Page 114 - Understanding Flight

CH04_Anderson 7/25/01 8:57 AM Page 101

Stability and Control 101

when the center of gravity is located at this point, there is neutral
stability.
When we add a horizontal stabilizer, the neutral point is changed
due to the lifting ability of the tail. If the center of gravity of the
airplane is at the neutral point, the plane has neutral stability. Now let
us look at some examples to illustrate the effects of the horizontal
stabilizer.
In Figure 4.3 a toy airplane is shown in its trim condition (i.e.,
straight-and-level flight) and then perturbed with an increased angle
of attack. Three examples are illustrated: stable, neutrally stable, and
unstable. The top example is chosen such that the center of gravity is
ahead of the neutral point. Looking at the picture, we see that the lift
of the wing produces a torque that wants to push the nose down since
the airplane rotates about its center of gravity. To counter this rotation
and thus balance the airplane, the horizontal stabilizer must produce
a downward force on the tail of the airplane. Notice how the
horizontal stabilizer is at a small negative angle of attack. Because the
tail is so far aft, there is a long lever arm so that the downward
force on the tail only needs to be on the order of 10 percent or The current $130 to $140
less of that of the wing (typically 6 to 8 percent). Because of million price tag for a Boeing
the downward force on the tail, the center of lift moves 777 is roughly the cost of
forward from the center of lift of the wing to coincide with the building an entire shopping mall.
center of gravity. The toy airplane is balanced.
The picture on the top right in Figure 4.3 shows the stable toy
airplane perturbed from its trim condition. Just as an example let us
say a gust has increased the angle of attack by 5 degrees. The wing
will create more lift because of the higher angle of attack. Meanwhile
the downward force on the tail will be reduced because of a decrease
in its negative angle of attack. The net increase in lift on the wing and
reduced downward force on the horizontal stabilizer results in the
center of lift moving aft. The airplane is no longer balanced but is
stable, because there is a restoring torque. So the airplane will pitch
back toward the balanced condition, where it will resume straight-
and-level and balanced flight.
The toy airplanes in the center of Figure 4.3 illustrate what happens
when the center of gravity is exactly at the neutral point. Notice that