Rotor control
Control of the helicopter in flight involves changing the magnitude of rotor thrust or its line of action or both. Almost the whole of the control task falls to the lot of the main rotor and it is on this that we concentrate. A change in line of action of the thrust would in principle be obtained by tilting the rotor shaft, or at least the hub, relative to the fuselage. Since the rotor is engine-driven (unlike that of an autogyro) tilting the shaft is impracticable. Tilting the hub is possible with some designs but the large mechanical forces required restrict this method to very small helicopters. Use of the feathering mechanism, however, by which the pitch angle of the blades is varied, either collectively or cyclically, effectively transfers to the aerodynamic forces the work involved in changing the magnitude and direction of the rotor thrust. Blade feathering, or pitch change, could be achieved in various ways. Thus Saunders (1975)1 lists the use of aerodynamic servo tabs, auxiliary rotors, fluidically controlled jet flaps, or pitch links from a control gyro as possible methods. The widely adopted method, however,is through a swashplate system,illustrated in Fig.4.14 which shows the operation with collective pitch while Fig. 4.15 shows the operation with cyclic pitch. Carried on the rotor shaft, this embodies two parallel plates, the lower of which does not rotate with the shaft but can be tilted in any direction by operation of the pilot’s cyclic control column and raised or lowered by means of his collective lever. The upper plate is connected by control rods to the feathering hinge mechanisms of the blades and rotates with the shaft, while being constrained to remain parallel to the lower plate. Raising the collective lever thus increases the pitch angle of the blades by the same amount all round
Control of the helicopter in flight involves changing the magnitude of rotor thrust or its line of action or both. Almost the whole of the control task falls to the lot of the main rotor and it is on this that we concentrate. A change in line of action of the thrust would in principle be obtained by tilting the rotor shaft, or at least the hub, relative to the fuselage. Since the rotor is engine-driven (unlike that of an autogyro) tilting the shaft is impracticable. Tilting the hub is possible with some designs but the large mechanical forces required restrict this method to very small helicopters. Use of the feathering mechanism, however, by which the pitch angle of the blades is varied, either collectively or cyclically, effectively transfers to the aerodynamic forces the work involved in changing the magnitude and direction of the rotor thrust. Blade feathering, or pitch change, could be achieved in various ways. Thus Saunders (1975)1 lists the use of aerodynamic servo tabs, auxiliary rotors, fluidically controlled jet flaps, or pitch links from a control gyro as possible methods. The widely adopted method, however,is through a swashplate system,illustrated in Fig.4.14 which shows the operation with collective pitch while Fig. 4.15 shows the operation with cyclic pitch. Carried on the rotor shaft, this embodies two parallel plates, the lower of which does not rotate with the shaft but can be tilted in any direction by operation of the pilot’s cyclic control column and raised or lowered by means of his collective lever. The upper plate is connected by control rods to the feathering hinge mechanisms of the blades and rotates with the shaft, while being constrained to remain parallel to the lower plate. Raising the collective lever thus increases the pitch angle of the blades by the same amount all round
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