Dihedral is the upward angle of the wings or tailplane of a fixed-wing aircraft. It is used to enhance the lateral stability of an aircraft. Dihedral angle has a strong influence on dihedral effect. Dihedral effect is the amount of roll moment produced in proportion to the amount of sideslip. Dihedral effect is a critical factor in the stability of an aircraft about the roll axis.
Military fighter aircraft often have near zero or even anhedral angle reducing dihedral effect and hence reducing the stability of the spiral mode. This increases maneuverability which is desirable in fighter-type aircraft. Anhedral angles are also seen on aircraft with a high mounted wing, such as the very large Antonov An-124 and Lockheed C-5 Galaxy cargo aircraft. In such designs, the high mounted wing is above the aircraft’s center of gravity which confers extra dihedral effect due to the keel. Such designs can be excessively stable, so anhedral angle on the wing is added to cancel out some of the dihedral effect so that the aircraft can be more easily maneuvered.