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Aviation History
1943
1943 - 0893.PDF
APRIL 8TH, 1943 FLIGHT 363 ROTAfcY-WINC AlRCRAFT blades out at the proper angle. An upper blade touched a lower and the machine crashed. But such was ever the fate of the pioneer. Control was achieved in the Breguet by varying the blade incidence. The Breguet helicopter made several flights, some in a straight line and some over closed circuits. A German aircraft firm which has always been willing to experiment with new ideas is the Focke-Wulf Flug- zeugbau, of Bremen. Many years ago the firm designed a tail-first, or Ente, type. The machine had many inter- esting features, but a crash in which its designer, Professor Wulf, was killed, led to the abandoning of work in that direction. Then came the Autogiro era, and the Focke- Wulf concern entered into an agreement with the Cierva Autogiro Co., Ltd., for licence rights. At the same time Professor Focke, the surviving partner in the firm, began to interest himself in the next step, pure helicopter flight. He realised that he would have to make full use of all the knowledge that existed at the time, and doubtless his agreement with Cierva was of considerable help. In fact, Professor Focke paid tribute to Cierva, and to such British aerodynamicists as Glauert and Lock, in an article we published in Flight of April 21st, 1938, in which he described some of the work on developing his FW61 heli- copter. Actually that machine was not built by Focke- Wulf, but by a subsidiary company', Focke, Achgelis and Co. Side-by-side Rotors Professor Focke, after examining the problem very care- fully, came to the conclusion that the most satisfactory solution, at least for a start, lay in the twin-rotor arrange- ment, with the rotors placed side by side. Not only was torque reaction cancelled out, but each rotor worked in undisturbed air, and was thus more efficient. Moreover, the arrangement avoided the vibrations sometimes encoun-^ tered with coaxial rotors. The result was the FVV61, which established a series of world's re-cords for helicopters. As a demonstration of the perfection of its' controls it was •flown inside the Deutschland Halle in Berlin by Friiulein Hanna Reitsch, who not only hovered stationary but flew slowly forwards and backwards. Control of the rotors and complete machine was by tilt- ing the rotor heads. The blades were, hinged, as in the Cierva Autogiro, so as to be free to " flap," and, in addi- tion, their incidence could be changed, but this change was used only for ensuring autorotation in case of engine failure, and not for flying control. A freewheel arrange- ment was, of course, incorporated in the drive. It is interesting to record in this connection that in this country also work has been going on with twin-rotor helicopters, The Sikorsky VS-300 has just alighted on wafer and issupported by flotation bags. The airscrew at the stern gives directional control and counteracts the torque from themain rotor. the firm in question being J. and G. Weir, Ltd., of Glasgow. But that is another story. One of the pioneers of helicopter design is the Hun- garian engineer, Mr. Oskar von Asboth. As long ago as 1918 he had built' a captive helicopter with which he succeeded in leaving the ground. Of Jiis political vicissi- tudes we need not speak here. He came to England a few years before the outbreak of the present war, and by X939 appeared to be in fair way to forge ahead, there being then some prospect of Air Ministry benevolence if not actual assistance. The basic feature of the Asboth helicopter, type A.H. X, shown in plan view, is the type of rotor hub used. The inventor was in the habit of referring to it as a four-function, hub. Actual details were not disclosed, but one of the three functions was control of the machine by tilting the planes in which the oppositely running rotors rotated. The other three functions were entirely automatic, including the change in pitch angle necessary to ensure autorotation. Twisted Blades From an aerodynamic point of view, the rotor blades were mainly interesting on account of their twisted inci- dence. They were, in fact, intermediate between the blades of an airscrew and the more orthodox untwisted blades of most gyroplanes. The inner ends, or roots, were at no angle of incidence. The middle portion had a small positive angle, and the tips a negative angle. Mr. von Asboth had calculated that this would give the best lift distribution, and consequently greatest efficiency. The arrangement had the further advantage that for autorota- tion the negative angles near the blade tips gave a power- ful forward thrust and thus assisted autorotation in con- ditions when blades of the constant-angle type would slow down. An unusual feature of the Asboth helicopter was that the blades were not hinged at the root. They were set rigidly at an upward ai-gle which corresponded to the resultant force (of centrifugal force and lift) when the machine was hovering. During forward speed of the whole machine there was a residue of unbalance, but the rotor head and blades were designed to take care of that. Finally we come to the Sikorsky VS-300 helicopter designed and built by the Vought-Sikorsky Division of the United Aircraft Corporation of America. Encouraged by the pioneer work on autogiros done by Senor de la Cierva, and materially assisted by the accumulated experience of Cierva and his associates, Igor Sikorsky began work (or more correctly resumed it, for he had worked on helicopters in Russia before the aeroplane was a practical preposition) on his helicopter in 1938. The first VS-300 was built the
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