Keel effect: Difference between revisions
Emphasizing that gravity has nothing to do with the keel effect; it arises from the geometry of the aerodynamic forces alone. |
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In [[aeronautics]], the '''keel effect''' (also known as the '''pendulum effect''' or '''pendulum stability'''<ref name="Hitchens2015">{{cite book|last1=Hitchens|first1=Frank|title=The Encyclopedia of Aerodynamics|date=2015|publisher=Andrews UK Limited|isbn=9781785383250|url=https://books.google.com/books?id=Zjv0CgAAQBAJ&q=keel+effect&pg=PT369|accessdate=13 September 2017|language=en}}</ref>) is the result of the sideforce-generating surfaces being above or below the [[center of gravity (aircraft)|center of gravity]] of the aircraft. Along with [[dihedral (aeronautics)|dihedral]], [[sweepback]], and [[weight distribution]], keel effect is one of the four main design considerations in aircraft [[Flight dynamics (aircraft)#Lateral modes|lateral stability]].<ref name="FAA2017">{{cite book|last1=Administration|first1=Federal Aviation|title=Pilot's Handbook of Aeronautical Knowledge|date=2017|publisher=Skyhorse Publishing, Inc.|isbn=9781510726185|url=https://books.google.com/books?id=L5TfDQAAQBAJ&q=%22Keel+effect%22&pg=PT266|accessdate=13 September 2017|language=en}}</ref> |
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In aeronautics, '''keel effect''' is the result of the sideforce-generating surfaces being above (or below) the [[center of mass]] (which coincides with the [[center of gravity (aircraft)|center of gravity]]) in ''any'' aircraft. Examples of such 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 (aircraft)|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. |
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==Mechanism== |
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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, the sidewards 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. 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. |
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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. |
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The effect is an important consideration in [[seaplane]] design where pontoon floats generate strong sideforces with a long moment arm.<ref name="Flying1945">{{cite book|title=Flying Magazine|date=December 1945|pages=82|url=https://books.google.com/books?id=7Uq_TImQyQYC&q=keel+effect&pg=PA82|accessdate=13 September 2017|language=en}}</ref> |
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{{reflist}} |
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[[Category:Aerodynamics]] |
[[Category:Aerodynamics]] |
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Latest revision as of 22:49, 11 November 2024
In aeronautics, the keel effect (also known as the pendulum effect or pendulum stability[1]) is the result of the sideforce-generating surfaces being above or below the center of gravity of the aircraft. Along with dihedral, sweepback, and weight distribution, keel effect is one of the four main design considerations in aircraft lateral stability.[2]
Mechanism
[edit]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.[3]
References
[edit]- ^ Hitchens, Frank (2015). The Encyclopedia of Aerodynamics. Andrews UK Limited. ISBN 9781785383250. Retrieved 13 September 2017.
- ^ Administration, Federal Aviation (2017). Pilot's Handbook of Aeronautical Knowledge. Skyhorse Publishing, Inc. ISBN 9781510726185. Retrieved 13 September 2017.
- ^ Flying Magazine. December 1945. p. 82. Retrieved 13 September 2017.
- Illman, Paul; The Pilot's Handbook of Aeronautical Knowledge; Fig 2.34