Blade Aerodynamic Forces

In order to understand the essence of main rotor operation in the auto­rotative regime, it is necessary to examine the aerodynamic forces which arise on the blade element. In the autorotative regime, each blade element has two velocities: the circumferential velocity u = шг and the vertical descent

velocity (Figure 74a). The sum of these velocities yields the resultant

velocity W = u + The air stream approaching the blade from the side has

a direction opposite the blade element resultant velocity vector.

As a result of this flow pattern, the air pressure on the bottom of the blade will be higher than that above the blade. The total aerodynamic force AR is created on the blade element. This force may be directed forward at the angle у relative to the main rotor hub rotation axis (Figure 74b); parallel to the hub axis (Figure 74c); or back at the angle у relative to the hub axis (Figure 74d). In the first case, the projection ( – AQ) of the force AR on the hub rotation plane will be directed along the rotation of the main rotor and forms a turning moment under the action of which the main rotor rpm will increase.

In the second case, the projection of the force AR on the hub rotation plane will be zero (AQ = 0). Therefore, the force AR will have no retarding or accelerating effect on the rotation of the main rotor.

In the third case, the projection AQ of the force AR on the hub rotation plane will be directed aft, opposite the rotor rotation, and creates a retard­ing moment under the influence of which the main rotor rpm decreases. This

means that the nature of the main rotor rotation is determined hy the direction of the elemental forces which develop on the blade.