Monday, June 29, 2015
FREE AVIATION STUDY: FREE AVIATION STUDY: Blade mean lift coefficient
FREE AVIATION STUDY: FREE AVIATION STUDY: Blade mean lift coefficient: FREE AVIATION STUDY: Blade mean lift coefficient : Blade mean lift coefficient Characteristics of a rotor obviously depend on the lift coeffi...
FREE AVIATION STUDY: Blade mean lift coefficient
FREE AVIATION STUDY: Blade mean lift coefficient: Blade mean lift coefficient Characteristics of a rotor obviously depend on the lift coefficient at which the blades are operating and it is...
FREE AVIATION STUDY: Tip loss
FREE AVIATION STUDY: Tip loss: Tip loss A characteristic of the actuator disc concept is that the linear theory of lift is maintained right out to the edge of the disc....
FREE AVIATION STUDY: Example of hover characteristics
FREE AVIATION STUDY: Example of hover characteristics: Example of hover characteristics Corresponding to CL/a and CD/CL characteristics for fixed wings, we have CT/qand Cp/CT for the helicopter...
FREE AVIATION STUDY: Rotor Mechanisms for Forward Flight
FREE AVIATION STUDY: Rotor Mechanisms for Forward Flight: Rotor Mechanisms for Forward Flight The edgewise rotor In level forward flight the rotor is edgewise on to the airstream,a basically u...
FREE AVIATION STUDY: Flapping motion
FREE AVIATION STUDY: Flapping motion: Flapping motion To examine the flapping motion more fully we assume, unless otherwise stated, that the flapping hinge is on the axis of rot...
FREE AVIATION STUDY: Rotor control
FREE AVIATION STUDY: Rotor control: Rotor control Control of the helicopter in flight involves changing the magnitude of rotor thrust or its line of action or both. Almost th...
FREE AVIATION STUDY: Equivalence of flapping and feathering
FREE AVIATION STUDY: Equivalence of flapping and feathering: Equivalence of flapping and feathering The performance of the rotor blade depends upon its angle of incidence to the tip-path plane. A gi...
Equivalence of flapping and feathering
Equivalence of flapping and feathering
The performance of the rotor blade depends upon its angle of incidence to the tip-path plane. A given blade incidence can be obtained with different combinations of flapping and feathering. Consider the two situations illustrated in Fig. 4.19: these are views from the left side with the helicopter in forward flight in the direction shown. In situation 1 the shaft axis coincides with the TPA; there is therefore no flapping but the necessary blade incidences are obtained from feathering according to Equation 4.8. Blade attitudes at the four quarter points of a rotation are as indicated in the diagram. In situation 2 the shaft axis coincides with the NFA.By definition this means that feathering is zero: the blade angles however are obtained from flapping according to Equation (4.4). It is seen that if the feathering and flapping coefficients B1 and a1 are equal, the blade attitudes to the tip-path plane are identical around the azimuth in the two situations. The blade perceives a change in nose-down feathering, via the swash-plate, as being equivalent to the same angle change in nose-up flapping.
The performance of the rotor blade depends upon its angle of incidence to the tip-path plane. A given blade incidence can be obtained with different combinations of flapping and feathering. Consider the two situations illustrated in Fig. 4.19: these are views from the left side with the helicopter in forward flight in the direction shown. In situation 1 the shaft axis coincides with the TPA; there is therefore no flapping but the necessary blade incidences are obtained from feathering according to Equation 4.8. Blade attitudes at the four quarter points of a rotation are as indicated in the diagram. In situation 2 the shaft axis coincides with the NFA.By definition this means that feathering is zero: the blade angles however are obtained from flapping according to Equation (4.4). It is seen that if the feathering and flapping coefficients B1 and a1 are equal, the blade attitudes to the tip-path plane are identical around the azimuth in the two situations. The blade perceives a change in nose-down feathering, via the swash-plate, as being equivalent to the same angle change in nose-up flapping.
Rotor control
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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