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Does the reference area changes with angle of attack?

Most sources says it is projected area. If it is projected area then it should depend on Angle of attack.

I want to calculate Lift and drag coefficient from a CFD simulation. I have got the Lift and drag force.

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  • $\begingroup$ That it is the projected and not the actual area is meant to eliminate dihedral. Then use the projection at AoA = 0°. $\endgroup$
    – Peter Kämpf
    Commented Jan 25, 2022 at 17:14

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No, it does not. It does not change even if the actual wing area does e.g. by extending Fowler flaps.

Lift is only very roughly proportional to wing area. It depends on the span and the chord and the profile in complex ways. By referencing to the wing area you are just getting a number that is of the same order for different wings so you can more easily see how good they are.

Referencing to different area for different angle of attack or different flap positions would just hide part of the effect of those on lift, so you reference to the same area for all flight regimes to have all the effect show in the lift coefficient.

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  • $\begingroup$ The idea of a coefficient is precisely that it contains all those factors likene geometry, angle of attack factors etc, and $\endgroup$
    – Apfelsaft
    Commented Jan 25, 2022 at 21:31
  • $\begingroup$ @CarlBerger, those factors are not separable from size. If you take a wing and scale it to double the area, it will not have the same coefficient of lift, nor the same critical angle of attack. $\endgroup$
    – Jan Hudec
    Commented Jan 25, 2022 at 21:39
  • $\begingroup$ my post was sent before ready. sorry. concerning the coefficients.. by definition: Lift = CL * p_Dyn * A, hence double the area means double the lift, when at the same lift coefficient. What you're trying to say is that e.g. lift coefficient is not constant when applying sheer geometric scaling, but noone questioned that. CL is anyway depending on lots of things (angle of attack, compressibility, ..) And yes to answer op's question, A remains constant. $\endgroup$
    – Apfelsaft
    Commented Jan 25, 2022 at 21:47
  • $\begingroup$ @CarlBerger, you kind of contradict yourself – since the coefficient does not stay the same when you apply scaling, double the area does not in practice mean double the lift. $\endgroup$
    – Jan Hudec
    Commented Jan 26, 2022 at 8:41
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The reference area is arbitrary, i.e. you can choose what your reference area is. But the idea of a reference area is that it is constant, and the influence of e.g. angle of attack on lift is reflected in the coefficient of lift. Some form of projected wing area is just the most common choice for airplanes (and there are certainly some good engineering reasons for choosing it).

If you compare drag coefficients for cars and airplanes, the former are usually based on frontal area, whereas the latter are based on what is roughly the wing area (typically including part of the fuselage), so you can't even compare dragginess between the two types of vehicles by just looking at the drag coefficient.

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