Rotorcraft Flying Handbook, FAA-H-8083-21

VERTICAL AUTOROTATION
HUB
Resultant
Relative Wind
Upward Airflow
(17 m.p.h. or 25 f.p.s.)
Rotational Airflow
(21 m.p.h. or 31 f.p.s.)
Rotor Speed: 300 r.p.m.
Upward Airflow
(17 m.p.h. or 25 f.p.s.)
TIP
Resultant Relative Wind
Rotational Airflow (267 m.p.h. or 393 f.p.s.)
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.
speed of 393 feet per second, or 267 m.p.h. The result
is a higher total relative wind, striking the blades at a
lower angle of attack. [Figure 16-3]
F
ROTOR DISC REGIONS
As with any airfoil, the lift that is created by rotor
blades is perpendicular to the relative wind. Because
the relative wind on rotor blades in autorotation shifts
from a high angle of attack inboard to a lower angle of
attack outboard, the lift generated has a higher forward
component closer to the hub and a higher vertical component
toward the blade tips. This creates distinct
regions of the rotor disc that create the forces necessary
for flight in autorotation. [Figure 16-4] The
autorotative region, or driving region, creates a total
aerodynamic force with a forward component that
exceeds all rearward drag forces and keeps the blades
spinning. The propeller region, or driven region, generates
a total aerodynamic force with a higher vertical
component that allows the gyroplane to remain aloft.
Near the center of the rotor disc is a stall region where
the rotational component of the relative wind is so low
that the resulting angle of attack is beyond the stall
limit of the airfoil. The stall region creates drag against
the direction of rotation that must be overcome by the
forward acting forces generated by the driving region.
AUTOROTATION IN FORWARD FLIGHT
As discussed thus far, the aerodynamics of autorotation
apply to a gyroplane in a vertical descent. Because
gyroplanes are normally operated in forward flight, the
component of relative wind striking the rotor blades as
a result of forward speed must also be considered. This
component has no effect on the aerodynamic principles
that cause the blades to autorotate, but causes a shift in
the zones of the rotor disc.
Driven Region
(Propeller)
Lift
Rotational
Relative Wind
Inflow Up
Through Rotor
TAF
Axis of
Rotation
VERTICAL AUTOROTATION
Total
Aerodynamic
Force Aft
of Axis of
Rotation
Drag
Chord Line
Resultant
Relative Wind
Driven Region
Driving Region
Stall
Region
Driving Region
(Autorotative)
Total
Aerodynamic
Force
TAF Forward
Lift of Axis of
Rotation
Drag
Inflow
Axis of
Rotation
Stall Region
(Blade is Stalled)
Inflow
TAF
Drag
Axis of
Rotation
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.
As a gyroplane moves forward through the air, the forward
speed of the aircraft is effectively added to the
Lift
16-2