From experiments conducted on wind
tunnel models and on full size airplanes, it has been determined that
as air flows along the surface of a wing at different angles of attack.
There are regions along the surface where the pressure is negative, or
less than atmospheric, and regions where the pressure is positive, or
greater than atmospheric.
This negative pressure on the upper surface
creates a relatively larger force on the wing than is caused by the
positive pressure resulting from the air striking the lower wing
surface. In the design of wing structures, this center of pressure
travel is very important, since it affects the position of the air
loads imposed on the wing structure in low angle-of-attack conditions
and high angle-of-attack conditions. The airplane's aerodynamic balance
and control ability are governed by changes in the center of pressure.
The
center of pressure is determined through calculation and wind tunnel
tests by varying the airfoil's angle of attack through normal operating
extremes. As the angle of attack is changed, so are the various
pressure distribution characteristics. Positive (+) and negative (–)
pressure forces are totaled for each angle of attack and the resultant
force is obtained. The resultant force vector represents the total
resultant pressure.
The
point of application of this force vector is termed the "center of
pressure" (CP). For any given angle of attack, the center of pressure
is the point where the resultant force crosses the chord line. This
point is expressed as a percentage of the chord of the airfoil. A
center of pressure at 30 percent of a 60- inch chord would be 18 inches
aft of the wing's leading edge. It would appear then that if the
designer would place the wing so that its center of pressure was at the
airplane's center of gravity, the airplane would always balance. The
difficulty arises, however, that the location of the center of pressure
changes with change in the airfoil's angle of attack.
In
the airplane's normal range of flight attitudes, if the angle of
attack is increased, the center of pressure moves forward; and if
decreased, it moves rearward. Since the center of gravity is fixed at
one point, it is evident that as the angle of attack increases. The
center of lift (CL) moves ahead of the center of gravity, creating a
force which tends to raise the nose of the airplane or tends to
increase the angle of attack still more. On the other hand, if the
angle of attack is decreased, the center of lift (CL) moves aft and
tends to decrease the angle a greater amount. It is seen then, that the
ordinary airfoil is inherently unstable, and that an auxiliary device,
such as the horizontal tail surface, must be added to make the airplane
balance longitudinally.
The
balance of an airplane in flight depends, therefore, on the relative
position of the center of gravity (CG) and the center of pressure (CP)
of the airfoil. Experience has shown that an airplane with the center
of gravity in the vicinity of 20 percent of the wing chord can be made
to balance and fly satisfactorily.
The
tapered wing presents a variety of wing chords throughout the span of
the wing. It becomes necessary then, to specify some chord about which
the point of balance can be expressed. This chord, known as the mean
aerodynamic chord (MAC), usually is defined as the chord of an
imaginary untapered wing, which would have the same center of pressure
characteristics as the wing in question.
Airplane
loading and weight distribution also affect center of gravity and
cause additional forces, which in turn affect airplane balance.
