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Shape (shape or design of the airfoil sections).
Speed (velocity of the air passing over the
airfoil).
Angle of attack (angle at which the air strikes
the airfoil).
Air density (amount of air in a given space).
Area and Shape
The specific shape and surface area of an airfoil are
determined by the aircraft manufacturer. An airfoil
may be symmetrical or unsymmetrical, depending on
specific requirements. A symmetrical airfoil is
designed with an equal amount of camber above and
below the airfoil chord line. An unsymmetrical airfoil
has a greater amount of camber above the chord
line. An airfoil with a smooth surface produces more
lift than one with a rough surface. A rough surface
creates turbulence, which reduces lift and increases
drag.
Speed
The speed of an airfoil can be changed by the speed
of the engine or by the angle of the blade. The lift
developed by an airfoil increases as speed increases.
However, there is a limit to the amount of lift because
the drag (resistance) of the airfoil also increases as
speed increases.
Angle of Attack
The angle of attack is the angle between the airfoil
chord and the direction of relative wind. Direction
of airflow in relation to the airfoil is called relative
wind. Lift increases as the angle of attack increases
up to a certain point. If the angle of attack becomes
too great, airflow over the top of the airfoil tends to
lose its streamlined path and break away from the
contoured surface to form eddies (burbles) near the
trailing edge. When this happens, the airfoil loses its
lift, and it stalls. The angle of attack at which burbling
takes place is called the critical angle of attack.
Air Density
The density (thickness) of the air plays an important
part in the amount of lift an airfoil is able to make.
The air nearest the earth’s surface is much denser
than air at higher altitudes. Therefore, an aircraft or
helicopter can achieve more lift near the ground than
at a high altitude. While keeping at the same speed
and angle of attack, an airfoil will slowly make less lift
as it climbs higher and higher.
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FM 1-514
AIRFOIL STABILITY
Center of Pressure
The resultant lift produced by an airfoil is the difference
between the drag and lift pressures of its
upper and lower surfaces. The point on the airfoil
chord line where the resultant lift is effectively concentrated
is called the center of pressure. The center
of pressure of a symmetrical airfoil remains in one
position at all angles of attack. When the angle of
attack of an unsymmetrical airfoil changes, the center
of pressure changes accordingly: the center of pressure
moves forward with an increase in angle of
attack, and the center of pressure moves backward
with a decrease in angle of attack.
Airfoil Aerodynamic Center
The aerodynamic center of an airfoil is the point
along the chord line about which the airfoil tends to
rotate when the center of pressure moves forward or
backward between the leading and trailing edges.
Torque
According to Newton’s third law of motion, for every
action there is an equal and opposite reaction. As a
helicopter main rotor or an airplane propeller turns
in one direction, the aircraft fuselage tends to rotate
in the opposite direction. This effect is called torque.
Solutions must be found to counteract and control
torque during flight. In helicopters torque is applied
in a horizontal rather than a vertical plane. The
reaction is therefore greater because the rotor is long
and heavy relative to the fuselage, and forward speed
is not always present to correct the twisting effect.
Gyroscopic Precession
If a force is applied against a rotating body, the
reaction will be about 90° from the point of application,
in the direction of rotation. This unusual fact is
known as gyroscopic precession. It pertains to all
rotating bodies. For example, if you push the 3-
o’clock point on a clockwise rotating wheel, the wheel
would move as if it had been pushed at the 6-o’clock
point. The rotors on helicopters act as gyroscopes
and are therefore subject to the action of gyroscopic
precession.
STRESS
Stress is a force placed on a body measured in
terms of force (pounds) per unit area (square
inches). Aircraft design engineers design aircraft
to meet – even to exceed – strength requirements
of military service. Since Army aircraft are
operated under combat conditions, they might exceed
these design limits. Therefore, maintenance
　
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