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Aviation History
1954
1954 - 0688.PDF
314 FLIGHT, 12 March 1954 DOMAIN OF THE HELICOPTER . . . Control Through Rotor Torque.—The rotor torque was a func tion of rotor speed and the mean blade incidence during rotation, i.e., collective pitch. As die rotor speed could not be varied quickly enough for control purposes, variation of the collective pitch remained the only means of varying the torque. With this variation there was, however, an unavoidably connected major variation of rotor thrust. Control Through Rotor Thrust.—As mentioned, rotor thrust was varied by the collective pitch of the rotor. If, then, the rotor speed was maintained constant, die collective pitch became an express control of rotor thrust. Such thrust variation was, how ever, associated with rotor torque variation—the argument being an inversion of the previous one—and consequently simultaneous adjustment by other suitable controls was necessary. Control Through Rotor Thrust Angle.—The rotor thrust vector acted substantially at right angles to the tip-path of the rotor, and tilting this plane in relation to the aircraft body would cause a corresponding tilt of the thrust vector. The tilting of the dp- path plane was effected by the cyclic or azimudi control. Even although the articulated rotor could—apart from rotor torque—only render forces at the rotor hub, it could nevertheless be employed effectively for the moment control of the helicopter. There were only two cases: (a) Moment Control Through Collective Pitch.—If a rotor was located in a manner such that its thrust vector substantially passed through the centre of gravity of die aircraft, men a varia tion of this thrust would have no moment about this point. If, however, there was a considerable lever arm of thrust vector about the e.g., then a change of dirust would be accompanied by a considerable change of moment. (b) Moment Control Through Cyclic Pitch.—Similarly, if a rotor hub was placed at or near the centre of gravity of the aircraft, then a tilt of the tip-path plane relative to the aircraft body would produce only a change in the balance of forces; it would not produce a material change in the moments. But if the rotor hub was placed at a considerable distance from die e.g., then a change in the direction of the rotor thrust vector would produce a substantial moment about mis point. Transmission of Power.—Power transmission might take many forms and even in helicopter engineering a variety of proposals had been made. These ranged from the ordiodox mechanical transmission to "gas shafts," between, on the one hand, a gas generator and a jet in the tip of a rotor blade, or on the other hand, a gas turbine converting the gas power into shaft power. Neverdieless, die autiior was convinced that the mechanical trans mission would stay with the helicopter for some time to come, simply because it was, for all but the largest rotors, the lightest and most efficient form by which power could be supplied in the form needed at the rotor. Investigations had shown that the optimum transmission speed for power shafting was between 3,000 and 4,000 r.p.m. In order to achieve the lightest transmission of power it was therefore necessary in the first instance to convert engine speed to the opti mum transmission speed in a gearbox close to the engine or, preferably, directly attached to it. At the other end of the power transmission—the rotor head—a second speed conversion, which was made in anomer gearbox, preferably forming one assembly with the rotor head, reduced transmission speed to rotor speed. Rotor Configuration.—In the following survey of the various rotor configurations attention was focused in the first instance on the need for the balance of forces and moments on die aircraft, secondly on its control and stability under various conditions of loading, thirdly on certain aerodynamic considerations relating to performance, fourthly on weight, and finally on the complexity of the transmission and similar factors. At the outset the differences between die tip-driven and the shaft-driven rotor must be recognized. The tip-driven rotor was torque-balanced within itself and only rotor dirust was transmitted to the aircraft body. The thrust vector of such a rotor could be made to balance the weight of the aircraft if the rotor was placed directlv above the centre of gravity, and diis simple arrange ment achieved die full equilibrium of forces and moments. As regards control, varying the magnitude of the rotor thrust vector produced (1) a vertical control force; a tilt of this vector in the longitudinal plane produced (2) a force in this plane; and a lateral tilt produced (3) a lateral force. If the hub was placed above the aircraft e.g., then the longitudinal and lateral components of tilt of the rotor thrust vector would also produce (4) a pitching and (5) a rolling moment respectively. There remained only the yawing moment. In a small helicopter this could be obtained conveniently by placing a control surface in the form of a rudder within the slipstream of the rotor. Driving at the blade tip therefore permitted the construction of a true single-rotor helicopter, and indeed it was the only prac tical form of single rotor helicopter. In respect of e.g. travel, how ever, the single rotor helicopter was necessarily rather limited. The tip-driven rotor had, nevertheless, a marked advantage on the score of weight. It had been shown that the weight of a rotor blade was controlled by the coning angle, and large rotor blades particularly must be mass-balanced at their tips. This parasitic weight could be avoided if the power unit was placed there also. Just as tip drive was natural to the single-rotor helicopter, so mechanical drive was natural to the multi-rotor helicopter. This conclusion was supported by the following reasoning: (a) Rotors were subject to limiting flight conditions. From this it was obvious that a multi-rotor helicopter would achieve the widest flight envelope when its rotors were so devised that their limiting conditions were reached simultaneously. (b) This was best achieved by turning the rotors at a rate such that their tip speed was the same. (c) This was best achieved by a mechanical interconnection in the form of shafting and suitable gearing. (d) It had been shown that the simplest rotor control was based on a variation of blade incidence, with the speed kept substantially constant. As in a multi-rotor helicopter all rotors were utilized for control purposes, the maintenance of constant speed in the whole rotor system was most desirable. (e) Such a transmission could be linked, at little additional expense in weight and complexity, with one or more sources of power. In shaft-driven multi-rotor configurations the balance of forces and moments was more complex than in the single rotor. An infinite number of combinations of vectors and moments was pos sible, but all these combinations fell into two groups: (a) Torque-balanced or twin-rotors. These were rotors with torque vectors which added up to zero, and (b) others. From the latter group only one form had come into prominence. This consisted of one large rotor supporting the whole weight of the aircraft, and a small rotor aft of me main rotor, producing a relatively small lateral thrust which, in conjunction with a large lever arm, balanced the main rotor torque. There were small addi tional forces and moments necessary in order to complete the balance and it was evident diat die force and moment diagrams were unsymmetrical and rather complicated. In particular a small change in any of the forces or moments, such as was caused by normal disturbances in flight, required adjustment almost all round and, as a result, this type of helicopter was more difficult to fly, for human and automatic pilots, than were symmetrical types. The necessarily different speeds of die unequal rotors produced a wider band of excitation frequencies than applied with eitiier a single rotor or equal rotors, and this increased the difficulties of avoiding resonant conditions. On the score of rotor weight there was also a penalty, because two rotors of different size weighed more than two equal rotors capable of carrying the same weight. There were various forms of torque-balanced twin rotors. Be sides being equal in torque they were generally identical in other respects (lift, diameter, speed), but rotated in opposite direc tions. We distinguished between eclipsed and non-eclipsed rotors. Eclipsed rotors were either concentric or they were closely inter- meshing. The balance of forces and moments in this configura tion was like that of the single rotor, except that in the former case torque (and thrust) could be increased in one rotor and decreased in the other (the total rotor thrust remaining constant), so pro ducing a net rotor torque which could be used for yaw control. There were two forms of non-eclipsed twin-rotors and both had been tried. The first was the side-by-side arrangement and the second was the arrangement in tandem. In both configurations the rotors had the full benefit of the total disc area in the hovering condition, but, as a quid pro: quo, die transmission system was more extensive and heavy than diat involved in eclipsed rotors. It would be seen that whilst these configurations, like all twin- rotor configurations, were symmetrical with respect to the normal direction of flight, the control was substantially different from that discussed previously. There was now an "axis of suspension" connecting the two rotor hubs and the two tiirust vectors, the centre of gravity of the aircraft lying below this axis. A considerable e.g. travel along the axis of suspension was per missible without seriously upsetting the balance of the aircraft. This represented a major advantage for the tandem helicopter over the other configuration, particularly as it could be arranged that the "axis of suspension" ran with the fuselage major axis. Whilst the aerodynamic performance in hovering was the same, there was a significant difference in forward flight between the side-by-side and the tandem configuration. The former had a large "wing span" over the two rotors and consequently its induced losses were relatively low, whereas the latter, with only half the span of the former, had a higher induced flow. Against this, allowance had to be made for the considerable weight and the parasitic drag of the lateral outrigger structure which side-by-side rotors required, whereas in the tandem arrangement the rotor supports were light and had negligible drag. The author believed that the increased complexity and the consequently reduced safety-standard of configurations involving more than two rotors were not acceptable, as these types could offer no material advantages over the two-rotor configurations. Power Supply.—After discussing the power requirements for hovering, cruising and take-off (see Fig. 1), and the technique of
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