Page 115 - Understanding Flight

CH04_Anderson 7/25/01 8:57 AM Page 102

102 CHAPTER FOUR

Stable

Wing lift
Net lift Wing lift Net lift

x

Weight Neutral H.S. Lift Weight H.S. Lift
point

Neutral
Wing lift
Net lift
Wing lift Net lift

H.S. Lift
Weight Weight H.S. Lift

Unstable
Wing lift
Wing lift Net lift Net lift

x
H.S. Lift
Neutral Weight H.S. Lift
point Weight

Fig. 4.3. Stability of a toy airplane.

the center of gravity is slightly aft of the center of lift of the wing
alone. Thus, to balance, the horizontal stabilizer must produce a small
positive lift. In this particular situation, the center of lift is
independent of the angle of attack. In other words, when the airplane
is perturbed from straight-and-level flight, the increased lift on the
wing and the increased lift on the tail balance such that the center of
lift remains stationary. This happens at the unique location where the
wing’s lift and tail’s lift change together in exact balance. So, when the