Static Stability. Besides being supported in flight by lift and propelled through the air by
thrust, an airplane is free to revolve or move around three axes. These axes may be thought of as
axles around which the airplane revolves much like a wheel does. Each axis is perpendicular to
the other two and all three intersect at the airplane's center of gravity (CG). The point around
which the airplane's weight is evenly distributed or balanced is considered the CG of the
airplane. Figure 2-2 depicts the axes about which the aircraft rotates.
Directional (Z) axis
Longitudinal (X) axis
Lateral (Y) axis
Figure 2-2 Aircraft Reference Axes
If an arrow having no feathered tail is shot from a bow, it will usually wobble or fall end-over-
end as it travels, since there is no force produced to bring it back to its original point first travel.
The arrow is made stable, however, by adding pieces of feather near the rear of its shaft. Then,
when the arrow is shot and begins to wobble, turn, or yaw, the air strikes the tail feathers and
turns the arrow back to a straight path. This correction action continues as long as the arrow has
sufficient forward motion.
An airplane wing, by itself, is also unstable. It would flip over and continue to flip end-over-end
as it flutters to the ground. Like the unstable arrow, the unstable wing needs some kind of "tail
feathers" to balance it and keep it on a straight course. Airplanes have their "tail feathers" in the
form of horizontal and vertical stabilizers located at the rear of the fuselage.
Again, like the feathered arrow, another most important factor producing directional stability is
the weather vaning effect created by the fuselage and vertical fin of the airplane. It keeps the
airplane headed into the relative wind. If the airplane yaws, or skids, the sudden rush of air
against the surface of the fuselage and fin quickly forces the airplane back to its original
direction of flight.
If all the upward lift forces on the wing were concentrated in one place there would be
established a center of lift, which is called the center of pressure (CP). Rarely, though, are the
CP and CG located at the same point. The locations of these centers in relation to each other
have a significant effect on the stability of the airplane. If the wing's CP is forward of the
airplane's CG, the airplane will always have a tendency to nose up and will have an inherent
INTRODUCTION TO T-34C AERODYNAMICS 2-5