a designer of aeroplanes. His heli- 

 copter, which was built about 1908, was different from 

 most if not all others, in being combined with an aero- 

 plane. It consisted of a large biplane on the outer 

 interplane struts of which a curious helicopter screw, 

 the construction of which is somewhat difficult to 

 describe, was fitted at each wing-tip. Each of these 

 screws was really composed of four small sets of 

 biplane wings at the extremities of two arms at right 

 angles to each other, and mounted at their intersection 

 on a circular frame, driven by gearing from the central 

 engine. The Brequet-Richet biplane-helicopter, as it 

 was called, was certainly quite different from any other, 

 and looking at it again in the light of M. Brequet's 

 subsequent career, it is not difficult to understand that 

 from it should have been evolved the biplanes which 

 are now so well known, as the machine was undoubtedly 

 more of an aeroplane than a helicopter, the latter part 

 of the design appearing more or less auxiliary to the 

 former. Another helicopter of the same period , designed 

 by M. Bertin, was unique in that the two superposed 

 lifting screws, revolving in opposite directions — a com- 

 mon feature in helicopter design — were of quite different 

 form and construction. The upper one, which was 

 driven direct from the 50 h.p. engine, was of a metal 

 type of construction common at that date, and had a 

 diameter of about 10 feet. The lower one, on the other 

 hand, which was carried on this same axis, was con- 

 siderably geared-down, being designed to run at a 

 very slow rate of revolution, and was composed of two 

 monoplane wings mounted at each extremity of a 

 horizontal arm, the total diameter being rather over 

 25 feet. A third interesting direct-lift machine of the 

 first decade of the century was also designed by a 

 Frenchman — M. Comu. The pilot of this machine 

 sat in a tubular steel framework, supported on four 

 bicycle wheels, with the engine in front of him. Carried 

 on steel tube outriggers running fore and aft from this 



frame were two large belt-driven pulleys with vertical 

 axis. Mounted on the periphery of each of these 

 pulleys were two fabric-covered monoplane surfaces, 

 the angle of which to the plane of rotation could be 

 altered at the will of the pilot. In addition to these, 

 a rudder and elevator were fitted in front and rear 

 respectively, though how these were expected to act 

 is not quite clear. 



From the purely theoretical standpoint, present-day 

 knowledge of the principles of helicopters in regard to 

 lifting force available and the possibilities of safe 

 descent in case of engine failure is probably due to 

 Colonel Renard, for many years the head of the French 

 Government's aeronautical experimental station at 

 Chalais-Meudon, more than to any other person. This 

 officer spent eighteen years on exhaustive researches 

 into the question of lifting screws, obtaining measure- 

 ments by means of aerodynamic balances, as a result 

 of which he was able in 1903 to read three papers on 

 the subject before the Academic des Sciences, which 

 still remain the classic literature of the subject. 



• It is in fact now, one will not say easy, but well 

 within the capacity of an aeronautical engineer to 

 design a helicopter which will be capable of raising itself 

 and its pilot from the ground and maintaining its 

 position in the air. Unfortunately, however, the 

 problem does not rest there, for if the machine is to be 

 of practical use, the vertical motion must be transfer- 

 able into a horizontal movement over the groimd, and, 

 further, it must have some measure of stability, be 

 controllable while flying, and be such that its rate of 

 descent is controllable under all conditions, with and 

 without the engine. The generally accepted method of 

 obtaining forward movement is to rotate the screws 

 about their axis, so that they can drive the machine 

 horizontally when it is partially supported by small 

 aeroplane surfaces ; or, where these are not provided, 

 to set the screws at such an angle that their upward lift 

 is still sufficient to maintain height while giving the 

 machine translational motion. The latter method 

 would appear to be one of obvious inefficiency, while 

 in both cases the weight of the necessary gearing 

 renders it difficult to imagine, in the writer's opinion, 

 that the helicopter can equal the efficiency of the aero- 

 plane on the basis of load can'ied per horse-power. 



There are two distinct types of helicopter of the modern 

 school : one having two screws mounted one above 

 the other on the same axis, and the other having 

 two or more screws each carried on a separate axis. 

 The first has the obvious advantages of greater mechan- 

 ical simplicity, for it involves possibly but one set of 

 gears, and so saves weight ; whilst the arrangement 

 embodying separate axes will undoubtedly ensure 

 greater efficiency of the screws, as there will be no 

 interference between the two slipstreams. In both 



