LATERAL STABILITY

LATERAL STABILITY

Lateral stability is stability around the longitudinal axis, or roll stability.
Lateral stability is achieved through (1) dihedral, (2) sweepback, (3) keel effect

Dihedral

 
The dihedral angle is the angle that each wing makes with the horizontal. The purpose of dihedral is to improve lateral stability. If a disturbance causes one wing to drop, the unbalanced force produces a sideslip in the direction of the downgoing wing. This will, in effect, cause a flow of air in the opposite direction to the slip. This flow of air will strike the lower wing at a greater angle of attack than it strikes the upper wing. The lower wing will thus receive more lift and the airplane will roll back into its proper position.
Since dihedral inclines the wing to the horizontal, so too will the lift reaction of the wing be inclined from the vertical. Hence an excessive amount of dihedral will, in effect, reduce the lift force opposing weight.
Some modern airplanes have a measure of negative dihedral or anhedral, on the wings and/or stabilizer. The incorporation of this feature provides some advantages in overall design in certain type of airplanes. However, it does have an effect, probably adverse, on lateral stability.

Keel Effect

Dihedral is more usually a feature on low wing airplanes although some dihedral may be incorporated in high wing airplanes as well.
Most high wing airplanes are laterally stable simply because the wings are attached in a high position on the fuselage and because the weight is therefore low. When the airplane is disturbed and one wing dips, the weight acts as a pendulum returning the airplane to its original attitude.

Sweepback

A sweptback wing is one in which the leading edge slopes backward. When a disturbance causes an airplane with sweepback to slip or drop a wing, the low wing presents its leading edge at an angle that is perpendicular to the relative airflow. As a result, the low wing acquires more lift, rises and the airplane is restored to its original flight attitude.

Sweepback also contributes to directional stability. When turbulence or rudder application causes the airplane to yaw to one side, the right wing presents a longer leading edge perpendicular to the relative airflow. The airspeed of the right wing increases and it acquires more drag than the left wing. The additional drag on the right wing pulls it back, yawing the airplane back to its original path.

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