Ideal Autorotation

The point of intersection of the induced-velocity curve with the line:

Vc + Vi = 0 (2.60)

is of particular interest because it defines what is termed the state of ideal autorotation[4] (IA in Figure 2.22), in which, since there is no mean flow through the rotor, the induced power is zero.

In round terms, values of Vc/V0 for ideal and real autorotation are about —1.7 and —1.8, respectively.

Pursuing the analogy of flow past a solid plate (turbulent wake state), the plate drag may be written:

1 2

D = – rVC • A • Cd

and if this is equated to rotor thrust we have:

from which we find:

(2.63)

Examining Figure 2.24, we see that with Vc/V0 = —1.7, Cd has the value 1.38 which is close to that for a solid plate. A slightly better analogy is obtained by taking the real value Vc/V0 =—1.8, which yields a Cd value of 1.23, close to the effective drag coefficient of a parachute.

Thus in autorotative vertical descent the rotor behaves like a parachute.