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Aviation History
1938
1938 - 1075.PDF
APRIL 21, 1938. FLIGHT. 381 Test rig for single rotor, including anelectric motor for the drive. Lift and power required at different speedswere measured simultaneously. or unofficial helicopter records were in force: distance i km; duration about 10 mins.; altitude 18 m. These figures were not very encouraging, and Pro- fessor Focke realised that what he had to do was to produce an aircraft worthy of the name, and in order to do this he had to take into account and benefit by all the accumulated experience of those experimenters who had preceded him. Professor Focke set up, in order of their importance, the following desiderata: Ability to land safely after engine failure; that meant incidence control of the rotor blades. Stability and controllability. General reliability. Sim- plicity of piloting. As good performance as possible. And last, but not least, the type of construction and general de- sign to be such that people accustomed to ordinary aeroplane manufacture could build the machine. The design of the rotor blades presented no particularly difficult problems. The three-bladed type was chosen for its efficiency, and its lift was ascertained by means of a model driven by a 3 h.p. electric motor. This model was tested in a wind tunnel, and the forces measured both in the driven condition and when autorotating. Great care had to be taken in making the measurements, particularly on account of the pro- nounced " ground influence." Torque One of the fundamental problems which actually determines the general design is that of taking care of the torque-reaction from the rotor, which may reach considerable magnitudes. After examining all the rotor arrange- ments possible, Professor Focke decided upon the side-by-side twin- rotor arrangement as being the most efficient (each rotor working in un- disturbed air) and free from the trouble with vibration encountered when one rotor is above the other. The price paid, in the form of aircraft drag, arises, of course, from the out- riggers which carry the rotors. -By running the two rotors in opposite directions, the torque-reaction is taken care of completely. Another advantage is that the only parts of the aircraft which are in the down-wash from the rotors are the outriggers. Direct as distinct from indirect control was chosen as being more in keeping with the helicopter ideal and effective during hovering flight. A great deal of time was spent in scheming out an arrangement of con- trol which would not only be natural for the pilot, but which would enable him to change instantly from direct lift to autorotation in case of engine failure. The only control surfaces of orthodox aircraft type are a fin and rudder, and a tailplane with adjust- able incidence for trimming pur- poses. Professor Focke admits that the mathematical calculations con- Rotor head with blade hinges and incidence control. nected with stability and control " reached terrifying pro- portions," but their justification came when, on its very first free flight, the machine reached a height of 400 metres (1,300ft.) and made a three-point landing. What made the procedure even more prolonged was the fact that repeatedly problems cropped up wiich would not be solved mathe- matically until certain test data had been obtained. How- 160 h.p. Siemens Sh 14a engine, with clutch and transmission.
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