Lift and Drag Which frame of reference?

[phys_student1]

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?

 

[Greg Bernhardt]

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?

 

[phys_student1]

Not really. I still do not have answer.

 

[boneh3ad]

It all depends on how C_D and C_L 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.

 

[phys_student1]

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.