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Stability and Control 107

changing the pitch angle. This comes under the category of handling
properties, which will be discussed later. So sizing a horizontal
stabilizer involves a combination of versatility in center of gravity,
weight, drag, and handling properties.

Directional Stability

In the previous section we discussed only stability in pitch, known as
longitudinal stability. In Chapter 1 you were introduced to two other
axes, roll and yaw. Roll stability, known as lateral stability, was covered
in detail in Chapter 3, on “Wings.” The effects of dihedral and sweep
were presented and will not be repeated here. Directional stability is
the stability in the yaw axis, and gives rise to the vertical stabilizer. The
vertical stabilizer and rudder serve the same function as the horizontal
stabilizer and elevator, except in yaw, instead of pitch. The rudder is
used for control and the vertical stabilizer is for stability. The main
function of the vertical stabilizer is to help the airplane weathervane
and keep the nose pointed into the direction of flight.
The desire for directional stability is to have the airplane always
line itself with the wind. So, if a gust temporarily perturbs the
direction the nose is pointed, the tail will have a nonzero
angle of attack with the airflow, as shown in Figure 4.6. This
The Spirit of St. Louis took less
causes a restoring force to realign the tail with the direction of
than 2 months to design and
travel. The effects of misalignment with the flight path are
build.
primarily high drag and poor turn coordination.
The size of the vertical stabilizer depends on several factors. For a
single-engine airplane, the requirement that sets the minimum size for
the vertical stabilizer is that the vertical area of the airplane aft of the
center of gravity be larger than the vertical area forward of the center
of gravity. This is the same requirement that puts feathers on arrows
for stability. A larger vertical stabilizer is needed to counter propeller
rotation effects and adverse yaw in a turn, which was discussed in
Chapter 3. A single-engine airplane can get away with the minimum-
size vertical stabilizer but will require more work on the pilot’s part.
For multiengine airplanes the size of the tail is dictated by the
torque caused by the loss of one engine. The net thrust being off center