Sometimes described as 'negative
dihedral', anhedral refers to an
aircraft wing configuration where the wing tips are lower than the
root, or center, where they meet each other or the
fuselage. Most
balsa wood toy airplanes, both
gliders and
rubber band powered types, have dihedral wings. The
AV-8B Harrier 'jump jet' is an example of a modern plane with anhedral wings. Dihedral contributes to the
inherent stability of an aircraft, while anhedral reduces inherent stability.
The
lift vector of a wing is generally
perpendicular to the plane of the wing. When an aircraft with dihedral wings is
banked to one side, the lower wing's lift vector is more perpendicular to
gravity than it was before, and much more than the other wing, so the lower wing tends to lift more - this
restoring force rolls the plane back to level flight (without further
control input). An aircraft with anhedral will conversely have the higher wing producing the most force perpendicular to gravity, and will thus tend to settle to a certain
bank angle to one side or the other (and almost certainly will thus be in a
turn as well).
Hang gliders employ very small amounts of anhedral in order to be able to be controlled in the
roll axis by the relatively small forces of pilot
weight shift. Wing
sweep and both physical
twist and
aerodynamic twist make hang gliders
stable in
yaw and
pitch, respectively, and also contribute to roll stability a bit, which is countered by anhedral. Control surfaces, such as
ailerons, are aerodynamic controls, and thus easily overcome the strong restorative force of dihedral. Hang gliders
by definition lack aerodynamic controls and so cannot use dihedral. By adjusting the mix of sweep, twist, and anhedral, a hang glider can be designed to be very stable, rolling back to level flight firmly with more roll resistance at higher angles, or it can be designed to prefer a given bank angle, or can even roll to level from
shallow bank angles and have very light roll pressure at higher bank angles. Since hang gliders spend a lot of time circling in lift (like
hawks and
eagles, which also exhibit anhedral (
vultures have dihedral)) such fine-tuned combinations are preferred.