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airfoils are asymmetrical in design, meaning the
upper and lower surface do not have the same camber.
Normally these airfoils would not be as stable, but this
can be corrected by bending the trailing edge to produce
the same characteristics as symmetrical airfoils. This is
called “reflexing.” Using this type of rotor blade allows
the rotor system to operate at higher forward speeds.
One of the reasons an asymmetrical rotor blade is not
as stable is that the center of pressure changes with
changes in angle of attack. When the center of pressure
lifting force is behind the pivot point on a rotor blade, it
tends to cause the rotor disc to pitch up. As the angle of
attack increases, the center of pressure moves forward.
If it moves ahead of the pivot point, the pitch of the
rotor disc decreases. Since the angle of attack of the
rotor blades is constantly changing during each cycle
of rotation, the blades tend to flap, feather, lead, and
lag to a greater degree.
When referring to an airfoil, the span is the distance
from the rotor hub to the blade tip. Blade twist refers to
a changing chord line from the blade root to the tip.
Figure 2-2. The upper and lower curvatures are the same on a
symmetrical airfoil and vary on an asymmetrical airfoil.
Asymmetrical
Symmetrical
Lift
Weight

Drag
Thrust

Figure 2-1. Four forces acting on a helicopter in forward flight.
2-2
Twisting a rotor blade causes it to produce a more even
amount of lift along its span. This is necessary because
rotational velocity increases toward the blade tip. The
leading edge is the first part of the airfoil to meet the
oncoming air. [Figure 2-3] The trailing edge is the aft
portion where the airflow over the upper surface joins
the airflow under the lower surface. The chord line is
an imaginary straight line drawn from the leading to
the trailing edge. The camber is the curvature of the airfoil’s
upper and lower surfaces. The relative wind is the
wind moving past the airfoil. The direction of this wind
is relative to the attitude, or position, of the airfoil and
is always parallel, equal, and opposite in direction to
the flight path of the airfoil. The angle of attack is the
angle between the blade chord line and the direction of
the relative wind.
RELATIVE WIND
Relative wind is created by the motion of an airfoil
through the air, by the motion of air past an airfoil, or by
a combination of the two. Relative wind may be
affected by several factors, including the rotation of the
rotor blades, horizontal movement of the helicopter,
flapping of the rotor blades, and wind speed and direction.
For a helicopter, the relative wind is the flow of air with
respect to the rotor blades. If the rotor is stopped, wind
blowing over the blades creates a relative wind. When
the helicopter is hovering in a no-wind condition, relative
wind is created by the motion of the rotor blades
through the air. If the helicopter is hovering in a wind,
the relative wind is a combination of the wind and the
motion of the rotor blades through the air. When the
helicopter is in forward flight, the relative wind is a
combination of the rotation of the rotor blades and the
forward speed of the helicopter.
BLADE PITCH ANGLE
The pitch angle of a rotor blade is the angle between its
chord line and the reference plane containing the rotor
hub. [Figure 2-4] You control the pitch angle of the blades
with the flight controls. The collective pitch changes each
rotor blade an equal amount of pitch no matter where it is
located in the plane of rotation (rotor disc) and is used to
change rotor thrust. The cyclic pitch control changes the
pitch of each blade as a function of where it is in the plane
of rotation. This allows for trimming the helicopter in
pitch and roll during forward flight and for maneuvering
in all flight conditions.
ANGLE OF ATTACK
When the angle of attack is increased, air flowing over
the airfoil is diverted over a greater distance, resulting
in an increase of air velocity and more lift. As angle of
attack is increased further, it becomes more difficult for
air to flow smoothly across the top of the airfoil. At this
point the airflow begins to separate from the airfoil and
enters a burbling or turbulent pattern. The turbulence
results in a large increase in drag and loss of lift in the
area where it is taking place. Increasing the angle of
attack increases lift until the critical angle of attack is
reached. Any increase in the angle of attack beyond this
point produces a stall and a rapid decrease in lift.
　
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