Stability about the airplane's vertical axis (the sideways
moment), is called yawing or directional stability.
|Yawing or directional stability is the more easily achieved stability
in airplane design. The area of the vertical fin and the sides of the fuselage
aft of the center of gravity are the prime contributors which make the
airplane act like the well known weathervane or arrow, pointing its nose
into the relative wind.
In examining a weathervane it can be seen that if exactly the same amount of surface were exposed to the wind in front of the pivot point as behind it, the forces fore and aft would be in balance and little or no directional movement would result. Consequently, it is necessary to have a greater surface aft of the pivot point that forward of it.
Similarly in an airplane, the designer must ensure positive directional stability by making the side surface greater aft than ahead of the center of gravity (Fig. 17-31). To provide more positive stability aside from that provided by the fuselage, a vertical fin is added. The fin acts similar to the feather on an arrow in maintaining straight flight. Like the weathervane and the arrow the farther aft this fin is placed and the larger its size, the greater the airplane's directional stability.
If an airplane is flying in a straight line and a sideward gust of air gives the airplane a slight rotation about its vertical axis (i.e., the right) the motion is retarded and stopped by the fin because while the airplane is rotating to the right, the air is striking the left side of the fin at an angle. This causes pressure on the left side of the fin, which resists the turning motion and slows down the airplane's yaw. In doing so it acts somewhat like the weathervane by turning the airplane into the relative wind.
The initial change in direction of the airplane's flightpath is generally slightly behind its change of heading. Therefore, after a slight yawing of the airplane to the right, there is a brief moment when the airplane is still moving along its original path, but its longitudinal axis is pointed slightly to the right.
The airplane is then momentarily skidding sideways, and during that moment (since we assume that although the yawing motion has stopped, the excess pressure on the left side of the fin still persists) there is necessarily a tendency for the airplane to be turned partially back to the left. That is, there is a momentary restoring tendency caused by the fin.
This restoring tendency is relatively slow in developing and ceases when the airplane stops skidding. When it ceases, the airplane will be flying in a direction slightly different from the original direction. In other words, it will not of its own accord return to the original heading; the pilot must reestablish the initial heading.
A minor improvement of directional stability may be obtained through sweepback. Sweepback is incorporated in the design of the wing primarily to delay the onset of compressibility during high speed flight. In lighter and slower airplanes, sweepback aids in locating the center of pressure in the correct relationship with the center of gravity. As we have learned, a longitudinally stable airplane is built with the center of pressure aft of the center of gravity.
Because of structural reasons, airplane designers sometimes cannot attach the wings to the fuselage at the exact desired point. If they had to mount the wings too far forward, and at right angles to the fuselage, the center of pressure would not be far enough to the rear to effect the desired amount of longitudinal stability. By building sweepback into the wings, however, the designers can move the center of pressure toward the rear. The amount of sweepback and the position of the wings then place the center of pressure in the correct location.
The contribution of the wing to static directional stability
is usually small. The swept wing provides a stable contribution depending
on the amount of sweepback but the contribution is relatively small when
compared with other components.