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Aviation History
1975
1975 - 0644.PDF
572 FLIGHT International. 3 April 197S Autogiro analysed DESPITE the remarkable short-field performance and engineering sim plicity of the autogiro, development of this type practically ceased with the advent of the production helicopter. There was a resurgence of the species when Igor Bensen produced his ultra- simple fixed pitch machines, and hun dreds of derivatives are currently fly ing in the USA. A few were built in the UK but the type did not achieve great popularity. A series of accidents aroused doubts about controllability although several specialists, notably Wg Cdr Wallis, amply demonstrated safe handling qualities. Accident analysis is a difficult task as hard evidence is rarely avail able, but the loss of control and ensuing crash of the autogiro at the Farnborough Show in 1970 was the subject of two good quality cine films. The Royal Aircraft Establishment (RAE) therefore had the basis for an analysis of the problems which can arise in the handling of these machines. The results of the analysis indicate that the machines have adequate stability and control as long as they are flown within their particular limi tations rather than those of a small fixed-wing aircraft. Film evidence The Accidents Investigation Branch of the Department of Trade includes in its publication on the Farnborough accident investigation* a report on the tests and research carried out by the RAE assisted by Wg Cdr Wallis: "The accident to G-AXAR was the fourth fatal accident to a light gyro plane to have occurred during the 18 months from March 9, 1969, and there appears to be some similarity between three of them. "The Helicopter Flight Group of the RAE was asked to assist in the inves tigation and to make two studies; first, a detailed analysis of the two films to provide information on the flight path and, second, an appraisal of the stability and control character istics of the Wallis WA-117 autogiro." A mathematical model was also con structed for computer analysis of the aircraft's pitching sequences. "The tests indicated that provided the WA-117 is flown within the limits set out in the Permit to Fly (that is below 65kt IAS) and under positive g loading, it exhibits no dangerous characteristics and is safe and pleas ant to fly. The theoretical stability analysis of the aircraft (without a tailplane) shows a marked stick-fixed instability as speed is increased. This is principally due to the unstable pitching moment from the cockpit fairing. This instability is not so noticeable in practice below 65kt be cause of a contribution from the air craft's unusually stable stick-free characteristics in which forces present at the control column tend to move the pilot's hand in the correcting direction. A likely further beneficial influence is the forward (chordwise) e.g. of the rotor blade near the tip, which, since the blades are flexible in torsion, produces a stabilising effect on the motion of the aircraft. This effect was not included in the theoret ical analysis." Private IFtDBDinr A Wallis WA-120 autogiro with an adjustable pitch stabiliser which was tried and eventually discarded. The mathematical model was set up for level flight speeds of 44, 68 and 95kt EAS, the latter being approx imately the speed at which the acci dent occurred. The starting point taken was that of trimmed flight at 95kt with a rotor speed of 500 r.p.m. and an aircraft weight of 7001b. The ensuing estimate of collective pitch was then used at the two lower air speeds giving the appropriate trimmed rotor speeds. Rotor speed was treated as an independent variable. The relatively large streamlined cockpit nacelle forward of the e.g. produces an unstable pitching move ment with respect to fuselage inci dence. The overall aircraft movement derivative depends on contributions from rotor, fuselage and propeller side forces. The contribution from the rotor is also dependent on the initial fuselage movement, which it has to trim and which becomes increasingly unstable with fuselage nose-down inci dence. The overall effect in the theo retical model stick-fixed was to pro duce an unstable aircraft which be comes increasingly unstable as speed increases. In the stick-free case movement of * Civil Aircraft Accident report 7/74, HMSO. Price 45p. the pilot's control column tilts the rotor spindle axis which is restrained by a trimmable spring. If the horizon tal spindle offset is sufficiently large the moment of the rotor forces about the pivot point becomes stabilising and tends to tilt the rotor disc and stick forward as incidence increases. In trimmed flight a constant force has to be supplied by the spring for equilibrium of rotor and fuselage, but the spring rate is relatively small and has little effect on stability. An analy sis of the equations of motion by a computer programme has been used to give stability and control response characteristics both in the stick-fixed and stick-free flight conditions. The major derivatives, some geometric parameters, and rotor inertia were varied systematically to study their effect on handling characteristics and to indicate trends and sensitivities. In the stick-free case all modes were found to be stable. The stick-fixed theoretical results showed the air craft to be dynamically unstable in several modes with a readily apparent trend of increasing instability with speed. Wg Cdr Wallis, who has carried out much of the engineering development and test flying of these aircraft, enlarges on the report: "G-AXAR was intended as a purely working aircraft, with a relatively roomy cockpit operation in a conserva-
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