|Angle of Attack and Lift
Angle of attack must not be confused with an airplane's attitude
in relation to the earth's surface, or with "angle of incidence" (the angle at
which the wing is attached relative to the longitudinal axis of the airplane).
Angle of attack is most frequently defined as the angle between the chord line
of the wing, and the relative wind. Generally, it is sufficient to say that
angle of attack is simply the angular difference between where the wing is
headed and where it is actually going. As can be seen from Figure 17-12, this
angle may be precisely the same for climbs, descents, and level flight, or can
be quite different even when maintaining the same altitude.
The feature that complicates this problem is that with certain
exceptions, we have no way of actually seeing the angle at which the wing meets
the relative wind. Angle of attack indicators usually are found only in the
turbojet powered airplanes.
In a very real sense the angle of
attack is what flight in airplanes is all about. By changing the angle of attack
the pilot can control lift, airspeed, and drag. Even the total load supported in
flight by the wing, may be modified by variations in angle of attack, and when
coordinated with power changes, and auxiliary devices such as flaps, slots,
slats, etc., is the essence of airplane control.
over the entire upper wing
surface. The negative pressure above the wing suddenly becomes almost equal to
atmospheric pressure in value with a resulting loss of lift and a sudden
increase in resistance or drag. These events show that Bernoulli's principle is
true only in a streamline or smooth airflow - not in a turbulent airflow. The
center of pressure at the point of stall is at its maximum forward position, and
the resultant force tilts sharply backward.
|The angle of attack of an airfoil directly controls the
distribution of pressure below and above it. When a wing is at a low but
positive angle of attack, most of the lift is due to the wing's negative
pressure (upper surface) and downwash. (Negative pressure is any pressure
less than atmospheric, and positive pressure is pressure greater than
From Fig. 17-7 it can be seen that the
positive pressure below the wing at a low angle of attack is very slight,
and it can be noted also that the negative pressure above the wing is
quite strong by comparison.
At any angle of attack, other than the angle at zero lift, all the
forces acting on the wing as a result of the pressure distribution
surrounding it may be summed up and represented as one force - the center
When the angle of attack increases to approximately 18 to
20 degrees (on most wings), the air can no longer flow smoothly over the
top wing surface. Because the airflow cannot make such a great change in
direction so quickly, it becomes impossible for the air to follow the
contour of the wing. This is the stalling or critical angle of attack, and
is often called the burble point. The burbling or turbulent flow of air
which begins near the trailing edge of the wing, suddenly spreads
One of the most important things a pilot should understand about
angle of attack is that for any given airplane the stalling or critical angle of
attack remains constant regardless of weight, dynamic pressure, bank angle, or
pitch attitude. These factors certainly will affect the speed at which the stall
occurs, but not the angle. The aerodynamicist may say that the stalling angle of
attack is not always an absolute constant, but for our purposes here it is a
valid, useful, and safe concept.
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