flow and negative stall regions similar to the 
powered flight condition. 
Autorotation is essentially a stable flight 
condition. If external disturbances cause the 
rotor to slow down, the autorotation region of 
the disc automatically expands to restore the 
rotor speed to the original equilibrium condi- 
tion. On the other hand, if an external 
disturbance causes the rotor to speed up, the 
propeller region automatically expands and 
tends to accelerate the rotor to the original 
equilibrium condition. Actually the stable 
autorotation condition will exist only when 
the autorotational speed is within certain 
limits. If the rotor speed is allowed to slow 
some excessive amount, then the rotor becomes 
unstable and the RPM will decrease even 
further unless the pilot immediately corrects 
the condition by proper control action. 
In case of engine failure, the fixed-wing 
airplane will be glided at maximum lift-drag 
ratio to produce maximum glide distance. If 
minimum rate of descent is desired in power-off 
flight rather than maximum glide distance, the 
fixed-wing airplane will be flown at some 
lower airspeed. Actually, the minimum rate 
of descent will occur at minimum power 
required. The helicopter exhibits similar char- 
acteristics but ordinarily the best autorota~tion 
speed may be considered that speed that results 
in the minimum rate of descent rather than 
maximum glide distance. The aerodynamic 
condition of the rotor which produces mini- 
mum rate of descent is: 
Maximum ratio of 
(Mean blade lift coetIicient)3” 
Mean blade drag coefficient 
It is this ratio which determines the auto- 
rotation rate of descent. Figure 6.18 illus- 
trates the variation of autorotation rate of 
descent with equivalent airspeed for a typical 
helicopter. Point A on this curve defines the 
point which produces autorotation with mini- 
mum rate of descent. Maximum glide distance 
NAVWEPS OO-BOT-BO 
APPLICATION OF AERODYNAMICS 
TO SPECIFIC PROBLEMS OF FLYl,NG 
during autorotation descent would be obtained 
at the flight condition which produces the 
greatest proportion between airspeed and rate 
of descent. Thus, a straight line from the 
origin tangent to the curve will define the 
point for maximum autorotative glide dis- 
tance. This corresponds to Point B of figure 
6.18. If the helicopter is being glided at the 
speed for maximum glide distance, a decrease 
in airspeed would reduce the rate of descent 
but the glide distance would decrease. If the 
helicopter is being glided at the speed for 
minimum rate of descent, the rate of descent 
(steady state) can not be reduced but the glide 
distance can be increased by increasing the 
glide speed to that for maximum distance. 
Weight and wind affect the glide character- 
istics of a helicopter the same way an airplane 
is affected. Ideally, the helicopter autorotates 
at a higher equivalent airspeed at higher gross 
weight or when autorotating into a headwind. 
In addition to aerodynamic forces which act 
on the rotor during autorotation, inertia forces 
are also important. These effects are usually 
associated with the pilot’s response time be- 
cause the rate a pilot reacts to a power failure 
is quite critical. The time necessary to reduce 
collective pitch and enter autorotation be- 
comes critical if the rotor inertia characteristics 
are such as to allow the rotor to slow down to 
a dangerous level before the pilot can react. 
With power on, the blade pitch is relatively 
high and the engine supplies enough torque to 
overcome the drag of the blades. At the 
instant of power failure the blades are at a high 
pitch with high drag. If there is no engine 
torque to maintain the RPM, the rotor will 
decelerate depending on the rotor torque and 
rotor inertia. If the rotor has high rotational 
energy the rotor will lose RPM less rapidly, 
giving the pilot more time to reduce collective 
pitch and enter autorotation. If the rotor has