Rotorcraft Flying Handbook, FAA-H-8083-21

percent of the radius. In the driven region, part A of figure
3-22, the total aerodynamic force acts behind the
axis of rotation, resulting in a overall drag force. The
driven region produces some lift, but that lift is offset
by drag. The overall result is a deceleration in the rotation
of the blade. The size of this region varies with the
blade pitch, rate of descent, and rotor r.p.m. When
changing autorotative r.p.m., blade pitch, or rate of
descent, the size of the driven region in relation to the
other regions also changes.
There are two points of equilibrium on the blade—one
between the driven region and the driving region, and
one between the driving region and the stall region. At
points of equilibrium, total aerodynamic force is
aligned with the axis of rotation. Lift and drag are produced,
but the total effect produces neither acceleration
nor deceleration.
The driving region, or autorotative region, normally
lies between 25 to 70 percent of the blade radius. Part
C of figure 3-22 shows the driving region of the blade,
which produces the forces needed to turn the blades
during autorotation. Total aerodynamic force in the
driving region is inclined slightly forward of the axis of
rotation, producing a continual acceleration force. This
inclination supplies thrust, which tends to accelerate
the rotation of the blade. Driving region size varies
with blade pitch setting, rate of descent, and rotor r.p.m.
By controlling the size of this region you can adjust
autorotative r.p.m. For example, if the collective pitch
is raised, the pitch angle increases in all regions. This
causes the point of equilibrium to move inboard along
the blade’s span, thus increasing the size of the driven
region. The stall region also becomes larger while the
driving region becomes smaller. Reducing the size of
the driving region causes the acceleration force of the
driving region and r.p.m. to decrease.
The inner 25 percent of the rotor blade is referred to as
the stall region and operates above its maximum angle
of attack (stall angle) causing drag which tends to slow
rotation of the blade. Part E of figure 3-22 depicts the
stall region.
A constant rotor r.p.m. is achieved by adjusting the collective
pitch so blade acceleration forces from the driving
region are balanced with the deceleration forces
from the driven and stall regions.
AUTOROTATION (FORWARD FLIGHT)
Autorotative force in forward flight is produced in
exactly the same manner as when the helicopter is
descending vertically in still air. However, because forward
speed changes the inflow of air up through the
rotor disc, all three regions move outboard along the
blade span on the retreating side of the disc where angle
of attack is larger, as shown in figure 3-23. With lower
angles of attack on the advancing side blade, more of
that blade falls in the driven region. On the retreating
side, more of the blade is in the stall region. A small
section near the root experiences a reversed flow, therefore
the size of the driven region on the retreating side
is reduced.
Retreating
Side
Driven
Region
Forward
Driving
Region
Advancing
Side
Stall
Region
Figure 3-23. Blade regions in forward autorotation descent.
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