Lift and Drag Which frame of reference?

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Discussion Overview

The discussion revolves around the appropriate frame of reference for expressing lift and drag forces acting on an airplane, specifically whether these forces are defined in a stability frame or a wind frame. The context includes theoretical considerations and potential implications for calculations in aerodynamics.

Discussion Character

  • Debate/contested
  • Technical explanation

Main Points Raised

  • One participant questions whether the equations for lift and drag are defined in the stability frame or the wind frame, noting that different rotation matrices would be required to convert to the body frame.
  • Another participant suggests that the validity of the lift and drag equations in either frame depends on how the coefficients C_L and C_D were calculated, emphasizing that dynamic pressure is not frame dependent.
  • It is mentioned that the reference area used is crucial, and the frame of reference primarily affects the direction of the force vectors.
  • A later reply introduces uncertainty regarding the calculation of C_D and C_L, indicating they are often derived from CFD software and may involve Taylor expansions.

Areas of Agreement / Disagreement

Participants express differing views on the appropriate frame of reference for lift and drag calculations, with no consensus reached on which frame is definitively correct.

Contextual Notes

The discussion highlights the dependence on how coefficients are derived and the implications of using different reference areas, but does not resolve the specific mathematical or conceptual steps involved.

phys_student1
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Lift and Drag...Which frame of reference?

Hi,

When expressing the Lift and Drag acting on an airplane as:

F_{lift}=qSC_L
F_{drag}=qSC_D

where q is the dynamic pressure, S is the wing ref. area, and C's are the lift and drag coefficient.

My question: Some sources, like this one, claim these equations are 'defined' in stability frame {S}, while other claim they are in wind frame {W}. In either case you'll use a rotation matrix to go back to body frame {B}, but this matrix will differ whether going from W->B or S->B, wo which is correct?
 
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I'm sorry you are not generating any responses at the moment. Is there any additional information you can share with us? Any new findings?
 
Not really. I still do not have answer.
 
It all depends on how CD and CL were calculated in the first place. The equations themselves are valid in either frame since the dynamic pressure is not frame dependent in the sense that it uses velocity magnitude of the air relative to the body. The only thing that really matters is what reference area was used. The only place the frame would matter is in determining which direction the vector for each force points within each respective frame.
 
Well, C_D and C_L are Taylor expanded into many components which are usually obtained from a CFD software, so I am not sure.
 

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