Stability about the airplane's longitudinal axis, which extends form nose to tail, is called lateral stability. This helps to stabilize the lateral or rolling effect when one wing gets lower than the wing on the opposite side of the airplane. There are four main design factors which make an airplane stable laterally - dihedral, keel effect, sweepback, and weight distribution. It will be seen in later discussions that these factors also aid in producing yawing or directional stability.
The most common procedure for producing lateral stability is to build the wings with a dihedral angle varying from one to three degrees. In other words, the wings on either side of the airplane join the fuselage to form a slight V or angle called "dihedral," and this is measured by the angle made by each wing above a line parallel to the lateral axis.
The basis of rolling stability is, of course, the lateral balance of forces produced by the airplane's wings. Any imbalance in lift results in a tendency for the airplane to roll about its longitudinal axis. Stated another way, dihedral involves a balance of lift created by the wings' angle of attack on each side of the airplane's longitudinal axis.
If a momentary gust of wind forces one wing of the airplane to rise and the other to lower, the airplane will bank. When the airplane is banked without turning, it tends to sideslip or slide downward toward the lowered wing (Fig. 17-29). Since the wings have dihedral, the air strikes the low wing at much greater angle of attack than the high wing. This increases the lift on the low wing and decreases lift on the high wing, and tends to restore the airplane to its original lateral attitude (wings level); that is, the angle of attack and lift on the two wings are again equal.
The effect of dihedral, then, is to produce a rolling moment tending to return the airplane to a laterally balanced flight condition when a sideslip occurs.
The restoring force may move the low wing up too far, so that the opposite wing now goes down. If so, the process will be repeated, decreasing with each lateral oscillation until a balance for wings level flight is finally reached.
Conversely, excessive dihedral has an adverse effect on lateral maneuvering qualities. The airplane may be so stable laterally that it resists any intentional rolling motion. For this reason, airplanes which require fast roll or banking characteristics usually have less dihedral than those which are designed for less maneuverability.
The contribution of sweepback to dihedral effect is important because of the nature of the contribution. In a sideslip the wing into the wind is operating with an effective decrease in sweepback while the wing out of the wind is operating with an effective increase in sweepback. The reader will recall that the swept wing is responsive only to the wind component that is perpendicular to the wing's leading edge. Consequently, if the wing is operating at a positive lift coefficient, the wing into the wind has an increase in lift, and the wing out of the wind has a decrease in lift. In this manner the swept back wing would contribute a positive dihedral effect and the swept forward wing would contribute a negative dihedral effect.
During flight, the side area of the airplane's fuselage and vertical fin react to the airflow in much the same manner as the keel of a ship. That is, it exerts a steadying influence on the airplane laterally about the longitudinal axis.
Such laterally stable airplanes are constructed so that
the greater portion of the keel area is above and behind the center of
gravity (Fig. 17-30). Thus, when the airplane slips to one side, the combination
of the airplane's weight and the pressure of the airflow against the upper
portion of the keel area (both acting about the CG) tends to roll the airplane
back to wings level flight.