Drag or
L/D max
L/Dmax—The maximum ratio
between total lift (L) and the total
drag (D). This point provides the
best glide speed. Any deviation
from best glide speed increases
drag and reduces the distance you
can glide.
Induced Drag
Figure 2-13. The formation of induced drag is associated with
the downward deflection of the airstream near the rotor
blade.
3-1
Once a helicopter leaves the ground, it is acted upon by
the four aerodynamic forces. In this chapter, we will
examine these forces as they relate to flight maneuvers.
POWERED FLIGHT
In powered flight (hovering, vertical, forward, sideward,
or rearward), the total lift and thrust forces of a
rotor are perpendicular to the tip-path plane or plane of
rotation of the rotor.
HOVERING FLIGHT
For standardization purposes, this discussion assumes
a stationary hover in a no-wind condition. During hovering
flight, a helicopter maintains a constant position
over a selected point, usually a few feet above the
ground. For a helicopter to hover, the lift and thrust
produced by the rotor system act straight up and must
equal the weight and drag, which act straight down.
While hovering, you can change the amount of main
rotor thrust to maintain the desired hovering altitude.
This is done by changing the angle of attack of the main
rotor blades and by varying power, as needed. In this
case, thrust acts in the same vertical direction as lift.
[Figure 3-1]
The weight that must be supported is the total weight of the
helicopter and its occupants. If the amount of thrust is
greater than the actual weight, the helicopter gains altitude;
if thrust is less than weight, the helicopter loses altitude.
The drag of a hovering helicopter is mainly induced drag
incurred while the blades are producing lift. There is,
however, some profile drag on the blades as they rotate
through the air. Throughout the rest of this discussion,
the term “drag” includes both induced and profile drag.
An important consequence of producing thrust is
torque. As stated before, for every action there is an
equal and opposite reaction. Therefore, as the engine
turns the main rotor system in a counterclockwise
direction, the helicopter fuselage turns clockwise. The
amount of torque is directly related to the amount of
engine power being used to turn the main rotor system.
Remember, as power changes, torque changes.
To counteract this torque-induced turning tendency, an
antitorque rotor or tail rotor is incorporated into most
helicopter designs. You can vary the amount of thrust
produced by the tail rotor in relation to the amount of
torque produced by the engine. As the engine supplies
more power, the tail rotor must produce more thrust.
This is done through the use of antitorque pedals.
TRANSLATING TENDENCY OR DRIFT
During hovering flight, a single main rotor helicopter tends
to drift in the same direction as antitorque rotor thrust. This
drifting tendency is called translating tendency. [Figure 3-2]
Thrust
Lift
Weight
Drag
Figure 3-1. To maintain a hover at a constant altitude, enough
lift and thrust must be generated to equal the weight of the
helicopter and the drag produced by the rotor blades.
Blade Rotation
Torque
Torque
Drift
Tail Rotor Thrust
Figure 3-2. A tail rotor is designed to produce thrust in a
direction opposite torque. The thrust produced by the tail
rotor is sufficient to move the helicopter laterally.
3-2
greater the centrifugal force. This force gives the rotor
blades their rigidity and, in turn, the strength to support
the weight of the helicopter. The centrifugal force generated
determines the maximum operating rotor r.p.m.
due to structural limitations on the main rotor system.
As a vertical takeoff is made, two major forces are acting
at the same time—centrifugal force acting outward
and perpendicular to the rotor mast, and lift acting
upward and parallel to the mast. The result of these two
forces is that the blades assume a conical path instead
of remaining in the plane perpendicular to the mast.
[Figure 3-4]
CORIOLIS EFFECT
(LAW OF CONSERVATION
OF ANGULAR MOMENTUM)
Coriolis Effect, which is sometimes referred to as conservation
of angular momentum, might be compared to
spinning skaters. When they extend their arms, their
rotation slows down because the center of mass moves
farther from the axis of rotation. When their arms are
retracted, the rotation speeds up because the center of
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