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Aviation History
1947
1947 - 1979.PDF
NOVEMBER I3TH, 1947 F LIG HY 347 Engine-off Landings ind to a much more considerable loss ol kinetic energy, since the initial speed was high and the kinetic energy is a function of the square of the speed. So long as a small increase in rotor in- cidence corresponds to a substantial loss of kinetic energy, a pull-out can at least be started; but consider the thick arrow in its present position: Here even a large increase in incidence corresponds to only' a small change in speed and a negligible change in kinetic energy, since the initial speed is low. Kven if the rotor were suddenly tipped up at right angles to the flight path, all that would happen would be that the arrow would swing down to indicate vertical descent and then it would be too short to reach the curve until the helicopter had settled down to a steady flight in the new dii o measure the length of the arrow in this diagram it is necessary to swing it on to one or other of the scales. If it is swung up to the top scale it can be seen that its length in the position shown corre- sponds to a glide approach at an indicated airspeed of 25 m.p.h. in the Hoverfly I, and it is clear that in these conditions the kinetic energy of forward motion is, for practical purposes, exhausted so that no sort of pull- out is possible from an approach glide at or below this speed. In fact, for the Hover- fly I there is no condition of steady gliding flight corresponding to an airspeed of less than about 23 m.p.h., although this is not indicated by an ordinary A.S.I, because the pitot tube is usually horizontal so that it does not register flight path speeds in steep descents. In engine-off landings from approach glides as steep as that indicated by the arrow, the elimination of downward velo- city is entirely dependent on the use of the collective pitch control as a means of extracting energy from the rotor to provide the required vertical force, and none of the helicopters in common use to-day is suit- able for landing gently at zero ground speed in still air from an approach glide of this kind. As the speed of the approach glide is increased above the minimum airspeed, it again becomes possible to commence a pull- out, but a considerable amount of energy is required to complete this manoeuvre and, in the case of the Hoverfly I, a simple pull-out cannot be completed until the speed of the approach glide has risen to somewhere between 45 and 50 m.p.h. I.A.S. Above this speed the collective pitch con- trol can no longer conveniently be used to assist in arresting downward velocity in an actual landing, and for this reason I will refer to the approach speed at which a simple pull-out is just possible, as the hange-over speed. Glide and Float Differences Above the change-over speed a horizontal ^at becomes possible after the pull-out. The minimum speed at the end of an or- dinary float occurs when the kinetic energy of the forward motion is exhausted, and it is basically the same speed as that repre- sented by the length of the thick arrow. There is, however, a difference between conditions in a glide and those in a horizontal float. This is illustrated by the small diagrams in the centre of the graph, which show that whereas, in a glide, the total air force is vertical and equal to the weight, in a float it is the iifi. which is vertical and equal to the weight. Since the disposition of the forces is otherwise identi- cal, it follows that they are slightly bigger in a float for the same rotor incidence, and the corresponding speed is approximately 10 per cent higher, so that for the Hoverfly I the minimum speed at the end of an ordinary float is about 28 m.p.h. The lower half of the graph merely sum- marizes part of what we have seen in con- sidering the top half. We have already noted that the kinetic energy of forward or linear motion depends on the square of the speed, and the lower curve shows how the initial energy of the approach glide in- creases with speed. It also indicates roughly the energy required for the pull-out and the rapid manner in which the energy available for the float increases with the speed of the approach glide. In an emergency the collective pitch con- trol is sometimes used at the commaoce- ment of an engine-off landing, but our practice of engine-off landings is still almost entirely based on the azimuth stick as the instrument for eliminating downward velocity. Therefore, if we consider only engine-off landings corresponding to engine failure at a sufficient height to allow the pilot full choice of his approach, I think I will not be treading on too many toes if I regard the flare-out as a special kind j>f pull-out and say that our present practice is based on the motions of pull-out, hori- zontal float, and final sit-down with the aid of the collective pitch control. In engine-off landings of the kinds usually practised, the float is sometimes absent and so, occasionally, is the use of the collective pitch control. Also the collective pitch control is sometimes used to cushion the fall of the helicopter after a flare-out some distance above the ground, and sometimes merely to hold it in the air while it con- tinues to lose forward speed after the end of a normal float. But before we consider these motions in detail I must first clear EXTRA FLOAT 3O LAS. 12 LAS Fig. 2. Engine-off landing from high- speed approach glide. The lower curve shows ballooning due to sharp pull-out. (Hoverfiy I). ... _; up a statement which I made earlier and which is repeated in Fig. 1, to the effect that the collective pitch control cannot conveniently be used to assist in arresting downward velocity above the change-over speed. At first sight this seems to be rather odd, because the collective pitch control is at all times a powerful means of arresting downward velocity, and whereas it is the only means of doing this from glides at the minimum airspeed, its effectiveness is also very considerably increased in forward flight. In fact, even if a fully loaded Hover- fly I is put into a glide at any A.S.I, read- ing between, say, 35 and 70 m.p.h., the alti- meter hand, which rotates quite fast in a steady glide, can easily be stopped momen- tarily by pulling up the pitch lever, even if the speed is kept constant and the throttle shut. At an A.S.I, reading of be- tween 40 to 60 m.p.h. the altimeter hand can not only be stopped, bu+ it can be held stationary for a brief period during which the machine is flying horizontally without power at more or less the original forward speed. On the other hand, if an attempt is made to eliminate downward velocity by means of the collective pitch control in a landing from an approach glide at more than the change-over speed, the pilot will find him- self in a dilemma because the only way in which he can reduce his ground speed is by tilting the helicopter backwards. If he starts to do this before using the collective pitch control, the result will be an ordin- ary pull-out or flare-out. If he pulls back the azimuth stick at the same time as he uses the pitch lever, he will find the heli- copter doing an excessively hard pull-out, with the result that his last ground speed will be Considerably greater than his first and he will have to down pitch and quickly think of some other manoeuvre. Finally, if the pitch lever is used first from a glide above the change-over speed, the velocity of the helicopter cannot subsequently be re- duced to less than the minimum steady air- speed, so that the forward component of this velocity can be appreciably reduced only at the expense of a heavy landing. Landings to be Practised We can now examine the kinds of engine- off landing which can be practised with the helicopters at present in common use, and in this connection I have in mind particu- larly the Hoverfly I, which is a very good machine for this kind of practice, since it gives the pilot manual control over the col- lective blade pitch, which I believe to be essential, and it is also so arranged that the main rotor can be safely inclined backward at quite large angles, even when the tail- whee! is touching the ground. Also at this point I must remark that, because a land- ing is a manoeuvre which is, by definition, conducted close to the ground and is always seen in relation to the ground, the effect of the wind speed on its appearance causes such confusion in the arguments which usually follow that it is absolutely essential to base our discussion strictly on no-wind conditions The upper part of Fig. 2 shows the essen- tial attitudes and motions of an engine-off landing from a high-speed approach. The speed figures quoted are subject to consider- able variation, but they are, in fact, typical of one kind of landing which has been ex- tensively practised at Beaulieu, where, in initial tests and in subsequent training and practice, well over 200, and probably by now nearer 300, of these landings have been made without mishap. I think this landing is ideal for initial training purposes because it is divided into a number of separate and distinct movements in which mistakes are easily noticed for correction in subsequent practice. It is useful for any ordinary forced landing in open country, and it can also be modified to suit many special cir- cumstances. In this landing the collective pitch control is used only in the "extra float," and the manoeuvre comprises a fairly fast approach glide, a gentle pull-out, and a horizontal float, so that it is very similar to an aeroplane landing. The feature which distinguishes it from other engine-off land- ings practised by rotating-wing aircraft is the deliberate inclusion of the horizontal float after the completion of the pull-out. The float is typically entered at a fairly high forward speed, with the fuselage sub- stantially horizontal, and we have already seen that conditions at this point are governed almost entirely by the speed which the pilot chooses for the approach glide. Thus the conditions of entry into the float are voluntary, but so long as the float re- mains level the subsequent deceleration of the aircraft, and the corresponding adjust- ment of ite, attitude, are governed exclusively by its aerodynamic characteristics and are therefore involuntary; with the important exception that the pilot can at any time dis- continue the float, either by allowing the aircraft to settle on to the ground or by using the collective pitch control to hold it in the air while it continues to decelerate without further alteration in attitude. The rate at which the rotor incidence, and the attitude of the fuselage, increase during a float is of particular interest in connection with the landing of helicopters in which a large backward inclination of the fuselage is not permissible near the ground because of the possibility of fouling the" tail rotor. The incidence of the rotor varies in- versely with the square of the speed, so that the change in rotor incidence, for a given reduction in speed is small when the
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