Keel effect

Mechanism

Examples of sideforce-generating surfaces are the vertical stabilizer, rudder, and parts of the fuselage. When an aircraft is in a sideslip, these surfaces generate sidewards lift forces. If the surface is above or below the center of gravity, the sidewards forces generate a rolling moment. This rolling moment caused by sideslip is dihedral effect. Keel effect is the contribution of these side forces to rolling moment as sideslip increases. 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 after the aircraft is put into a bank. It 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.

The effect is an important consideration in seaplane design where pontoon floats generate strong sideforces with a long moment arm.

References

• Illman, Paul; The Pilot's Handbook of Aeronautical Knowledge; Fig 2.34

References

1. (2015). "The Encyclopedia of Aerodynamics". Andrews UK Limited.
2. (2017). "Pilot's Handbook of Aeronautical Knowledge". Skyhorse Publishing, Inc..
3. (December 1945). "Flying Magazine".