Figure of Merit

The ideal power derived from momentum considerations is the power required by a rotor consisting only of an idealized actuator disc without regard for the drag of the actual blades. In this sense, the ideal power is analogous to the induced drag of an airplane wing and, as a matter of fact, is generally called induced power. The actual power, of course, is higher than the induced, or ideal, power. The ratio of induced power to actual power is known as the Figure of Merit (F. M.):

Induced power

F-M – = -7——— r-^——–

Actual power

It will later be shown that the value of the Figure of Merit is a function of the rotor geometry and of the rotor operating conditions. Whirl tower tests of conventional helicopter rotors have shown that the maximum Figure of Merit that can be expected in practice is 0.75 to 0.80.