Page 117 - Understanding Flight

CH04_Anderson 7/25/01 8:57 AM Page 104

104 CHAPTER FOUR

The horizontal stabilizer’s job is airplane’s center of gravity is within limits. Unfortunate
to maintain stability. accidents have resulted from pilots inadvertently taking off
with an airplane loaded such that the center of gravity was
behind the neutral point. In this situation, no effort by the pilot can
save the airplane from catastrophic consequences.
The purpose of the horizontal stabilizer is frequently
misunderstood. Frequently, the horizontal stabilizer is said to control
pitch. Actually, as discussed in Chapter 1, it is the elevator, which is
part of the horizontal stabilizer, that controls pitch. The horizontal
stabilizer’s job is to maintain stability.

Trim

An airplane must be able to be balanced and stable for a variety of
loads. For example, a commercial airplane may face two extremes, one
where the airplane is half full of passengers who are all sitting in the for-
ward seats and one where the airplane is half full and all of the passen-
gers are sitting in the aft seats. The two seating arrangements will move
the center of gravity of the airplane. We assume that in both cases the
center of gravity is ahead of the neutral point so that it is stable. But the
airplane must also be balanced. This is where elevator trim plays a role.
Depending on where the center of gravity is, the horizontal
stabilizer’s lift must be adjusted to balance the airplane. The pilot
could achieve this by holding the elevator in position, but this
would be tiring on a long flight. So, instead, the pilot trims the
In 2001, NASA will test active
airplane to balance it. In Chapter 1 the trim tab was
aeroelastic wings that will use
introduced. This small flap on the elevator is adjusted from
wing warping rather than control
the cockpit so that the horizontal stabilizer produces the right
surfaces. The first to use this
amount of force to balance the airplane without moving the
method were the Wright brothers,
elevator. On many airplanes, such as the large commercial
who developed wing warping for
transports, the entire horizontal stabilizer is rotated so that its
roll control before 1903.
angle of attack can be adjusted for different flight conditions.
Now, what happens when passengers move about in the airplane?
The center of gravity moves and the airplane will want to pitch nose
up or down. So the horizontal stabilizer trim is adjusted to
compensate. But, if the pilot wishes to purposely change the pitch, or
angle of attack, the pilot will command the elevators. Thus, the