Page 112 - Understanding Flight

CH04_Anderson 7/25/01 8:57 AM Page 99

Stability and Control 99

interested in the stability of the airplane as a whole. However, The all-African-American
to introduce the concept of stability we first focus on the lon- squadron from Tuskegee had
gitudinal stability of a wing. Only after the stability of a wing trouble convincing superiors that
is understood will you be introduced to the purpose of the they were qualified for battle.
horizontal stabilizer and stability of the airplane as a whole. When they finally were sent to
Europe, they distinguished
Stability of a Symmetric Wing
themselves by not allowing a
We have chosen to use a symmetric wing section for this single bomber to be lost under
example to simplify the explanation. The description does not their escort.
change for a nonsymmetric wing, but a few additional ideas
would have to be introduced, which would only confuse the issue.
The wing has a center of gravity (c.g.) as marked in Figure 4.2. The
center of gravity is where the wing balances. If you could combine the
distributed weight of the wing into a single point, the location of this
point would be the center of gravity, which is also at the center of
balance. Similarly, the center of lift is the point where we would place
the lift if we took the distribution of lift over the wing and placed it at
a single point.
Figure 4.2 shows three situations, a wing that is stable, neutrally
stable, and unstable. For the stable wing the center of gravity is ahead
of the center of lift. Let us say that a gust of air increases the angle of
attack. The greater angle of attack will increase the lift. This increase
in lift will cause the wing to rotate about the center of gravity and
reduce the angle of attack. In other words, there is a rotational torque
that rotates the wing back in the direction from where it started. So,
we say this wing is stable.

Lift Lift
c.g.
Increase Increase
Stable in lift Unstable in lift

Rotational Rotational
torque torque
Lift
Increase
Neutral
in lift

Fig. 4.2. Stability of a symmetric wing.