In aeronautics, the keel effect (also known as the pendulum effect or pendulum stability) is the result of the sideforce-generating surfaces being above (or below) the

center of mass In physics, the center of mass of a distribution of mass in space (sometimes referred to as the balance point) is the unique point where the weighted relative position of the distributed mass sums to zero. This is the point to which a force may ...

(which coincides with the

center of gravity In physics, the center of mass of a distribution of mass in space (sometimes referred to as the balance point) is the unique point where the weight function, weighted relative position (vector), position of the distributed mass sums to zero. Thi ...

) in an aircraft. Along with dihedral,

sweepback A swept wing is a wing that angles either backward or occasionally forward from its root rather than in a straight sideways direction. Swept wings have been flown since the pioneer days of aviation. Wing sweep at high speeds was first investigate ...

, and

weight distribution Weight distribution is the apportioning of weight within a vehicle, especially cars, airplanes An airplane or aeroplane (informally plane) is a fixed-wing aircraft that is propelled forward by thrust from a jet engine, propeller, or rocke ...

, keel effect is one of the four main design considerations in aircraft lateral stability.

Mechanism

Examples of sideforce-generating surfaces are the vertical stabilizer, rudder, and parts of the fuselage. When an aircraft is in a

sideslip A slip is an aerodynamic state where an aircraft is moving ''somewhat'' sideways as well as forward relative to the oncoming airflow or relative wind. In other words, for a conventional aircraft, the nose will be pointing in the opposite directio ...

, these surfaces generate sidewards

lift Lift or LIFT may refer to: Physical devices * Elevator, or lift, a device used for raising and lowering people or goods ** Paternoster lift, a type of lift using a continuous chain of cars which do not stop ** Patient lift, or Hoyer lift, mobile ...

forces. If the surface is above or below the

, the sidewards lift 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), 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 In physics and mechanics, torque is the rotational equivalent of linear force. It is also referred to as the moment of force (also abbreviated to moment). It represents the capability of a force to produce change in the rotational motion of th ...

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 A seaplane is a powered fixed-wing aircraft capable of takeoff, taking off and water landing, landing (alighting) on water.Gunston, "The Cambridge Aerospace Dictionary", 2009. Seaplanes are usually divided into two categories based on their tec ...

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