ROTORCRAFT FLYING HANDBOOK1(19)

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acting force) and drag (the rearward acting or retarding
force of inertia and wind resistance). [Figure 3-11]
In straight-and-level, unaccelerated forward flight, lift
equals weight and thrust equals drag (straight-and-level
flight is flight with a constant heading and at a constant
altitude). If lift exceeds weight, the helicopter climbs;
if lift is less than weight, the helicopter descends. If
thrust exceeds drag, the helicopter speeds up; if thrust
is less than drag, it slows down.
As the helicopter moves forward, it begins to lose altitude
because of the lift that is lost as thrust is diverted
forward. However, as the helicopter begins to accelerate,
the rotor system becomes more efficient due to the
increased airflow. The result is excess power over that
which is required to hover. Continued acceleration
causes an even larger increase in airflow through the
rotor disc and more excess power.
TRANSLATIONAL LIFT
Translational lift is present with any horizontal flow of
air across the rotor. This increased flow is most noticeable
when the airspeed reaches approximately 16 to 24
knots. As the helicopter accelerates through this speed,
the rotor moves out of its vortices and is in relatively
undisturbed air. The airflow is also now more horizontal,
which reduces induced flow and drag with a corresponding
increase in angle of attack and lift. The additional
lift available at this speed is referred to as “effective
translational lift” (ETL). [Figure 3-12]
When a single-rotor helicopter flies through translational
lift, the air flowing through the main rotor and over the
tail rotor becomes less turbulent and more aerodynamically
efficient. As the tail rotor efficiency improves,
more thrust is produced causing the aircraft to yaw left
in a counterclockwise rotor system. It will be necessary
to use right torque pedal to correct for this tendency on
takeoff. Also, if no corrections are made, the nose rises
or pitches up, and rolls to the right. This is caused by
combined effects of dissymmetry of lift and transverse
flow effect, and is corrected with cyclic control.
Resultant
Resultant
Lift
Thrust
Helicopter

Movement
Weight
Drag
Figure 3-11. To transition into forward flight, some of the vertical
thrust must be vectored horizontally. You initiate this by
forward movement of the cyclic control.
No Recirculation

of Air
More Horizontal

Flow of Air
Reduced

Induced Flow

Increases

Angle of Attack
Tail Rotor Operates in

Relatively Clean Air
16 to 24

Knots
Figure 3-12. Effective translational lift is easily recognized in
actual flight by a transient induced aerodynamic vibration
and increased performance of the helicopter.
Thrust
Lift
Weight
Drag
Vertical Ascent
Figure 3-10. To ascend vertically, more lift and thrust must be
generated to overcome the forces of weight and the drag.
3-6
Translational lift is also present in a stationary hover if
the wind speed is approximately 16 to 24 knots. In normal
operations, always utilize the benefit of translational
lift, especially if maximum performance is needed.
INDUCED FLOW
As the rotor blades rotate they generate what is called
rotational relative wind. This airflow is characterized
as flowing parallel and opposite the rotor’s plane of
rotation and striking perpendicular to the rotor blade’s
leading edge. This rotational relative wind is used to
generate lift. As rotor blades produce lift, air is accelerated
over the foil and projected downward. Anytime a
helicopter is producing lift, it moves large masses of air
vertically and down through the rotor system. This
downwash or induced flow can significantly change
the efficiency of the rotor system. Rotational relative
wind combines with induced flow to form the resultant
relative wind. As induced flow increases, resultant relative
wind becomes less horizontal. Since angle of
attack is determined by measuring the difference
between the chord line and the resultant relative wind,
as the resultant relative wind becomes less horizontal,
angle of attack decreases. [Figure 3-13]
TRANSVERSE FLOW EFFECT
As the helicopter accelerates in forward flight, induced
flow drops to near zero at the forward disc area and
increases at the aft disc area. This increases the angle
of attack at the front disc area causing the rotor blade to
flap up, and reduces angle of attack at the aft disc area
causing the rotor blade to flap down. Because the rotor
　
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