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Rotorcraft Flying Menu >Aerodynamics
of Flight >Autorotation
> Autorotation (Vertical Flight)
Most autorotations are performed with forward
speed. For simplicity, the following aerodynamic explanation
is based on a vertical autorotative descent (no forward 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 helicop-ter airspeed is not a factor.
During vertical autorotation, the rotor disc
is divided into three regions as illustrated in figure 3-21—the
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.
Figure 3-21. Blade regions in vertical
autorotation descent.
Figure 3-22. Force vectors in vertical
autorotation descent.
The driven region, also called the propeller
region, is nearest the blade tips. Normally, it consists of
about 30 percent of the radius. In the driven region, part Aof
fig-ure 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 rota-tion 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 pro-duced, 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. Aconstant rotor r.p.m. is achieved by adjusting the
col-lective pitch so blade acceleration forces from the driv-ing
region are balanced with the deceleration forces from the driven
and stall regions.
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