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Aviation History
1951
1951 - 0075.PDF
FLIGHT, 11 January 195. 49 THE HELICOPTER . . . similar arrangement, causes the pitch of each blade to vary periodically with its position round the disc; the amount and the position in azimuth are determined by the position into which the stick is moved. The stick is connected to a non- rotating plate, mounted universally on the rotor shaft, and above this is a second plate mounted on bearings and rotat- ing with the rotor. The control horns of the blades are linked to the rotating plate. When the stick is central no cyclic-pitch is imposed; but if, for instance, the stick is moved in any azimuth, the swash- plate tilts and the blade angle decreases in the direction of the stick movement, then increases as it passes the opposite point 180 deg away. As the lift is greater where the angle is higher, the blades move on their flapping hinges in such a way that the plane of the .rotor disc tilts in the direction of the stick movement, and the resultant rotor lift, which is always substantially along the effective disc axis, also tilts in the same direction, giving control in the plane of the resul- tant lift, i.e., in the direction in which the stick is moved. The way in which cyclic-pitch control is used varies some- what according to the helicopter configuration, and should be explained— With the single rotor, cyclic-pitch control is used about both pitching and rolling axes, and it should be noted that in itself the tilting of the rotor disc produces movement only in the plane of the resultant lift, due to there being a component of the rotor lift in that plane. Even though the lift may be varying, no moment can be transmitted to the fuselage because of the blade hinges; and it is only because the centre of gravity of the helicopter is a considerable distance below the rotor that a moment about the e.g. is fig. I. Hub of the West- land Sikorsky S-51. The numbers refer to the author's explanation of itsoperation. obtained. If the e.g. were located at the centre of the rotor hub no control could be obtained from cyclic-pitch change. In normal flight the control acts in the same way as the elevator and ailerons of a fixed-wing aircraft. When hover- ing, small corrective displacements of the stick will hold the helicopter on an even keel, and above a given spot; but if the stick is moved and held the helicopter will move [in the direction of the stick displacement. Thus, if the stick is moved sideways and held, the helicopter will fly sideways, at a speed depending on the stick displacement. In the side-by-side or tandem arrangement, using two rotors, it is usual to employ cyclic-pitch control only about the axis in the plane of the rotor centres, control about the axis at right angles being obtained by the differential use of collective pitch between the rotors. Thus, in the tandem arrangement lateral control is by cyclic-pitch; but to raise fhe nose or reduce flying speed (for instance) the rearward Movement of the stick causes an increase in the lift of the front rotor by increasing collective pitch, and a reduction •n the lift of the rear rotor. This same arrangement is used on the three-rotor Air Horse, cyclic-pitch being applied to the two side-by-side rear rotors for lateral control, while differential collective pitch between the single front and the two rear rotors is used for longitudinal control. In directional control, again, we must distinguish between the single- and multi-rotor arrangements. In the former, the commonest method of compensating the torque reaction, and at the same time obtaining directional control, is exem- plified in the well-known Sikorsky type, where a vertical rotor is used at the tail. The tail rotor is of variable-pitch type, with an orthodox "rudder" bar or pedals connected to the pitch-changing mechanism. When the "rudder" is central the tail-rotor pitch is set to give torque compensation under cruising conditions. It should be noted that this arrangement does not give a pure yawing couple, but only a moment about the heli- copter centre of gravity, and a side force. To balance the latter it is necessary to introduce a compensating force by tilting the rotor resultant lift. One variant of the tail rotor which ought to be men- tioned is that used on the French Sud-Est Type 3101, where two tail rotors are employed, carried on booms at 45 deg to the vertical. This not only gives torque compensation and directional control, as with the single-tail rotor, but also longitudinal control, so eliminating the normal cyclic- pitch control about the pitching axis. The tail rotor, while a very effective control and acting as fin surface to stabilize the body in yaw, has the demerit that it absorbs power otherwise available for lift, and com- plicates the overall control of the helicopter. Thus, to turn the helicopter in the sense opposite to the torque reaction absorbs more power, so that to maintain height the throttle must be adjusted, thus altering the torque reaction and in- volving further "rudder" correction. Conversely, ny alteration in the throttle or collective pitch alters the torque reaction and involves the use of directional control. In sideways flight, opposite "rudder" must be applied to keep the aircraft heading correctly, and this again involves throttle and /or pitch correction to maintain height. As regards multi-rotor systems, in the side-by-side or tandem arrange- ment directional control is obtained by differential use of cyclic-pitch control, the resultant lift of one rotor being tilted in one direction, as appropriate, and that of the other rotor in the opposite sense. In the Air Horse three-rotor system the same method is used on the two rear side-by-side rotors. In a superimposed contra-rotating configuration the torque balance of the two rotors is unbalanced by differen- tially changing the collective pitch of the two rotors in the desired sense. Hub Arrangements.—We have seen that the essential requirements of a helicopter of the flapping-blade type are that the blades shall be so mounted on the hub that they are free to move about a hinge in the plane at right angles to the mechanical axis of rotation of the hub, in order that they may automatically take up a position where the moments of lift, blade weight and centrifugal force are in equilibrium, so as to balance out the rolling movement due to the asymmetry of lift in forward flight. This, unfor- tunately, is not the whole requirement in a flapping-blade system, since there are fluctuating forces in the plane of rotation which make it necessary to introduce a further hinge, known as the "drag hinge," in order to relieve the blade roots of the heavy bending moments imposed. The oscillation of the blade about the drag hinge is unstable unless the movement about the drag hinge is damped or restrained by springs, and we commonly find that friction or hydraulic dampers are provided. The blades must also be mounted in the drag-hinge fittings in such a way as to be capable of rotation for pitch change, and we must have means of controlling the blade angle, both collectively and cyclically. Typical hub arrangements to satisfy these re- quirements are described in what follows.
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