The spinning main rotor of a helicopter acts like a
gyroscope. It has the properties of gyroscopic action,
one of which is precession. Gyroscopic precession is
the resulting action occurring 90 degrees from the
applied force. A downward force to the right of the
disc area will cause the rotor to tilt down in front.
This action is true for a right-to-left (counterclockwise)
turning rotor. The cyclic control applies force to the
main rotor through the swashplate.
To simplify directional control, helicopters use a
mechanical linkage that places cyclic pitch change
90 degrees ahead of the applied force. Moving the
cyclic control forward will cause high pitch on the
blades to the pilots left. At the same time, low pitch
occurs on the blades to his/her right. This combina-
tion of forces results in the rotor tilting down in front.
If not for this offset linkage, the pilot would have
to move the cyclic stick 90 degrees out of phase. In
other words, the pilot would have to move the stick to
the right when attempting to tilt the disc forward.
He/she would move the cyclic stick forward when
attempting to tilt the disc area to the left, and so on.
This is due to the loss of the ground cushion caused by
the changing direction or vector of the rotors thrust.
As the helicopter continues to accelerate, the rotor
will be introduced to larger masses of air. The rotor
will become more efficient and the thrust vector of the
rotor will become more stable. Without increasing
power (thrust), the helicopter will begin to climb and
continue to accelerate. This changing relationship of
power (thrust) available and power required is called
translational lift. The speed that a helicopter passes
out of translational lift into forward flight can vary,
but generally it is equal to approximately one-half the
rotor diameter in knots, or approximately 25 knots for
a 50-foot diameter rotor.
Autorotation occurs when the main rotor rotates by
air passing up through the rotor system instead of by the
engine. The rotor disengages automatically from the
engine during engine failure or shutdown.
autorotation, the rotor blades turn in the same direction
as when engine driven. The air passes up through the
rotor system instead of down. This action causes a
slightly greater upward flex or coning of the blades.
Ground effect can be achieved when a helicopter is
in a hover or forward flight while in close proximity to
the ground or some other hard flat surface. When a
helicopter is in a hover or moving slowly, the main rotor
is developing thrust that is being vectored, or directed
down toward the surface. The surface resists this
airflow (thrust) by building up air pressure between the
rotor and the surface, thus providing ground cushion.
When the helicopter is in forward flight, the cushion is
not as great as the thrust that is being vectored down and
aft of the helicopter. This ground cushion will provide
additional lift without additional power, and will be
apparent when the helicopter is hovering or flying at an
altitude of approximately one-half the main rotor
diameter or below. The closer the helicopter is to the
ground, the greater the cushion effect. This will be
indicated by the reduced power required to maintain
flight or hover.
The maximum cushion effect is
achieved at zero airspeed.
As a helicopter begins the transition from a hover
to forward flight, at approximately 10-15 knots, it will
experience a loss of lift and settle slightly and seem to
loose power, without an actual reduction in power.
Stalling, as applied to fixed-wing aircraft, will not
occur in helicopters.
However, power settling may
occur in low-speed flight. Power settling is the uncon-
trollable loss of altitude. This condition may occur due
to combinations of heavy gross weights, poor density
conditions, and low forward speed. During low
forward speed and high rates of descents, the
downwash from the rotor begins to recirculate. The
downwash moves up, around, and back down though
the effective outer disc area. The velocit y of this recir-
culating air mass may become so high that full collec-
tive pitch cannot retard or control the rate of descent.
TYPES OF HELICOPTERS
Learning Objective: Identify the two basic
types of helicopters and recognize the
advantages of each.
Two basic types of helicopters are the single-rotor
and multirotor types. The single main rotor with a
vertical or near vertical tail rotor is the most common
type of helicopter. The SH-60 and SH-2, shown in
figure 10-5, are examples of single-rotor helicopters.