LouW

The purpose of dihedral is to provide lateral stability. Lateral stability is the tendency of an airplane to roll away from a sideslip. In normal flying if a wing drops, the airplane will begin to slip toward the low wing. If it is laterally stable it will roll back wings level. An airplane that is laterally stable is easier to fly since it wants to right itself when disturbed by a gust or an inadvertent control input. Most training airplanes have positive dihedral.

The tendency to roll away from a sideslip is also called roll coupling. In aerobatic flight such as knife-edge flight, where the airplane is in a continuous sideslip, it requires aileron input opposing the roll tendency. For this reason aerobatic aircraft are usually designed with no dihedral. Inverted flight is another area where little or no dihedral is desired. While dihedral makes flying right side up easier, it makes inverted flight more difficult.

Lateral stability is affected by other factors than physical dihedral of the wing. The placement of the wing, height of the vertical tail, landing gear size and location all affect lateral stability somewhat. All else being equal, a high wing placement tends to have some lateral stability even with a flat wing. On the other hand a low wing placement tends to have negative lateral stability with a flat wing. A tall vertical fin will tend to have some lateral stability and a low tail will not, even with a flat wing.

An airplane with negative dihedral (anhedral) will roll in the same direction as the sideslip. Such an unstable aircraft can be flown but it takes constant attention on the controls since it not only doesn’t correct itself but also tends to diverge further.

With models, the amount of dihedral is largely a matter of personal preference. Beginners and sport flyers enjoy positive lateral stability where those into extreme aerobatics prefer none.

As for full-scale aircraft, FAR part 23 under which all small aircraft are presently certified requires positive lateral stability in all flight regimes. Many full-scale aerobatic aircraft are certified in the experimental and home built category and are not required to meet normal standards. A full-scale aircraft would be a real pain to fly (not to mention dangerous) without some lateral stability. Sure the Wright brothers did it as did many others in the early days, which is why the present standards are what they are. They are the result of often bitter experience.