Equivalence of Flapping and Feathering
The performance of the rotor blade depends upon its angle of incidence to the TPP. A given blade incidence can be obtained with different combinations of flapping and feathering. Consider the two situations illustrated in Figure 4.20.
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 4.9. 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 4.5. It is seen that if the feathering and flapping coefficients B1 and a1 are equal, the blade attitudes to the TPP are identical around the azimuth in the two situations. The blade perceives a change in nose-down feathering, via the swashplate, as being equivalent to the same angle change in nose-up flapping.
A pilot uses this equivalence in flying the helicopter, for example to trim the vehicle for different positions of the centre of gravity (CG). The rotor thrust, in direction and magnitude, depends upon the inclination of the TPP in space and the incidence of the blades relative to it. The same blade incidence can be achieved, as we have seen, either with nose-up flapping or with the same degree of nose-down feathering, or of course with a combination of the two. By adjusting the relationship, using the cyclic control stick, the pilot is able to
Direction of Flight
Figure 4.20 Equivalence of flapping and feathering. (Blade chordwise attitudes are shown in the plane of the diagram for azimuth angles of 90° and 270° and normal to the diagram for 0° and 180°)
compensate for different nose-up or nose-down moments in the helicopter, arising from different CG positions. The angle of the shaft axis to the vertical, hence the attitude of the helicopter in space, varies with the CG position but the TPP remains at a constant inclination to the direction of flight.