Autorotation is the process by which a helicopter can descend and land without engine power by using the airflow coming up from the ground or air during descent to turn the rotor blades and maintain control. During autorotation, the helicopter trades altitude for the energy required to spin the rotor blades at a speed that provides enough lift to control the aircraft. The pilot lowers the collective to reduce blade drag and tilt the total aerodynamic force vector forward to maintain rotor RPM as the helicopter descends.

1. Autorotation
The ability to maintain and control rotor RPM in the
event of an engine malfunction so controlled flight
may be continued to the ground.
Airflow during helicopter descent provides the
necessary energy to overcome blade drag and to turn
the rotor.
The aviator gives up altitude at a controlled rate in
return for the needed energy to turn the rotor at an
RPM that provides aircraft control. Stated another
way, the helicopter has potential energy by virtue of
its altitude.

2. UH OH!
POOF!
In powered flight, rotor drag is overcome with engine
power. When the engine fails, or is otherwise
disengaged from the rotor system, some other force
must sustain rotor RPM so controlled flight can be
continued to the ground.

3. If a loss of power should occur
with the helicopter in this
condition, RPM decay is rapid.
To prevent RPM decay, the
collective must be lowered
immediately to reduce the drag and
incline the TAF vector forward
toward the axis of rotation

4. Entry and Descent
Specific entry technique may vary
and will be determined by such
factors as airspeed, gross weight,
density altitude and altitude above
the landing surface.

5. Entry and Descent cont..
From cruise altitudes and airspeeds, the
collective must be reduced and the cyclic
adjusted to achieve an airspeed that
maintains RRPM while affording a
reasonable glide distance and rate of
descent.

6. Entry and Descent cont..
Once a steady state autorotation has been
achieved, any movement of the cyclic will
affect Rotor RPM.
Aft cyclic will initially increase R-RPM and
forward cyclic will reduce RRPM. R-RPM will
stabilize at some other value once cyclic inputs
are stopped.

7. Maximum Glide Distance
•Best Glide Distance is determined through flight
tests
•The specific speed at which a power-off glide will
cover the maximum distance
•Typically 4 to 1 (4 feet forward for every 1 foot of
descent) Or One NM per 1,500’AGL
•Rotor RPM Approximately 90%
•Airspeed Approximately 75 KIAS

8. Minimum rate of descent
•For each aircraft, there is an airspeed that
will result in the minimum rate of descent.
•The values for minimum rate of descent are
determined through flight tests.
•For the R-22 - 53KIAS
•Values are very close to the airspeed for
minimum drag.

9. Driven Region Driving Region
30% of radius 45% of radius
Blade regions in a
vertical autorotation
Stall Region
25% of radius

10. Stall Region
•That area inboard of the 25% radius
•Operates above the critical angle of attack
•Contributes little vertical lift but some
rotational drag

11. Stall Region
TAF
L
D

12. Driving Region
•That blade region between approximately 25% and
70% radius
•Operates at comparatively high angles of attack
•Resultant aerodynamic force is inclined slightly
forward of axis of rotation in the direction of rotation
•Inclination of the total aerodynamic force provides
horizontal thrust in the direction of rotation and tends
to increase RRPM

13. Driving Region
TAF
L
D

14. Driven Region
•The blade region outboard of the 70% radius
•Operates at slightly less angle of attack than Driving
region
•Because of higher relative wind speed, provides
most of the vertical lift opposing weight
•Inclination provides horizontal drag, opposite the
direction of rotation, which tends to decrease RRPM

15. Driven Region
L TAF
D

16. Forward
Driven
Driving Autorotative regions
Stall in forward flight.
Regions incline
towards the retreating
side

17. Driven
A Region
A
B Point of
Equilibrium
B, D Driving
C
Region
C D Point of
Equilibrium
E Stall
E Region

18. The rotor disk TAF is tilted
well forward providing the
necessary thrust to propel
the helicopter at the desired
airspeed
However, the individual blade
segment TAF is inclined well
aft of the axis of rotation.
The engine is needed to
overcome the drag forces
generated by this situation.

19. Requirements
The rotor system must be decoupled from
the engine(s)
This occurs if an engine malfunctions, or if
the pilot retards the throttle, as in a simulated
engine failure.
The collective must be lowered so the angle
of attack will not become so excessive that
RPM will be lost.

20. Questions?