Page 121 - Understanding Flight

CH04_Anderson 7/25/01 8:57 AM Page 108

108 CHAPTER FOUR

Wind

α

Restoring force

Fig. 4.6. Directional stability.

causes the airplane to want to yaw. A large vertical stabilizer, with
trim, can compensate for this. That is why twin-engine commercial
transports have such large vertical stabilizers.
The FAA dictates limits on directional stability. Modern airplanes
now have vertical stabilizers that are so effective as to make the use of
the rudder for small corrections almost unnecessary.
The B-2 bomber, which has no vertical stabilizer,
accomplishes directional stability by using differential drag.
On April 12, 1989, a British
The ailerons on the side you wish to turn will split, deploying
Airways Concorde landed in
both up and down. Thus, they increase the drag on that side,
Sydney, Australia, with half of its
pulling the nose around. This can be seen on the right wing in
top rudder missing.
Figure 4.4.

Dynamic Stability

Static stability deals with the tendency of the airplane when disturbed
to return to its original flight attitude. Dynamic stability deals with
how the motion caused by a disturbance changes with time. The three
types of dynamic stability are shown for the longitudinal stability of
an airplane in Figure 4.7. The first flight path shows positive dynamic
stability. When the airplane pitched up, there was a restoring force
(statically stable). The path oscillates through the original altitude and