Game Development Reference
In-Depth Information
Stability
The previous section introduced the concept of trim, which is a condition where the moment
through the center of gravity of an airplane is equal to zero. What happens when the angle of
attack deviates from the trim angle defines the
stability
of the airplane. In this section we will
discuss pitch stability, in which moments applied to the airplane cause the nose of the airplane
to rotate up or down. Moments applied to an airplane can also cause the nose of the airplane to
rotate side-to-side (yaw) or cause the airplane to roll.
The pitch stability of an airplane can be assessed by determining how the pitching moment
about the center of gravity changes with changing angle of attack. Consider the plot of moment
coefficient about the center of gravity,
C
M,CG
, versus angle of attack,
a
, shown in Figure 10-30.
A negative moment coefficient indicates a moment that will push the nose of the airplane
down. A zero moment coefficient signifies the trim condition.
Let's say there are two airplanes flying at their trim angle of attack,
a
trim
. The solid line
indicates the moment coefficient curve for the first airplane. If the angle of attack of the first
airplane increases beyond its trim angle, that is, the nose of the airplane pitches upwards, the
resulting moment coefficient is negative. The negative moment acts to pitch the nose of the
airplane back downwards. If the nose of the airplane pitches down, decreasing the angle of
attack, the resulting moment is positive, which acts to pitch the nose of the airplane back up.
Therefore, any moments that are generated for the first airplane are restoring moments in that
they work to return the airplane to its trim angle of attack. This situation is known as
pitch stability
.
Figure 10-30.
Stability depends on the slope of the moment-angle of attack curve.
On the other hand, let's say the dashed line signifies the moment coefficient curve for the
second airplane. The slope of the moment coefficient curve is now positive. If the angle of
attack of the second airplane increases beyond the trim value, a positive moment is generated.
This moment acts to pitch the nose of the airplane even higher, increasing the angle of attack
even more. This action generates an increased positive moment, which pitches the nose up,
and so on, and so on. The airplane will continue to pitch upwards until it tumbles out of control.
A moment coefficient curve with a positive slope therefore represents an unstable situation.
