Dihedral determines lateral stability. So what is lateral stability and why and when would you want it?

Lateral stability is the tendency to roll away from a sideslip. A sideslip is when the airplane isn’t moving directly forward, but also a little to one side. When an airplane encounters a gust that causes one wing to drop, it will sideslip toward the low wing. If it has dihedral, it will roll away from the slip raising the wing back up. This tends to keep the wings level without input from the pilot.

When rudder only is applied, the airplane sideslips opposite the applied rudder. If it has dihedral it will roll in the same direction as the applied rudder (away from the sideslip). This is how an airplane is made to roll into a turn with rudder only.

When an airplane is flown in “knife edge” flight it is in a continuous sideslip and if it is laterally stable it will continue to try to roll out, and that tendency must be resisted with aileron. Lateral stability in this cause is often referred to as “roll coupling”.

An airplane that has lateral stability when upright due to dihedral becomes laterally unstable when flying inverted.

For a beginning pilot and/or for just relaxing easy flights, some lateral stability is desired. For better aerobatic performance, a minimum of lateral stability is desired.

Dihedral is the usual way of achieving lateral stability. When the wing is mounted above the fuselage, dihedral effect is enhanced, so that there is some lateral stability even with a flat wing. If the wing is mounted below the fuselage, dihedral effect is reduced so that a flat wing may have no lateral stability or even be slightly unstable. This is why low wing airplanes are usually built with more dihedral than high wing airplanes.

Another way to provide lateral stability is sweepback. A low wing airplane with a flat wing and some sweepback may be laterally stable. An advantage to this configuration is that the airplane is also stable when inverted. (This is one reason many pattern models have low mounted swept wings.)

The amount of dihedral is determined by the purpose of the airplane, and for experienced flyers is largely a matter of personal preference. For a low wing training or rudder only machine, Johng’s suggestion is a good starting place as is 2-3 degrees for a high wing model.

If you fly coordinating aileron and rudder (to produce a pure rolling motion), dihedral doesn’t have much effect on roll rate. (It doesn’t really effect roll damping,) However if you fly banking the wings with ailerons only, the resulting adverse yaw (which is actually a sideslip) will cause an opposing moment that will reduce aileron effectiveness.