right when attempting to tilt the disc forward. He/she
autorotation, the rotor blades turn in the same direction
would move the cyclic stick forward when attempting
as when engine driven. The air passes up through the
to tilt the disc area to the left, and so on.
rotor system instead of down. This action causes a
slightly greater upward flex or coning of the blades.
GROUND EFFECT
POWER SETTLING
Ground effect can be achieved when a helicopter is
in a hover or forward flight while in close proximity to
Stalling, as applied to fixed-wing aircraft, will not
the ground or some other hard flat surface. When a
occur in helicopters. However, power settling may
helicopter is in a hover or moving slowly, the main
occur in low-speed flight. Power settling is the
rotor is developing thrust that is being vectored, or
uncontrollable loss of altitude. This condition may
directed down toward the surface. The surface resists
occur due to combinations of heavy gross weights,
this airflow (thrust) by building up air pressure
poor density conditions, and low forward speed.
between the rotor and the surface, thus providing
During low forward speed and high rates of descents,
ground cushion. When the helicopter is in forward
the downwash from the rotor begins to recirculate. The
flight, the cushion is not as great as the thrust that is
downwash moves up, around, and back down though
being vectored down and aft of the helicopter. This
the effective outer disc area. The velocity of this
ground cushion will provide additional lift without
recirculating air mass may become so high that full
additional power, and will be apparent when the
collective pitch cannot retard or control the rate of
helicopter is hovering or flying at an altitude of
descent.
approximately one-half the main rotor diameter or
Q17-1. What produces the lift required for helicopter
below. The closer the helicopter is to the ground, the
flight?
greater the cushion effect. This will be indicated by the
reduced power required to maintain flight or hover.
Q17-2. When the camber is the same on both surfaces
The maximum cushion effect is achieved at zero
of an airfoil and results in a fixed center of
airspeed.
pressure, what term describes the helicopter
airfoil?
TRANSLATIONAL LIFT
Q17-3. What determines the amount of lift generated
As a helicopter begins the transition from a hover
by a helicopter's rotor?
to forward flight, at approximately 10-15 knots, it will
Q17-4. What is the result of highly polished rotor
experience a loss of lift and settle slightly and seem to
blades?
loose power, without an actual reduction in power.
This is due to the loss of the ground cushion caused by
Q17-5. Where is density altitude at its greatest?
the changing direction or vector of the rotor's thrust.
Q17-6. On a single main rotor helicopter, the tail
As the helicopter continues to accelerate, the rotor will
rotor is used to counteract what force?
be introduced to larger masses of air. The rotor will
become more efficient and the thrust vector of the rotor
Q17-7. Other than horizontal flight, how is rotor
will become more stable. Without increasing power
blade dissymmetry created?
(thrust), the helicopter will begin to climb and continue
Q17-8. What type of main rotor allows individual
to accelerate. This changing relationship of power
rotor blades to move in both a vertical and
(thrust) available and power required is called
horizontal plane to reduce vibration caused
"translational lift." The speed that a helicopter passes
by blade flapping?
out of translational lift into forward flight can vary, but
Q17-9. The upward bending of rotor blades is known
generally, it is equal to approximately one-half the
by what term?
rotor diameter in knots, or approximately 25 knots for a
50-foot diameter rotor.
Q17-10. When a helicopter is close to the ground, will
it require more or less power to maintain a
AUTOROTATION
hover?
Autorotation occurs when the main rotor rotates by
Q17-11. What term refers to main rotor rotation
air passing up through the rotor system instead of by
caused by air passing through the main rotor
the engine. The rotor disengages automatically from
blades instead of being powered by the
the engine during engine failure or shutdown. During
engine?
17-4
