Dihedral (aeronautics)
Dihedral effect is a critical factor in the stability of an aircraft about the roll axis (the spiral mode).
Longitudinal dihedral is a comparatively obscure term related to the pitch axis of an airplane.
Dihedral effect[1] of an aircraft is a rolling moment resulting from the vehicle having a non-zero angle of sideslip.
In aviation, the usage "dihedral" evolved to mean the positive, up angle between the left and right wings, while usage with the prefix "an-" (as in "anhedral") evolved to mean the negative, down angle between the wings.
The aerodynamic stabilizing qualities of a dihedral angle were described in an influential 1810 article by Sir George Cayley.
[note 3] The dihedral effect does not contribute directly to the restoring of "wings level", but it indirectly helps restore "wings level" through its effect on the spiral mode of motion described below.
Aircraft designers may increase dihedral angle to provide greater clearance between the wing tips and the runway.
Dihedral effect is defined simply to be the rolling moment caused by sideslip and nothing else.
Rolling moments caused by other things that may be related to sideslip have different names.
Since dihedral effect is noticed by pilots when "rudder is applied", many pilots and other near-experts explain that the rolling moment is caused by one wing moving more quickly through the air and one wing less quickly.
Dihedral effect helps stabilize the spiral mode by tending to roll the wings toward level in proportion to the amount of sideslip that builds up.
Yaw stability created by the vertical fin opposes the tendency for dihedral effect to roll the wings back level by limiting sideslip.
Each increases or decreases total aircraft dihedral effect to a greater or lesser degree.
For instance, two small biplanes produced from the 1930s to 1945 by Bücker Flugzeugbau in Germany, the Bücker Jungmann two-seat trainer and the Bücker Jungmeister aerobatic competition biplane, were designed with sweepbacks of approximately 11 degrees, which provided significant dihedral effect – in addition to their small dihedral angles having a similar but lesser effect.
This is caused by the center of lift and drag being further above the CG and having a longer moment arm.
The dihedral effect created by the very low vertical CG more than compensates for the negative dihedral effect created by the strong anhedral[note 6] of the necessarily strongly downward curving wing.
This can be unpleasant to experience, or in extreme conditions it can lead to loss of control or can overstress an aircraft.
Such designs can have excessive dihedral effect and so be excessively stable in the spiral mode, 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.
Some older aircraft such as the Beriev Be-12 were designed with gull wings bent near the root.
[citation needed] Others, such as the Vought F4U Corsair, used an inverted gull wing design, which allowed for shorter landing struts and extra ground clearance for large propellers and external payloads, such as external fuel tanks or bombs.
Modern polyhedral wing designs generally bend upwards near the wingtips (also known as tip dihedral), increasing dihedral effect without increasing the angle the wings meet at the root, which may be restricted to meet other design criteria.
The McDonnell Douglas F-4 Phantom II is one such example, unique among jet fighters for having dihedral wingtips.
This was added after flight testing of the flat winged prototype showed the need to correct some unanticipated spiral mode instability – angling the wingtips, which were already designed to fold up for carrier operations, was a more practical solution than re-engineering the entire wing.
