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Why are the aerodynamic characteristics of a finite wing different from the properties of its airfoil section?

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  • $\begingroup$ Can you clarify what you have in mind by giving an example of a characteristic different between wing and airfoil? $\endgroup$ – mins Jan 22 '17 at 18:38
  • $\begingroup$ To be honest I dont really know...this was a question from my previous exam but I suppose it's related with lift and moment coefficient, for example $\endgroup$ – B737lover Jan 22 '17 at 18:47
  • $\begingroup$ The airfoil is a 2D representation of a wing slice, but the wing is a 3D object, non rectangular, usually swept and twisted, with different airfoils from root to tip, with non constant angle of attack. Wing characteristics are "composite". $\endgroup$ – mins Jan 22 '17 at 18:59
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Because the wing is finite, while the airfoil is assumed to stretch out sideways indefinitely. The consequence is that on a wing you get spanwise flow outside of the plane of symmetry, while on the airfoil all flow speeds are in the airfoil's plane only.

Spanwise flow is caused by the pressure difference between lower and upper side which can equalise around the wingtips. This lets the pressure difference diminish towards the tips and especially in the rear part of the wingtips. The consequences are:

  • Lift is smaller on a "real" wing relative to a wing of the same airfoil with no such sideways flow.
  • The center of pressure of a "real" wing is ahead of the one of the airfoil.
  • On a swept wing the air transverses the surface differently than it transverses the airfoil. Basically, a sideways speed component is added to the speed around the airfoil.
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  • $\begingroup$ Thank you so much for your help Peter!!! You made my day! $\endgroup$ – B737lover Jan 23 '17 at 21:53

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