Dihedral (aeronautics)

Lore & Background

In geometry, dihedral angle is the angle between two planes, but aviation usage evolved to mean the positive up angle between left and right wings, with 'anhedral' for negative down angle. The aerodynamic stabilizing qualities of dihedral angle were described in an influential 1810 article by Sir George Cayley. Longitudinal dihedral is a comparatively obscure term related to the pitch axis, defined as the angle between the zero-lift axis of the wing and the zero-lift axis of the horizontal tail, also often called decalage.

Dihedral angle has important stabilizing effects on flying bodies because it strongly influences dihedral effect. Dihedral effect is the rolling moment resulting from a non-zero sideslip angle, and increasing dihedral angle increases this effect. However, many other parameters also influence dihedral effect, including wing sweep, vertical center of gravity, and the height and size of anything that changes sidewards force with sideslip. Aircraft designers may increase dihedral angle for wing clearance from the runway, especially on swept-wing aircraft, or to adjust overall dihedral effect when other design elements are harder to change.

Common confusions exist: dihedral effect is defined solely as rolling moment caused by sideslip, not by yaw rate or sideslip rate. It is not roll stability itself, but a contributing factor to spiral mode stability. The dihedral angle creates dihedral effect by causing greater angle of attack on the forward-yawed wing during sideslip, producing more lift on that wing and a rolling moment that tends to level the wings.

Reader's Guide

Dihedral angle and dihedral effect are central to aircraft stability analysis. Dihedral effect is a stability derivative (Clβ) representing change in rolling moment coefficient per degree of sideslip. Its purpose is to contribute to stability in the roll axis, particularly the spiral mode, sometimes called 'roll stability.' Dihedral effect does not directly restore wings level but indirectly helps through its effect on the spiral mode. During design, changing dihedral angle is a relatively simple way to adjust overall dihedral effect, compensating for other elements like wing sweep or wing mounting height. For example, low-wing aircraft typically have greater dihedral angle than high-wing aircraft because the 'highness' of a wing naturally creates more dihedral effect. The legacy of dihedral is its role in enabling stable flight across diverse aircraft types, from early biplanes like the deHavilland DH-4 to modern jets like the Airbus A380, where increased dihedral provides wingtip clearance. Understanding dihedral prevents common confusions among pilots and near-experts who may misattribute rolling moments from yaw rate or sideslip rate to dihedral effect.

Did You Know?

• Dihedral angle is also used in some types of kites such as box kites.
• Longitudinal dihedral is also often referred to as decalage.
• Dihedral effect is not caused by yaw rate nor by the rate of sideslip change.