Lateral Stability about the Longitudinal Axis Lateral Stability about the Longitudinal Axis

Lateral stability is the stability displayed around the longitudinal axis of the airplane. An airplane that tends to return to a wings-level attitude after being displaced from a level attitude by some force such as turbulent air is considered to be laterally stable.

 Three factors that affect lateral stability are:
• Dihedral
• Sweepback
• Keel Effect
Dihedral is the angle at which the wings are slanted upward from the root to the tip. [Figure 1-27] The stabilizing effect of dihedral occurs when the airplane sideslips slightly as one wing is forced down in turbulent air. This sideslip results in a difference in the angle of attack between the higher and lower wing with the greatest angle of attack on the lower wing. The increased angle of attack produces increased lift on the lower wing with a tendency to return the airplane to wings-level flight. Note the direction of the relative wind during a slip by the arrows in figure 1-27.
Figure 1-27.—Effect of dihedral.
 Sweepback is the angle at which the wings are slanted rearward from the root to the tip. The effect of sweepback in producing lateral stability is similar to that of dihedral, but not as pronounced. If one wing lowers in a slip, the angle of attack on the low wing increases, producing greater lift. This results in a tendency for the lower wing to rise, and return the airplane to level flight. Sweepback augments dihedral to achieve lateral stability. Another reason for sweepback is to place the center of lift farther rearward, which affects longitudinal stability more than it does lateral stability. [Figure 1-28]
Figure 1-28.—Effect of sweepback.
Keel Effect
 Keel effect depends upon the action of the relative wind on the side area of the airplane fuselage. In a slight slip, the fuselage provides a broad area upon which the relative wind will strike, forcing the fuselage to parallel the relative wind. This aids in producing lateral stability. [Figure 1-29]
Figure 1-29.—Keel effect.