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Aviation History
1966
1966 - 0897.PDF
516 FLIGHT (nternot/ona,, 3j STOL take-off attitude (above) at full-power, stick hard back and rotor drive disengaged. Hands-off landing (below) shows WA-II6 docility THE WALL1S AUTOGYROS ... under an Air League scheme, are introducing a wide cross- section of the private flying community to rotary-winged flight (Flight, August 12, 1965, page 264). At such an early stage in the development of an advanced new concept in flying machines it would be surprising if there had not been operating difficulties and in fact both the privately owned WA-116s have been damaged in ground accidents at various times. But it is significant that in several thousand flights in under five years by a total of over 80 pilots of every conceivable kind of previous experience, there has not been a single accident associated with the flying characteristics of the aircraft. Invariably the trouble has been incorrect pilot action on the ground: stick too far forward on take-off; taxying too fast downwind and trying to turn; and too much speed on the ground. With correct handling, it is claimed, anyone of average skill should be able to fly a WA-116 from any surface over which it can be taxied. The WA-117 could well be the smallest aircraft ever to get a C of A, but it will not be the cheapest; there may not be much substance to an autogyro, but what there is needs to be made with watch-like precision. The WA-116 with a permit- to-fly sold for £1,950, but WA-117s will cost nearer £2,750 apiece. Although autogyros are expensive compared with ultra- light aeroplanes, their slow-flying ability, manoeuvrability and STOL performance (better than that of any fixed-wing aircraft) are key factors for many aerial-work applications. In the first instance, Wallis Autogyros Ltd are concentrating on single- seaters, since it is the ability to put a man in the sky as cheaply as possible that seems to offer the greatest unchallenged market The WA-116 Most pre-war autogyros had tractor engine/ propeller arrangements. A pusher system was chosen for the WA-116 on the score of compactness, because it gave the pilot a better view, and because the noise and smells of the were behind him. The consequent difficulties of engine ^ and the effect of power variations on heading (because th° f force of the slipstream blows over the rudder) were con 'ri less important. Very little can be done to cut the overall h ^ of any autogyro, for it is largely fixed by the propeller diamet the pusher arrangement is not the best in this respect be of the necessary rearwards inclination of the rotor disc CTh^ factor is really troublesome and leads to inefficiency wh comes to installing a more powerful engine: a four-bfri" propeller is regretfully envisaged for the WA-117. One-, two-, or three-bladed rotor? A rigid hub or an all flapping arrangement? All of these alternatives were con" sidered before it was decided to aim for absolute simplicity —whereupon the choice fell on a two-bladed rigitfrotor arrangement with a simple teetering action. Pre-take-off spin- up of the rotor normally implies mechanical complication but this highly desirable feature was arranged in an ingeniously simple manner. A rigid rotor tends to have vibration problems but Wallis considered that simplicity was more important— in the event the problem has yielded in the face of careful design and construction. Structural testing has included strain- gauging the main fuselage and control tubes for the measure- ment of in-flight loads. Rotor blade functions have been assessed photographically as described on page 517. Rotor Head The rotor head is at the heart of any rotary- winged aircraft and, together with the control system, is the outstanding feature of the WA-116 design. The gimbal head and the control system are so proportioned that rotor drag and dynamic movements of the non-rotating structure counter- act each other throughout the flight regime. For the moderate speed range envisaged in this case, a simple fixed-geometry offset gimbal is sufficient. The suspension geometry is the result of considerable trial and error—inherent stability is dis- played at all times and the "stick force per g" characteristics follow normal aeroplane values. The Timken taper roller main bearing is of the "dead-axle" kind to avoid the fatigue problems of a small-diameter revolv- ing shaft, the axis of which is located some 2in aft of the point where the main suspension plate is pivoted from the supporting pylon. The twin control rods are linked through self-aligning bearings to the suspension plate a further 2in aft of the main axle. The roll spindle axis is below that of the pitch spindle. The rotation plane of the blades is displaced by parallel movement of the control rods (pitch) and by opposite move- ment (roll) or any combination of the two (page 518). Stick loads are alleviated by springs attached to the control rods. Ball, roller and self-aligning bearings are used throughout the control system and there is no lost motion. To compensate for the natural unequal lift distribution between the advancing and retreating blades the rotor is, of course, free to pivot (teeter) in relation to the rotor head disc. Teeter action is limited during pre-take-off up to about 200 rotor r.p.m., whereupon two spring-loaded limit-stops fly out under centrifugal action to give full flight-teeter freedom. Re- engagement of the stops occurs at around 180 r.p.m. This prevents any possible damage to the rotor head while the air- craft is being taxied over rough ground. If there was ever an in-flight tendency to exceed the teeter limits, the stops would take the initial load—providing some warning to the ^| —before the intentionally weak pivots would shear. Thus, there is some additional movement provided, but so far tne stops have never been touched in flight. The patented teeter stop arrangement is fail-safe—should a bob-weight or lever arm break free in flight the pivot bolt would shear to prevem engagement of the limit stops. A simple hand-operated friction- strap rotor-brake works on the main suspension plate. A high-speed low-torque flexible-shaft system weighing » mere 51b total, transmits engine power to the main suspen plate to spin the rotor to about 280 r.p.m. prior to take- - considerably more spin-up is available for occasional «' short take-offs. A commercial epicyclic gearbox is usea reduce the high shaft-rotation speed at the rotor head ^ output pinion engages on an internally toothed wheeI o ^ suspension plate/rotor spindle. At the engine end the an^ ^ made by running a plain rubber wheel on to a drum a propeller hub. There is no risk of torque reaction airec
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