(17 m.p.h. or 25 f.p.s.)
TIP
Rotor Speed: 300 r.p.m.
F
Resultant
Relative WindRotational Airflow
(21 m.p.h. or 31 f.p.s.)
Upward Airflow
(17 m.p.h. or 25 f.p.s.)
HUB
VERTICAL AUTOROTATION
Figure 16-3. Moving outboard on the rotor blade, the rotational velocity increasingly exceeds the upward component of airflow,
resulting in a higher relative wind at a lower angle of attack.
Driven Region
Driving Region
Stall
Region
Driven Region
(Propeller)
Driving Region
(Autorotative)
Stall Region
F
VERTICAL AUTOROTATION
Rotational
Relative Wind
Lift
Lift
TAF
TAF
Total
Aerodynamic
Force Aft
of Axis of
Rotation
Drag
Inflow Up
Chord Line
Through Rotor Resultant
Relative Wind
Total
Aerodynamic
Force
Forward
of Axis of
Rotation
Drag
Inflow
Axis of
Rotation
Axis of
Rotation
Axis of
Rotation
(Blade is Stalled)
TAF
Drag
Inflow
Lift
Figure 16-4. The total aerodynamic force is aft of the axis of
rotation in the driven region and forward of the axis of rotation
in the driving region. Drag is the major aerodynamic
force in the stall region. For a complete depiction of force
vectors during a vertical autorotation, refer to Chapter 3—
Aerodynamics of Flight (Helicopter), Figure 3-22.
16-3
relative wind striking the advancing blade, and subtracted
from the relative wind striking the retreating
blade. To prevent uneven lifting forces on the two sides
of the rotor disc, the advancing blade teeters up,
decreasing angle of attack and lift, while the retreating
blade teeters down, increasing angle of attack and lift.
(For a complete discussion on dissymmetry of lift, refer
to Chapter 3—Aerodynamics of Flight.) The lower
angles of attack on the advancing blade cause more of
the blade to fall in the driven region, while higher
angles of attack on the retreating blade cause more of
the blade to be stalled. The result is a shift in the rotor
regions toward the retreating side of the disc to a degree
directly related to the forward speed of the aircraft.
[Figure 16-5]
REVERSE FLOW
On a rotor system in forward flight, reverse flow occurs
near the rotor hub on the retreating side of the rotor
disc. This is the result of the forward speed of the aircraft
exceeding the rotational speed of the rotor blades.
For example, two feet outboard from the rotor hub, the
blades travel in a circle with a circumference of 12.6
feet. At a rotor speed of 300 r.p.m., the blade speed at
the two-foot station is 42 m.p.h. If the aircraft is being
operated at a forward speed of 42 m.p.h., the forward
speed of the aircraft essentially negates the rotational
velocity on the retreating blade at the two-foot station.
Moving inboard from the two-foot station on the
retreating blade, the forward speed of the aircraft
increasingly exceeds the rotational velocity of the
blade. This causes the airflow to actually strike the
trailing edge of the rotor blade, with velocity increasing
toward the rotor hub. [Figure 16-6] The size of the
area that experiences reverse flow is dependent primarily
on the forward speed of the aircraft, with higher
speed creating a larger region of reverse flow. To some
degree, the operating speed of the rotor system also has
an effect on the size of the region, with systems operating
at lower r.p.m. being more susceptible to reverse
flow and allowing a greater portion of the blade to
experience the effect.
RETREATING BLADE STALL
The retreating blade stall in a gyroplane differs from
that of a helicopter in that it occurs outboard from the
rotor hub at the 20 to 40 percent position rather than at
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