ROTORCRAFT FLYING HANDBOOK1(20)

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acts like a gyro, maximum displacement occurs 90° in
the direction of rotation. The result is a tendency for
the helicopter to roll slightly to the right as it accelerates
through approximately 20 knots or if the headwind
is approximately 20 knots.
You can recognize transverse flow effect because of
increased vibrations of the helicopter at airspeeds just
below effective translational lift on takeoff and after
passing through effective translational lift during landing.
To counteract transverse flow effect, a cyclic input
needs to be made.
DISSYMMETRY OF LIFT
When the helicopter moves through the air, the relative
airflow through the main rotor disc is different on the
advancing side than on the retreating side. The relative
wind encountered by the advancing blade is increased
by the forward speed of the helicopter, while the relative
wind speed acting on the retreating blade is
reduced by the helicopter’s forward airspeed.
Therefore, as a result of the relative wind speed, the
advancing blade side of the rotor disc produces more
lift than the retreating blade side. This situation is
defined as dissymmetry of lift. [Figure 3-14]
If this condition was allowed to exist, a helicopter with
a counterclockwise main rotor blade rotation would roll
to the left because of the difference in lift. In reality, the
main rotor blades flap and feather automatically to
equalize lift across the rotor disc. Articulated rotor systems,
usually with three or more blades, incorporate a
horizontal hinge (flapping hinge) to allow the individual
rotor blades to move, or flap up and down as they
rotate. A semirigid rotor system (two blades) utilizes a
teetering hinge, which allows the blades to flap as a
unit. When one blade flaps up, the other flaps down.
Figure 3-13. A helicopter in forward flight, or hovering with a headwind or crosswind, has more molecules of air entering the aft
portion of the rotor blade. Therefore, the angle of attack is less and the induced flow is greater at the rear of the rotor disc.
Resultant Relative Wind
Resultant Relative Wind
10 to 20

Knots
A
A
B
B
Induced

Flow
Induced

Flow
Angle of

Attack Angle of

Attack
Rotational Relative Wind Rotational Relative Wind
3-7
As shown in figure 3-15, as the rotor blade reaches the
advancing side of the rotor disc (A), it reaches its maximum
upflap velocity. When the blade flaps upward,
the angle between the chord line and the resultant relative
wind decreases. This decreases the angle of attack,
which reduces the amount of lift produced by the blade.
At position (C) the rotor blade is now at its maximum
downflapping velocity. Due to downflapping, the angle
between the chord line and the resultant relative wind
increases. This increases the angle of attack and thus
the amount of lift produced by the blade.
The combination of blade flapping and slow relative wind
acting on the retreating blade normally limits the maximum
forward speed of a helicopter. At a high forward
speed, the retreating blade stalls because of a high angle of
attack and slow relative wind speed. This situation is
called retreating blade stall and is evidenced by a nose
pitch up, vibration, and a rolling tendency—usually to the
left in helicopters with counterclockwise blade rotation.
You can avoid retreating blade stall by not exceeding
the never-exceed speed. This speed is designated VNE
and is usually indicated on a placard and marked on the
airspeed indicator by a red line.
During aerodynamic flapping of the rotor blades as they
compensate for dissymmetry of lift, the advancing blade
RelativeWind
RelativeWind
Direction

of Flight
Advancing

Side
Blade Tip

Speed Plus

Helicopter

Speed

(400 KTS)
Blade Tip

Speed Minus

Helicopter

Speed

(200 KTS)
Retreating

Side
Forward Flight

100 KTS
Blade

Rotation
Figure 3-14. The blade tip speed of this helicopter is approximately
300 knots. If the helicopter is moving forward at 100
knots, the relative wind speed on the advancing side is 400
knots. On the retreating side, it is only 200 knots. This difference
in speed causes a dissymmetry of lift.
Figure 3-15. The combined upward flapping (reduced lift) of the advancing blade and downward flapping (increased lift) of the
retreating blade equalizes lift across the main rotor disc counteracting dissymmetry of lift.
Direction of Rotation
Chord Line
Resultant RW
　
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