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Aviation History
1953
1953 - 1465.PDF
6 November 1953 619 lift with the collective-pitch lever will bring small changes in torque to be corrected with rudder. When facing down-wind the stick must come back to prevent drifting down wind. Then comes the turn back into wind with full weathercock action to be countered. If quick anticipatory action is not taken the helicopter, all in a second or two, will whip round to head into wind, climb, and start to move backwards. The engine, too, will probably overspeed if not checked at once. Having tried this out for oneself one appreciates a whole lot more the efforts of die pilots who demonstrate widi such precision these whirls and other manoeuvres at displays and exhibitions. I am reminded here that one question I was required to answer was: what prevents rotor blades, which are hinged at the roots for up-and-down movement, from coning and folding upwards in flight? There are, of course, no "up" stops as such. It seems that there are two answers, of which one is subsidiary. Centri fugal force is the main reason for die blades remaining in the spread attitude and gravity helps to some extent. A "see-saw" rigidity within certain rotor systems of die Bell type would make a contribution. Another question was asked in regard to torque correction. The effect of torque, a couple, is counteracted in die helicopter by a tail rotor producing a moment. This leaves a sideways force apparently uncorrected unless it is by the pilot holding the stick out of centre in flight. Pilots are not required to do this, and in fact, this force is looked after by rigging the rotor in such a way diat with stick central an appropriate amount of opposing sideways thrust is produced. To avoid complications when auto rotating with torque absent, this compensating device is linked up mechanically widi die collective-pitch lever so diat, as torque increases with die raising of die lever, so does die sideways thrust. And when the lever is down for autorotation no sideways dirust is produced. For some unaccountable reason I find it intriguing to taxi a helicopter—perhaps, perversely, because there seems to be no apparent reason for doing so or even being able to do so. In any case, it is only just fair to call die evolution taxying—in fact, it comes near to being flying widi your wheels dragging. But cer tainly with stick forward and some of the weight taken on your elbows as it were when you pull the lever up to about half-way, you can move around freely enough on die ground and turn in the normal manner. Perhaps I should have mentioned diat normally the collective-pitch lever would always be down on its lower stop when the rotor is turning and die machine is on the ground. One of the things to be learned is how to make a limited-power take-off. The main use of diis action would be at high-altitude airfields, because a helicopter can be flown with forward speed at heights where it would be unable to hover—a matter of transla- tional lift again. A W.S.51 might be able to hover for a landing or take-off widi pilot only at, say, 14,000ft, but to take off at diis height widi a passenger or two would require a technique rather similar to mat for a fixed-wing machine. A forward speed of somewhere between 20 and 40 m.p.h. on the ground, particularly widi some wind to help, should produce sufficient extra lift to let the machine fly off. The W.S.51's hovering ceiling, with ground effect and at its maximum all-up weight of 5,700 lb, is 7,000ft. Run-on landings are usually practised for a different reason, namely, following an engine failure. They are no more dian a glide approach and landing under other names. While I do not suggest that one should make a practice of cutting fuel, switching- off and autorotating in, for an experienced pilot this is no trouble and, in fact, I was shown several and allowed to practise a few dual. Experience indicates that most pupils, if confronted widi an engine failure, manage a run-on landing without serious damage. The fact that G-ALIK is still available for instruction after several years and more than one engine failure adds some weight to this contention. What happens if die engine stops? First, the automatic clutch and free-wheel come into use and for a second or two the rotor fans on. An obliging student will instantly lower the lever and make an autorotative (or glide) descent, turning into wind with bank and adjusting length of glide within die rather small limits (judged by fixed-wing standards) by altering the forward speed between about zero and 70 m.pi. He should require no more than a small, not too rough, field for landing. The lengdi of run will depend on wind strength, and die touch-down speed should be 20-40 m.p.h. On die other hand, I hate to think of die consequences to die obtuse pupil who fails to autorotate—quite quickly his rotors will slow down and then fold like an umbrella blown inside out—and he will descend no slower than if he were hanging on its handle. Before leaving the subject of autorotation I would add that autorotative descents, aiming at a predetermined touch-down point, are practised down to 200 or 300ft, just as forced-landing approaches might be learned in a fixed-wing trainer. On recover ing and opening-up there is also the same risk of over-revving for a few seconds, and die swing of die nose must be corrected—or better, anticipated. Once before, in an early article, it was mentioned that the essential difference between autorotation and powered rotation is diat in die former the air moves up "through" the rotor blades instead of down and that the blades are turned by die air stream rather than beating on it. The two little sketches reproduced here serve to show what must be done to the pitch of the blades to keep diem fanning after removal of the engine power. They are intended to indicate simply the forces acting on the blades. POWER AUTOROTATION Vertical Vcrticol Autorotation: The position of the resultant in these total air reaction diagrams shows why the rotor blades keep turning in autorotation after the engine has failed. On the left, the blade angle is high and both lift and drag are considerable. On the right, with blades autorotating (and resultant ahead of the vertical) the angle is small. The drag is thus comparatively low and the lift is somewhat less than in the first diagram. With the resultant ahead of the vertical, the blades will accelerate. To sum up, die preceding paragraphs are included to dispel the impression which many people still have that die loss of all engine power means disaster for the helicopter. Far from it; this flying machine can glide like any odier and at a much lower forward speed. Even the failure of a tail rotor does not necessarily spell curtains, because with engine out and blades in autorotation A willing beast, and to some even a handsome one, G-ALIK, gaily painted, relaxes in the sun. "Flight" photograph
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