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speed) in still air. Under these conditions, the forces
that cause the blades to turn are similar for all blades
regardless of their position in the plane of rotation.
Therefore, dissymmetry of lift resulting from helicopter
airspeed is not a factor.
During vertical autorotation, the rotor disc is divided
into three regions as illustrated in figure 3-21—the
Figure 3-20. During an autorotation, the upward flow of relative wind permits the main rotor blades to rotate at their normal
speed. In effect, the blades are “gliding” in their rotational plane.
Figure 3-21. Blade regions in vertical autorotation descent.
Figure 3-19. The horizontal component of lift accelerates the
helicopter toward the center of the turn.
Centripetal Force

(Horizontal Component of Lift)
Vertical

Component

of Lift
Bank

Angle
Resultant

Lift
Weight
Centrifugal

Force (Inertia)
Normal Powered Flight Autorotation
Direction
of Flight
Direction
of Flight
Stall

Region 25%
Driven

Region 30% Driving

Region 45%
3-10
driven region, the driving region, and the stall region.
Figure 3-22 shows four blade sections that illustrate
force vectors. Part A is the driven region, B and D are
points of equilibrium, part C is the driving region, and
part E is the stall region. Force vectors are different in
each region because rotational relative wind is slower
near the blade root and increases continually toward
the blade tip. Also, blade twist gives a more positive
angle of attack in the driving region than in the driven
region. The combination of the inflow up through the
rotor with rotational relative wind produces different
combinations of aerodynamic force at every point
along the blade.
The driven region, also called the propeller region, is
nearest the blade tips. Normally, it consists of about 30
Figure 3-22. Force vectors in vertical autorotation descent.
B & D
Rotational

Relative Wind
Lift
TAF
TAF
Total

Aerodynamic

Force Aft

of Axis of

Rotation
Total

Aerodynamic

Force Forward

of Axis of

Rotation
Angle of

Attack 2° Drag
Chord Line
Inflow Up

Through Rotor
Resultant

Relative Wind
Equilibrium
Drag
Inflow TAF
Angle of

Attack 6°
Drag Driving

Region
Inflow
Axis of
 

Rotation
Angle of

Attack 24°
(Blade is Stalled)
TAF
Drag
Stall

Region
Inflow
Driven

Range
C
Driven

Region
Drag
Point of

Equilibrium
Point of

Equilibrium
Driving

Region
Stall

Region
Drag
Autorotative Force
A
C
E
E
D
B
A
Lift
Lift
Lift
3-11
percent of the radius. In the driven region, part Aof 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
　
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