Mechanism
Examples of sideforce-generating surfaces are the vertical stabilizer, rudder, and parts of the fuselage. When an aircraft is in a Angle of sideslip, sideslip, these surfaces generate sidewards Lift (force), lift forces. If the surface is above or below the center of gravity (aircraft), center of gravity, the sidewards lift forces generate a rolling moment. This "rolling moment caused by sideslip" is "Dihedral (aircraft), dihedral effect". Keel effect is the contribution of these side forces to rolling moment (as sideslip increases), i.e. keel effect is the contribution of the side forces to dihedral effect. Sideforce producing surfaces ''above'' the center of gravity will ''increase'' dihedral effect, while sideforce producing surfaces below the center of gravity will decrease dihedral effect. Increased dihedral effect (helped or hindered by keel effect) results in a greater tendency for the aircraft to return to level flight when the aircraft is put into a bank. Or, reduces the tendency to diverge to a greater bank angle when the aircraft starts wings-level. Keel effect is also called "Pendulum Effect" because a lower center of gravity increases the effect of sideways forces (above the center of gravity) in producing a rolling moment. This is because the ''moment arm is longer'', ''not'' because of gravitational forces. A low center of gravity is like a pendulum (which has a ''very'' low center of gravity). The effect is an important consideration in seaplane design, where pontoon floats generate strong sideforces with a long moment arm.References
{{reflist * Illman, Paul; The Pilot's Handbook of Aeronautical Knowledge; Fig 2.34 Aerodynamics