The Operating Regime Coefficient of the Main Rotor

A special quantity is introduced to characterize the operating regimes

(*)

of the main rotor — the operating regime coefficient

The operating regime coefficient of a main rotor, y, is defined as the ratio of the projection of the flight velocity vector on the plane of rotation of the hub to the circular velocity of the blade tip. The projection of the flight velocity vector, or the undisturbed flow, on the plane of rotation of the hub is equal to the product of V cos A (Figure 13).

This is the tip speed ratio.

Then we have

In the axial flow regime, when V = 0, or cos A = 0(A = 90°), у = 0.

Consequently, the equation у = 0 indicates the axial flow regime. If у > 0, this is the index of the trans­verse flow regime. The larger the coefficient y, the larger the effect of transverse flow. The coefficient у for contemporary helicopters varies from 0 to 0.4. In most cases the angle of attack of the main rotor does not exceed 10°. Since cos 10° as 1, then it is possible to define у by the approximate formula

V