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 (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:
D = – rVC • A • Cd
and if this is equated to rotor thrust we have:
from which we find:
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.