large increase in drag and rearward shift of the 
airfoil aerodynamic center, compressibility ef- 
fects on the helicopter increase the power te- 
quired to maintain rotor RPM and cause rotor 
roughness, vibration, stick shake, and an un- 
desirable structural twisting of the blade. 
Since compressibility effects become more 
severe at higher lift coefficients (higher blade 
angles of attack) and higher Mach numbers, 
the following operating conditions represent 
the most adverse conditions from the stand- 
point of compressibility: 
1. High airspeed 
2. High rotor RPM 
3. High gross weight 
4. High density altitude 
5. Low temperature-the speed of sound 
is proportional to the square root of 
the absolute temperature. Therefore, 
sonic velocity will be more easily 
obtained at low temperatures when 
the sonic speed is lower. 
6. Turbulent air-sharp gusts momen- 
tarily increase the blade angle of 
attack and thus lower the critical 
Mach number to the point where 
compressibility effects may be en- 
countered on the blade. 
Compressibility effects will vanish by de- 
creasing the blade pitch. The similarities in 
the critical conditions for retreating blade 
stall and compressibility should be noticed 
but one basic difference must be appreciated- 
compressibility occurs at HIGH RPM while 
retreating blade stall occurs at LOW RPM. 
Recovery technique is identical for both with 
the exception of RPM control. 
AUTOROTATION CHARACTERISTICS. 
One of the unique characteristics of helicopters 
is their ability to take part of the energy of 
the airstream to keep the rotor turning and 
glide down to a landing with no power. 
Consideration of the rotor during a vertical 
autorotation will provide an understanding of 
why the rotor continues to rotate without 
power. During autorotation, the flow of air 
NAVWEPS CID-ROT-R0 
APP,LlCATlON OF AERODYNAMICS 
TO SPECIFIC PROBLEMS OF FLYmING 
is upward through the rotor disc and there is 
a vertical velocity component equal to the 
rate of descent of the helicopter. In addition, 
there is a velocity component due to rotation 
of the rotor. The vector sum of these two 
velocities is the relative wind for the blade 
element. The forces resulting from the relative 
wind on each particular blade section will 
provide the reason why the rotor will continue 
to operate without power. First, consider, a 
blade element near the tip of the blade as illus- 
trated in figure 6.17. At this point there is 
a lift force acting perpendicular to the relative 
wind and a drag force acting parallel to the 
relative wind through the aerodynamic center. 
Since the rotation of the rotor is affected only 
by forces acting in the plane of rotation, the 
important forces are components of the lift 
and drag force in the plane of rotation. In 
this low angle of attack high speed tip section, 
the net in-plane force is a drag force which 
would tend to retard the rotor. Next, con- 
sider a blade section at about the half-span 
position as illustrated in figure 6.17. In this 
case, the same forces are present, but the iti- 
plane component of lift force is greater than 
the drag force and this results in a net thrust 
or forward force in the plane of rotation which 
tends to drive the rotor. 
During a steady autorotation, there is a 
balance of torque from the forces along the 
blade so that the RPM is maintained in equi- 
librium at some particular value. The region 
of the rotor disc where there is a net drag force 
on the blade is called the “propeller region” 
and the region of the rotor disc where there 
is a net in-plane thrust force is called the 
“autorotation region.” These regions are 
shown for vertical autorotation and forward 
speed (or normal) autorotation in figure 6.17. 
Forces acting on the rotor blades in forward 
flight autorotation are similar to those in 
vertical autorotation but the difference will 
consist mainly of shifts of the autorotation 
region to the left and the addition of reverse