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Aviation History
1953
1953 - 0123.PDF
23 January 1953 121 2 deg to plus 14 deg. Increase or decrease of collective pitch also automatically actuates the engine throttle mechanism, feeding in sufficient power to the main rotor to be absorbed by increase of pitch, and reducing power with decrease. The twist-grip throttle on this lever is used to make fine adjustments in the rotor revo lutions necessary for any particular flight condition. Movements of the collective-pitch lever are closely related to movements of the control column. Response to collective-pitch changes is immediate, and it is a more powerful control than is the cyclic pitch from the control column. When hovering in still air conditions, where only slight movements of the stick are necessary to keep the machine over a given spot, collective-pitch changes are hardly discernible. Hovering over a spot facing a wind of say 15 m.p.h. is a different matter, as will be explained. First it should be said, in parenthesis, that the spinning rotor behaves in exactly the same way as the fixed-aerofoil section of a mainplane, deriving increased lift from an airstream passing over it, or from an increase in angle of tilt (angle of attack) in relation to the airstream. The individual blades become the "advancing" and "retreating" blades as they move through their cycle of rotation. Lift on the blades, varying with the square of their velocity, increases to a greater extent on the advancing side of the rotor than it decreases on the retreating side, resulting in an overall gain. The individual blades need not be considered, as it is the overall effect on the rotor which affects control move ments. A tendency to drift backwards while hovering into wind is corrected by a forward movement of the control column. Thit> movement provides the forward tilt to counteract the backward drift, but it must be accompanied by an increase of collective pitch to offset the loss of lift resulting from the decrease in the angle of attack of the rotor in relation to the airstream. Con versely, while a backward movement of the stick will check a forward drift of the machine, it will increase the angle of attack of the rotor and necessitate a decrease of collective pitch to prevent the helicopter rising. Loss of lift caused by tilting the total lift vector out of the vertical accentuates the sink of the machine with forward tilt, but is not sufficient to offset the increase in lift result ing from the increased angle of attack of the rotor with backwards tilt. Sideways movements are corrected in a similar way. The lateral thrust of the tail rotor does affect the controls to a certain degree in sideways flight, but it is not proposed here to confuse the issue by introducing this further complication. It is evident from the foregoing that hovering a helicopter is a full time, two-handed job. The fixed-wing pilot will have a good idea of what is involved if he considers the amount of concen tration required—and the degree of co-ordination of elevator, aileron, and rudder controls—during the hold-off period imme diately before a good three-point landing. It is in exactly that position that the helicopter must be held in hovering flight for an extended period. When one recalls some of the bumpy landings one made when first learning to fly a fixed-wing machine it is not surprising that the helicopter pupil sometimes needs an area half the size of a football pitch to learn to hover. The third primary control, the rudder pedals, fortunately pre sent the easiest problem. It is common knowledge that the tail rotor provides a lateral thrust at the end of the tail boom to counteract the torque of the main rotor and so prevent the fuselage turning in the opposite direction. The tail rotor blades have a variable collective-pitch mechanism which the pedals operate through cable and pulleys to give control in the yawing plane. Increase or decrease of power to the main rotor, occasioned by movement of the collective-pitch lever, varies the amount of torque reaction, and causes the fuselage to turn about its own axis beneath the rotor. Opposite rudder applied in the natural sense compensates by varying the tail-rotor thrust. Rudder pedal movements are therefore related to movements of the collective-pitch lever, and in like manner the use of the last It can be shown mathematically that the resultant of all the angles of attack of the individual blades in their cycle of rotation is equal to the angle between the tip-path plane and the airstream. During hovering into a wind or when in forward flight, backward movement of the stick has the effect of increasing the disc angle of attack, and thus the lift. DISC ANCLE-OF-AT lACK EQUIVALENT POWER" AND FUEL CYCLIC PITCH Typical of modern helicopter controls is the layout for the Sikorsky S-55 shown above. Dual controls are provided in the cockpit, but the aircraft is usually flown from the starboard side. primary control, the twist-grip throttle, is related to movements of the rudder pedals. The tail rotor is geared directly to the main rotor and runs at a speed ratio constant to it—usually about five times the main rotor speed. Any increase or decrease of pitch on the tail rotor blades draws power away from the main rotor, or allows more power to be absorbed by it. The main-rotor revolutions will fluctuate slightly with rudder pedal movements, and the fine adjustments necessary to maintain the constant rotor-speed applicable to any particular flight condition are made with the twist-grip throttle. Correct co-ordination of these four primary controls will result in perfectly stationary flight. As with all aircraft, the control movements do resolve themselves into pressures rather than deliberate movements, as one becomes accustomed to anticipating what the next movement of the helicopter will be. It is true, though, that to hover a helicopter accurately for any lengthy period is apt to be an exacting and tiring business. While it is not desired in any way to exaggerate the so-called difficulties of helicopter flying (though there is nothing about them which cannot be learned), it would be unwise to minimize the intricacies involved. There is compensation, however, in the fascination peculiar to this form of flight. Any mechanical device the function of which depends on the close inter-relation of its component parts, pre sents a source of interest; in this the helicopter excels. Before passing on from hovering to conditions applicable in forward flight, it is as well to refer back to the control-column movements. When a stick movement is made in any direction from the central position in hovering flight to correct a movement of the machine in the opposite direction (caused possibly by out side air disturbances), the fine of thrust of the rotor is tilted away from its original position. This will correct the undesired move ment; but if the stick is held in the new position it will induce a new movement of the machine in the direction of the corrective tilt. Immediately after corrective movements, therefore, the con trol column should always be returned to the central hovering position. There is no similarity to this technique in any fixed- wing control movement, except perhaps the necessity to take off bank in a turn, though in that case the cause is different. Even this is not a good parallel, as, bearing in mind the time-lag in response to helicopter-stick movements, the return movement of the stick may actually be made before the effect of the corrective tilt of the rotor is felt in the fuselage. Helicopter take-offs and landings are closely akin to hovering and, in training, are practised as part of that exercise rather than as part of circuits and landings. For take-off, the control column is held central, in the hovering position, and collective pitch and power gradually increased with the other hand. As the machine becomes light on its wheels it is possible to see which way it is going to unstick, and a small adjustment in the control-column position may, if necessary, be made to ensure that it rises vertically. A further slight increase in pitch and power from this stage will lift the machine gently into hovering flight. On landing the proce dure is reversed. A slight decrease in collective pitch from the hovering position will allow the helicopter to settle gently on to the ground, after which pitch is reduced to the bottom stop and the engine allowed to slow back to idling speed. The control column remains central on landing and must not be pulled back— a common early fault with fixed-wing pilots—as this will make the machine try to sit on its tail rotor. WIND DIRECTION
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