Page 113 - Understanding Flight

CH04_Anderson 7/25/01 8:57 AM Page 100

100 CHAPTER FOUR

For the neutrally stable wing the center of gravity and the center of
lift are colocated. So, if the wing is disturbed by a gust, increasing the
angle of attack, the lift will increase but will not result in any restoring
torque around the center of mass. Since there is no rotational torque,
this wing will tend to remain just where the gust puts it. Thus it is
neutrally stable.
The unstable wing has its center of gravity behind the
The Wright brothers flew from
center of lift. Now, if a gust increases the angle of attack, the
Kill Devil Hill, a few miles south
lift increases and there is a net rotational torque, which works
of Kitty Hawk.
to increase the angle of attack even further. Any disturbance
will be amplified. This situation is unstable.

Balance

The above description illustrates stability. As mentioned before, sta-
bility is the tendency to rotate back in the direction toward its initial
state. Balance, on the other hand, occurs when there is no net torque
on the airplane. Therefore, to be balanced, the center of lift and the
center of gravity must coincide. For the example of the symmetric
wing in the above section, only the neutrally stable wing is balanced.
Our desire is to have an airplane that is balanced and stable. But if
the only balanced wing is neutrally stable, then we cannot have both
stability and balance at the same time. This is where the horizontal
stabilizer enters the picture.

The Horizontal Stabilizer
Introducing the horizontal stabilizer allows adjustment of the center of
lift. If the horizontal stabilizer is lifting upward, it moves the center
of lift aft. If it is pulling down, it moves the center of lift forward. So
the horizontal stabilizer can be used to balance the airplane. But how
does it contribute to stability? Before we can answer this question, you
must understand the concept of the neutral point.
The example of the wing in the previous section showed stable,
neutrally stable, and unstable configurations. This was determined by
the location of the center of gravity with respect to the center of lift of
the wing. For the symmetric wing described, the center of lift remains
fixed and is called the neutral point of the wing. This means that