|Lateral control is obtained through the use of ailerons,
and on some airplanes the aileron trim tabs. The ailerons are movable surfaces
hinged to the outer trailing edge of the wings, and attached to the cockpit
control column by mechanical linkage. Moving the control wheel or stick
to the right raises the aileron on the right wing and lowers the aileron
on the left wing. Moving the control wheel or stick to the left reverses
this and raises the aileron on the left wing and lowers the aileron on
the right wing. When an aileron is lowered, the angle of attack on that
wing will increase, which increases the lift. This permits rolling the
airplane laterally around the longitudinal axis. [Figure 1-30]
Many airplanes are equipped with an aileron trim tab which is a small movable part of the aileron hinged to the trailing edge of the main aileron. These trim tabs can be moved independently of the ailerons. Aileron trim tabs function similar to the elevator trim tabs. Moving the trim tabs produces an effect on the aileron which in turn affects the entire airplane. If the trim tab is deflected upward, the aileron is deflected downward, increasing the angle of attack on that wing, resulting in greater lift on that wing. The reverse is true if the trim tab is deflected downward.
Directional stability is displayed around the vertical axis and depends
to a great extent on the quality of lateral stability. If the longitudinal
axis of an airplane tends to follow and parallel the flightpath of the
airplane through the air, whether in straight flight or curved flight,
that airplane is considered to be directionally stable.
Directional stability is accomplished by placing a vertical stabilizer or fin to the rear of the center of gravity on the upper portion of the tail section. The surface of this fin acts similar to a weather vane and causes the airplane to weather vane into the relative wind. If the airplane is yawed out of its flightpath, either by pilot action or turbulence, during straight flight or turn, the relative wind would exert a force on one side of the vertical stabilizer and return the airplane to its original direction of flight.
Wing sweepback aids in directional stability. If the airplane is rotated about the vertical axis, the airplane will be forced sideways into the relative wind. Because of sweepback this causes the leading wing to present more frontal area to the relative wind than the trailing wing. This increased frontal area creates more drag, which tends to force the airplane to return to its original direction of flight.
The combined effects of the vertical stabilizer (fin) and sweepback can be compared with feathers of an arrow. It would be difficult to imagine an arrow traveling through the air sideways at any appreciable rate of speed.