Page 116 - Understanding Flight

CH04_Anderson 7/25/01 8:57 AM Page 103

Stability and Control 103

airplane pitches up due to some disturbance, as shown with the toy
airplane on the right, there is no restoring torque about the center of
gravity. This toy airplane is neutrally stable, but balanced.
Finally, the bottom example in Figure 4.3 shows what happens if
the center of gravity is behind the neutral point. In straight-and-level
flight, as shown on the left, the lift on the wing is balanced by the lift
on the tail. But, in its perturbed state, for example, 5 degrees nose
pitch up, the lift on the wing grows faster than the lift on the tail. The
result is a rotational torque that rotates the toy airplane farther from
its initial straight-and-level flight state. This of course is unstable.
One question which you might ask is how the lift on the wing
The accident rate for general-
can grow faster than that on the tail since lift is just proportional
aviation aircraft is a little over 7
to the angle of attack. There are two reasons. First, the tail is
per 100,000 hours flown.
generally less efficient. On a typical airplane, the tail has about
half the aspect ratio of the wing. You learned in Chapter 2 the
importance of aspect ratio on efficiency of the wing. The same is true
with the horizontal stabilizer. The other factor that affects the horizontal
stabilizer’s lift is that it is flying in the downwash of the wing. If the wing
is generating more lift, it has a greater downwash. The net effect of the
downwash is that the horizontal stabilizer sees a lower relative angle of
attack than it would if there were no downwash. The wing may
experience a 10-degree change in the angle of attack while the horizontal
stabilizer only sees a 6-degree change in the direction of the air.
The lesson is that for longitudinal (pitch) stability it is crucial that
the center of gravity of an airplane be forward of the neutral point of
the airplane. That is, the plane is nose-heavy. Such an airplane, when
disturbed by a gust of air or a sudden control movement, will tend to
return to the original attitude. If the center of gravity is behind the
neutral point, the airplane is not flyable. Any disturbance will be
magnified and will tend to increase. Thus, if the nose pitches up just
a little, the airplane will want to exaggerate this motion. In such a
situation the controls will not respond at all. So the airplane must be
loaded ahead of the neutral point. That is why children put paper clips
on the nose of their paper airplanes.
Pilots must determine the center of gravity before each flight,
depending on fuel, passengers, and payload, to ensure that the