ROTORCRAFT FLYING HANDBOOK1(13)

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[Figure 2-5]
Angle of attack should not be confused with pitch
angle. Pitch angle is determined by the direction of the
relative wind. You can, however, change the angle of
attack by changing the pitch angle through the use of
the flight controls. If the pitch angle is increased, the
angle of attack is increased, if the pitch angle is
reduced, the angle of attack is reduced. [Figure 2-6]
Axis-of-Rotation—The imaginary
line about which the rotor rotates.
It is represented by a line drawn
through the center of, and perpendicular
to, the tip-path plane.
Tip-Path Plane—The imaginary
circular plane outlined by the
rotor blade tips as they make a
cycle of rotation.
Aircraft Pitch—When referenced
to a helicopter, is the movement of
the helicopter about its lateral, or
side to side axis. Movement of the
cyclic forward or aft causes the
nose of the helicopter to move up
or down.
Aircraft Roll—Is the movement of
the helicopter about its longitudinal,
or nose to tail axis. Movement
of the cyclic right or left causes the
helicopter to tilt in that direction.
Figure 2-3. Aerodynamic terms of an airfoil.
Trailing

Edge
Chord Line
Angle of
Attack
FLIGHT PATH
RELATIVE WIND
Upper

Camber
Lower

Camber
Leading

Edge
Axis of Rotation
Reference Plane
Pitch

Angle
Chord Line
Figure 2-4. Do not confuse the axis of rotation with the rotor
mast. The only time they coincide is when the tip-path plane
is perpendicular to the rotor mast.
2-3
LIFT
MAGNUS EFFECT
The explanation of lift can best be explained by looking
at a cylinder rotating in an airstream. The local velocity
near the cylinder is composed of the airstream velocity
and the cylinder’s rotational velocity, which decreases
with distance from the cylinder. On a cylinder, which is
rotating in such a way that the top surface area is rotating
in the same direction as the airflow, the local velocity at
the surface is high on top and low on the bottom.
As shown in figure 2-7, at point “A,” a stagnation point
exists where the airstream line that impinges on the surface
splits; some air goes over and some under. Another
stagnation point exists at “B,” where the two air
streams rejoin and resume at identical velocities. We
now have upwash ahead of the rotating cylinder and
downwash at the rear.
The difference in surface velocity accounts for a difference
in pressure, with the pressure being lower on the
top than the bottom. This low pressure area produces
an upward force known as the “Magnus Effect.” This
mechanically induced circulation illustrates the relationship
between circulation and lift.
An airfoil with a positive angle of attack develops air
circulation as its sharp trailing edge forces the rear
stagnation point to be aft of the trailing edge, while the
front stagnation point is below the leading edge.
[Figure 2-8]
BERNOULLI’S PRINCIPLE
Air flowing over the top surface accelerates. The airfoil
is now subjected to Bernoulli’s Principle or the “venturi
effect.” As air velocity increases through the constricted
portion of a venturi tube, the pressure decreases.
Axis of Rotation
Reference Plane
Pitch

Angle Chord Line
Angle of
Attack
RELATIVE WIND
Figure 2-6. Angle of attack may be greater than, less than, or
the same as the pitch angle.
Figure 2-5. As the angle of attack is increased, the separation
point starts near the trailing edge of the airfoil and progresses
forward. Finally, the airfoil loses its lift and a stall
condition occurs.
LIFT

 

STALL
8°
12-16° Figure 2-7. Magnus Effect is a lifting force produced when a
rotating cylinder produces a pressure differential. This is the
same effect that makes a baseball curve or a golf ball slice.
B A
Increased Local Velocity

(Decreased pressure)
Decreased Local Velocity

 

Downwash Upwash
Figure 2-8. Air circulation around an airfoil occurs when the
front stagnation point is below the leading edge and the aft
stagnation point is beyond the trailing edge.
Leading Edge

Stagnation Point
Trailing Edge

Stagnation Point
B
A
Steady-State Flight—A condition
when an aircraft is in straightand-
level, unaccelerated flight,
and all forces are in balance.
2-4
ward. According to Newton’s Third Law of Motion,
　
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