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Chapter 2 - Aerodynamics: The Wing Is the Thing
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Stability and the Neutral Point

Can you imagine any flight school hiring an instructor who wasn't stable? If you were kidding around and said that
this instructor's new shirt is proof that there's a family in Costa Mesa without curtains, he'd go ballistic and you'd
never be able to calm him down. That's a lack of "emotional" stability. Not surprisingly, stability also applies to air-
planes, but in the mechanical sense.
For instance, suppose an airplane is trimmed for level flight at a constant airspeed. The airplane is in a state of
equilibrium. If the airplane encounters a sudden gust or the pilot bumps the flight controls, a stable airplane tends to
return to its previous undisturbed pitch attitude (Figure 101). We call this static stability. On the other hand, an air-
plane that's unstable tends to deviate from its original pitch attitude once it's disturbed. We call this static instability
(Figure 102). And in both of these examples, we're focusing on
pitch stability that's expressed along the "long" axis of the
airplane, its longitudinal axis, otherwise known as longi-
tudinal stability. So let's see what can make an airplane
statically stable or statically unstable.
Let's say our airplane is trimmed for straight and level
unaccelerated flight with its CG located slightly ahead of the
wing's center of pressure (CP), also known as its center of
lift. As we've already learned, the wing's lift is expressed at the
center of pressure, which is located aft of the CG. This pro-
duces a nose-down pitching moment about the CG (Figure
103, position A). To prevent the airplane from nosing over,
the tail (the horizontal stabilizer) has a slight negative angle
of attack. This produces lift by the tail directed in a down-
Fig. 101
ward direction (Figure 103, position B). Since the tail is
located a relatively long distance from the CG, it only
needs to produce about 1/10th of the wing's total lift to
produce a nose-up pitching moment sufficient to counter
the wing's nose-down pitching moment. This airplane is
trimmed for level flight and is in a state of equilibrium.
Now let's see how airplane A in Figure 104 behaves
when it encounters a vertical gust of air. The vertical gust
of air temporarily increases the wing’s angle of attack and
temporarily decreases the tail’s angle of attack (Figure 104,
Fig. 102
position B). That's right. The vertical gust of air means that the
relative wind striking the wing now has a slight upward component
of wind added to it. This bends the wing’s relative wind upward
slightly, temporarily increasing the wing’s angle of attack, increasing
lift, and causing the nose to pitch upward.
Since the horizontal stabilizer is attached to this airplane to give it
a negative angle of attack, any vertical gust acts to decrease the lift on
the tail. Why? The fixed portion of the tail is, after all, a miniature wing.
A gust of air from below the horizontal stabilizer adds a slight upward
component of wind (gust) to the relative wind, thus reducing the
tail's angle of attack and reducing the downward lift it pro-
duces. Any reduction in the downward-acting lift produced
by the tail, allows the tail to rise, which temporarily
increases the airplane’s nose-down pitching moment.
Fig. 103 The net result in the airplane's pitch attitude tends to
move in the direction of its previous undisturbed con-
dition. In other words, the airplane wants to return
to a level flight attitude. We call an airplane that
responds this way a statically stable airplane.
Now you know why the tail is designed to be large
enough and positioned aft far enough so that any change in
lift on this surface compensates for a change in lift on the