9 O'Clock Position
Angle of Attack at
3 O'Clock Position
Angle of Attack over
Tail
Angle of Attack over
Nose
A
B
C
D
D
A
B
C
RW = Relative Wind
= Angle of Attack
VNE —The speed beyond which an aircraft should never be
operated. VNE can change with altitude, density altitude, and
weight.
3-8
ward. Drag now acts forward with the lift component
straight up and weight straight down. [Figure 3-18]
TURNING FLIGHT
In forward flight, the rotor disc is tilted forward, which
also tilts the total lift-thrust force of the rotor disc forward.
When the helicopter is banked, the rotor disc is
tilted sideward resulting in lift being separated into two
components. Lift acting upward and opposing weight is
called the vertical component of lift. Lift acting horizontally
and opposing inertia (centrifugal force) is the
horizontal component of lift (centripetal force).
[Figure 3-19]
As the angle of bank increases, the total lift force is tilted
more toward the horizontal, thus causing the rate of turn
to increase because more lift is acting horizontally. Since
the resultant lifting force acts more horizontally, the
effect of lift acting vertically is deceased. To compensate
for this decreased vertical lift, the angle of attack of
the rotor blades must be increased in order to maintain
altitude. The steeper the angle of bank, the greater the
angle of attack of the rotor blades required to maintain
altitude. Thus, with an increase in bank and a greater
angle of attack, the resultant lifting force increases and
the rate of turn is faster.
AUTOROTATION
Autorotation is the state of flight where the main rotor
system is being turned by the action of relative wind
Centripetal Force—The force
opposite centrifugal force and
attracts a body toward its axis of
rotation.
Resultant
Lift
Thrust
Drag
Resultant
Weight
Helicopter
Movement
Figure 3-18. Forces acting on the helicopter during rearward
flight.
achieves maximum upflapping displacement over the
nose and maximum downflapping displacement over the
tail. This causes the tip-path plane to tilt to the rear and is
referred to as blowback. Figure 3-16 shows how the rotor
disc was originally oriented with the front down following
the initial cyclic input, but as airspeed is gained and
flapping eliminates dissymmetry of lift, the front of the
disc comes up, and the back of the disc goes down. This
reorientation of the rotor disc changes the direction in
which total rotor thrust acts so that the helicopter’s forward
speed slows, but can be corrected with cyclic input.
SIDEWARD FLIGHT
In sideward flight, the tip-path plane is tilted in the direction
that flight is desired. This tilts the total lift-thrust
vector sideward. In this case, the vertical or lift component
is still straight up and weight straight down, but the
horizontal or thrust component now acts sideward with
drag acting to the opposite side. [Figure 3-17]
REARWARD FLIGHT
For rearward flight, the tip-path plane is tilted rearward,
which, in turn, tilts the lift-thrust vector rear-
Helicopter
Movement
Weight
Drag
Resultant
Thrust
Lift
Figure 3-17. Forces acting on the helicopter during sideward
flight.
Figure 3-16. To compensate for blowback, you must move
the cyclic forward. Blowback is more pronounced with higher
airspeeds.
3-9
rather than engine power. It is the means by which a
helicopter can be landed safely in the event of an
engine failure. In this case, you are using altitude as
potential energy and converting it to kinetic energy during
the descent and touchdown. All helicopters must
have this capability in order to be certified.
Autorotation is permitted mechanically because of a
freewheeling unit, which allows the main rotor to continue
turning even if the engine is not running. In normal
powered flight, air is drawn into the main rotor system
from above and exhausted downward. During
autorotation, airflow enters the rotor disc from below
as the helicopter descends. [Figure 3-20]
AUTOROTATION (VERTICAL FLIGHT)
Most autorotations are performed with forward speed.
For simplicity, the following aerodynamic explanation
is based on a vertical autorotative descent (no forward
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