Figure 2-2. Aircraft Reference Axes

T-34C CONTACT

CHAPTER TWO

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

Yaw

Roll

Longitudinal (X) axis

Pitch

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