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18 AIRCRAFT ENGINEERIN G January, 1942 Some Elementary Considerations on Efficiency and Control By J. Lockwood Taylor, D.SC. EPORT S from America speak of flight component of thrust , as is needed to give forward separate control, a not very acceptable tests of two types of screw-lift aircraft, the complication. motion in the absence of a separate propelling Sikorsky multi-rotor machine, and a twin- screw. Rotor tilting is perhaps neater, and Assuming two main lifting screws, there is a rotor type. From this it would seem that the choice between tandem .and side by side posi e.g. shift, which may only need to be quite day of the practical helicopter, so Ions fore tioning. The latter is probably better acro- small, probably simpler mechanically, at any shadowed, is not far distant. The following dynamically, giving less interference between rate in a small machine. general notes on design considerations may the two when under way, and less screening Simultaneous pitch changing of the blades of therefore be sufficiently topical to be of interest. b y the fuselage when lifting vertically. The both rotors in normal flight is merely an altern There is no special difficulty in designing an former has the important advantage of not ative, and not apparently a very attractive one, airscrew to give the necessary static and climb requiring a special structure to be built out to to the use of the throttle, although it may seem ing thrust, equal to the weight of the loaded support the rotor bearings, which may be car an obvious refinement in these days of variable- aircraft. To get the static lift, the main re ried from either end of the body. Mutual pitch airscrews. Synchronization of the pitch quirement is sufficient disk area, which in interference with the tandem arrangement may changes would be necessary to a high degree practice involves gearing down to low r.p.m. tend to disturb the balance of torques when the of accuracy to maintain equal thrusts. As a and low tip speed. The latter ensures a value machine is moving horizontally, but an exact means of allowing auto-rotation in the event of of relative advance (v/nd) high enough to give neutralization is not so important under these engine failure, without reversal of the direction a reasonable efficiency at maximum vertical conditions. Control considerations are fairly of "rotation, this type of pitch control seems climbing speed in still air, although on the low evenly balanced, comparing the two lay-outs, much more attractive, if not essential, although side judged by ordinary airscrew standards, the and on the whole there is probably not a great there is then a limitation of the radial pitch dis principal restrictions being diameter and weight deal to choose between them. tribution desirable for maximum efficiency, of rotor and gearing, and the necessity to avoid under driven and windmilling conditions re too low a ratio of circumferential to maximum spectively, which may enforce the use of an Performance horizontal speed. With the relatively low tip untwisted and therefore inefficient blade. I t is evident that quite a good climb perform speed, and relief of bending moment by hing Differential pitch changing of the two rotors, ance can be expected from the helicopter, even ing at the root, as in the case of the autogiro, a in so far as this is allowable without destroying without exceptional rotor efficiency, seeing that high aspect ratio of the individual blades is the torque reaction balance, has possibilities so little power is expended in overcoming drag, permissible, rigidity and flutter considerations for control, either lateral or fore and aft, extra to that of the rotor itself—which is of being the limiting factors. There appears to according to the rotor arrangement; a still course included in the efficiency. These re be a case for a multi-blade rotor with a view to further refinement, in the shape of cyclic pitch marks apply to a vertical climb in still air, but keeping down diameter and allowing a higher change, i.e. pitch varied mechanically as the there is some reason to expect an even better gear ratio, even at a slight sacrifice of efficiency. blade moves round the disk, gives a fairly direct rate of climb at a moderate forward speed, Wooden blades, with their low density and high control of rotor centre of pressure; when using corresponding with the optimum climbing internal damping, seem ideal. rigid blades, the effect is less direct, and may be speed of an ordinary aeroplane. The drag is partly cancelled out, with the more probable still fairly low, and the rotor efficiency is actual arrangement of flapping blades. Torqu e Reaction ly increased owing to the fact that with forward Pendulum action may be expected to contri The bugbear of helicopter designers has speed the screw is continually acting on a fresh but e to the stability in the longitudinal and column of air, resulting in an improved momen always been the elimination of torque reaction lateral directions, though this effect alone will tu m efficiency. Since the rotor blade drag from the rotor upon the aircraft, which if hardly furnish a complete picture of the stabil loss remains constant , to a first order of approxi unbalanced would give the machine a steady ity characteristics, since aerodynamic influences mation, the net efficiency is increased by an spin in the opposite direction to the screw. must also enter. It may be that the lateral amoun t which may be estimated from the Twin rotors revolving in opposite directions stability will be such as to enable a separate larger volume of air acted upon. The effect is provide a partial solution, but it is not easy lateral control to be dispensed with, a very t o ensure absolute equality of torque even with analogous to the decrease in induced drag of an desirable simplification. Too great an amount symmetrically arranged lifting screws, and less aeroplane with increased speed. of longitudinal stability would be detrimental to so with co-axial airscrews, which might other Nothing exceptional in the way of top speed control, as in the case of an ordinary aircraft. wise seem a suitable arrangement. A differen is to be expected, however careful the stream As regards directional or yawing stability, this tial drive from a single engine might be pro lining, since when th e horizontal velocity reaches appears to require model or full-scale investiga vided, but there is still the difficulty of possible a figure comparable with that for the circum tion; in fact this applies to th e stability generally inequality in bearing friction, apart from the ferential speed at, say, 70 per cent of the screw as it is here tha t data on which to base predic sacrifice of a degree of independence of control radius, there is an almost total loss of lift on tions is most lacking. of the two rotors, which may be disadvantage one side of the disk, which must result in a ous. The question is bound up with that of sharp drop in efficiency; and, as has already directional control; if complete control at low Conclusion been seen, the r.p.m. and tip speed cannot be forward speeds could be dispensed with, the made too 'nigh without loss of climb. In prac The helicopter as a specialized aircraft cap ordinary fin and rudder would obviously be tice it will probably be found necessary to com able of taking-off from and landing in the most applicable. Otherwise it seems necessary to promise between climb and top speed by a confined areas, since it can rise and descend add such features as differential tilt of the two suitable choice of disk area, just as in an ordin vertically, irrespective of wind, having a high rotors, giving a yawing moment at the expense ar y aircraft it is necessary to choose between rat e of climb and capable also of hovering and of a small rolling moment, or differential pitch to p speed and take-off, by varying the wing exceptionally accurate manoeuvring, has un control, enabling the pilot to equalize the torques area and wing loading. doubted attractions and should find an opera with possible slight resulting variation in the tional field of its own. These remarks apply Directional control has already been men lift generated by the two rotors; these suggested to the inherent possibilities of the type, assum tioned in discussing the torque reaction prob solutions apply to rotors with a common prime ing development difficulties to have been over lem. Fore-and-aft control in the vertical plane mover. Separately driven screws can in theory come. Little is known about stability, though can be got, in the absence of sufficient forward have their torques adjusted to equality by something can no doubt be learnt from the speed to make an elevator effective, either by throttle movement, but in practice this would autogiro in this connection. Control, particu tilting the rotor axis relative to the aircraft probably be very tricky and difficult, if not larly the twin problems of directional control (the ultimate effect of which will be to tilt both impossible, especially as the running conditions and torque reaction elimination, seems to be the rotor and aircraft relative to the horizontal) or of the two engines would hardly be likely to outstanding question requiring most attention; b y fore and aft shift of the centre of gravity remain absolutely steady after adjustment. th e complete answer may lie in some combina relative to the rotor, producing a bodily pitch There remains the expedient of torque balanc tion of the various methods outlined, or may ing displacement. Either of these alternatives ing by means of auxiliary rotors under involve the use of gyroscopic stabilization. is a possible means of applying a horizontal
Aircraft Engineering and Aerospace Technology – Emerald Publishing
Published: Jan 1, 1942
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