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Aviation History
1954
1954 - 0687.PDF
FLIGHT, 12 March 1954 313 DOMAIN of the HELICOPTER Raoul Hafner's Bleriot Lecture: A Comprehensive Survey IN Paris on Wednesday last, March 10th, the Royal Aeronautical Society's seventh Louis Bleriot memorial lecture was due to be given by Mr. Raoul Hafner, A.F.R.Ae.S. Entitled The Domain of the Helicopter, the paper reviews the historical development and fundamental design-principles of rotating-wing aircraft, and analyses their configurations, performance and economy. It appears certain that Mr. Hafner's lecture will be regarded as a most significant contribution to helicopter knowledge, and the reporting of it in this special issue of Flight is there fore particularly appropriate. Particular attention has been paid, in the abridged version which follows, to Mr. Hafner's considerations of rotor systems, of the three basic types of helicopter (in which a novel convertible helicopter is described) and of the helicopter in civil air transport. After being introduced by Mr. Hafner, the lecture was to be read, in translated digest form, by M. Morian, chief engineer of the S.N.C.A.S.O. helicopter division. It is noticeable that the author of the paper has been characteristically reticent about his own achievements in die field of helicopter design, which he first entered as long ago as 1927. In collaboration with J. B. Coats, he produced the R-l, which flew in Austria in 1930, and the R-2, which he brought to this country in 1933. Two years later the Hafner ARIII gyroplane appeared; though not a true helicopter, it introduced the now-established system of control by cyclic and collective pitch. His recent work with die Bristol Com pany—he joined diem in 1944—is well known. Historical Introduction.—The paper began by giving a short summary of die historical development of the helicopter. It first traced the development of human flight through the ages, and the successive emphasis laid on the three types of wing, flapping, fixed and rotating, with which it was thought this could be achieved. While Nature achieved success solely with the flapping wing, man successively tried all three. Successful controlled flight, however, came with the application of the fixed wing. But this led progressively to the large, modern aircraft with its principal dangers and limitations of high speed and the need for elaborate ground facilities. The rotating-wing development had continued slowly in parallel with fixed-wing progress, and apparendy initially with little success. Several helicopters were built from the earliest days, Juan de la Cierva successfully proved the Autogiro configuration; and, finally, in 1936 and 1937, the Breguet-Dorand and Focke-Achgelis demonstrated helicopter practicability. It remained for Igor Sikorsky in 1939 to achieve the production and application of the helicopter on a commercial basis. After comparing these developments with the "fantastic per formances" of modern fixed-wing aircraft, Mr. Hafner oudined the major disadvantages of the latter, and indicated the possi bilities of the helicopter for transport over the shorter routes. The Korean war had finally proved the helicopter's worth, and it remained now to apply, by practical design and operation, the lessons learned. The main purpose of the lecture was, in the light of this, to define the domain of the helicopter. To avoid confusion which might arise owing to the great variety of terms applied to the helicopter, Mr. Hafner gave the four definitions which we quote below, and which he proposed not as new terms for general use, but to establish some basic defini tions for use in his paper. He defined a helicopter as an aircraft heavier than air, capable of sustained hovering and embodying wings, which (at least during the period of hovering) rotated in a free airstream, and thus supplied substantially the total lift support. He then gave the following definitions: — Pure Helicopter: a helicopter the wings of which rotated in a substantially horizontal plane and, throughout the flight, sup plied the aerodynamic forces for sustentation and propulsion. Compound Helicopter: a helicopter the rotating wings of which supplied at all times during flight the major pan of the lift, but which in addition embodied substantially aerodynamic Mr. Raoul Hafner, A.F.R.Ae.S., is chief designer to the heli copter division of the Bristol Aeroplane Co., Ltd. components, such as wings and/or airscrews that, mainly at higher translational speeds, supplemented the action of the rotating wings. Convertible Helicopter: a helicopter capable of conversion during flight, such that the total lift was substantially trans ferred from the rotating wings to other wings, and vice versa. Rotor Systems.—The rotor system of a helicopter comprised die rotors, their hubs and control mechanism and, in the case of shaft-driven helicopters, that part of the transmission which joined die rotors together. The rotor system was driven by one or more power units through branches of the transmission, with a freewheel in each branch, located preferably at the point of junc ture with the rotor system. The rotor system was the critical department in the helicopter. It had been said that the predominant hazard of the fixed wing lay in its high minimum flying speed which, combined with bad visibility or a power failure, was liable to lead to serious con sequences. The hazard in the helicopter, on the other hand, was the mechanical complexity of the rotor system. There was, however, no collective safety in a number of rotors. We could not afford to lose one rotor—or even one blade—and expect to continue to fly safely with the remainder. The rotor system was like a chain—if one link failed, the whole failed. Therefore, everything else being equal, die fewer rotors and blades there were in a rotor system, the greater was its safety. As with the fixed wing, so the rotating wing was exposed to two tvDes of loading: a limited number of applications of extreme load and frequent applications of medium load. It was thus neces sary to establish on the one hand an envelope of ultimate load conditions where die material might be stressed close to the proof stress figure, and on the other hand to establish the so-called fatigue envelope, for which there was a much reduced safe stress level. The ultimate stresses arose generally from manoeuvre and gust loads, but as these were always coupled wiui inertia effects such as blade oscillations or torsional oscillations in the trans mission, their evaluation was made somewhat difficult. Even more complex were the loading systems that produced the fatigue stresses, for these were due mainly to fluctuating air loads on the blades or torque fluctuation in the engines. Besides the problem of stress in the rotor system, there was that of wear in bearings and other detail components. These con siderations in tow had pointed to the inadequacy of purely theore tical work alone and had, in consequence, led to a procedure of ground testing in which the flight conditions were simulated with as much accuracy as was possible. Rotor Control.—The rotor provided not only lift but also con trol of the helicopter. This must necessarily be so because the rotor blade was the only aerodynamic surface which maintained high speed in all flight conditions, including hovering, and was therefore capable of receiving aerodynamic forces adequate for the purpose of control. We must, however, distinguish between die rigid and the articulated rotor. At its hub, the rigid rotor could render a force—the rotor thrust—as well as three independent moments—the rotor torque and two transverse moments. On the other hand, the articulated rotor could, apart from the rotor torque, produce three forces—the thrust and two transverse com ponent forces. Rotation of the rotor axis produced in the rigid rotor a gyroscopic moment of considerable magnitude which was transmitted to the aircraft body. In the articulated rotor this gyroscopic moment was balanced by an aerodynamic moment, and no transverse moment was transmitted through the rotor hub. The articulation of rotor blades had been found to overcome many of the difficulties which beset the early helicopters and, since the successful performance of the autogyro demonstrated the value of articulation, it had been adopted almost universally in helicopter engineering. The following considerations of various forms of control applied therefore only to the articulated rotor: G
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