Force components acting on an airfoil

  • Thread starter FEAnalyst
  • Start date
31
7

Summary:

Are these formulas for lift and drag correct ?

Main Question or Discussion Point

Hi,

I have a simple question but I want to be 100% sure that my reasoning is correct. Take a look at this picture showing forces acting on an airfoil:

airfoil forces.PNG

Green forces (X and Y components) are known from CFD software but I need the values of blue components (lift and drag). Of course for zero angle of attack they will be equal to each other but I need formulas for nonzero angle. In the literature I've found the following equation for lift: $$F_{L}=F_{X} \sin{\alpha} - F_{Y} \cos{\alpha}$$ From this I figured out the formula for drag: $$F_{D}=F_{X} \cos{\alpha} + F_{Y} \sin{\alpha}$$ Are these equations correct ? If not then how they should look like ?

Thanks in advance for your help
 

Answers and Replies

319
140
Y axis should be inverted in diagram above for equation you provide to be correct.
 
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It's always the part that one is most confident about that turns out to be wrong. So be careful of any "of course" statements. :>)
 
31
7
Thanks for reply. Is the first formula (for lift) correct and second one (for drag) wrong ? Could you tell me what should be changed ? Only sign so that it becomes ##F_{D}=F_{X} \cos{\alpha}- F_{Y} \sin{\alpha}## ?
 
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Either change your diagram or change every sign of ##F_Y## in your equations.

PS. Always test the cases of ##\alpha=0## and ##\alpha=90## to make sure that your sign convention is correct.
 
31
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So, just to make sure, for the diagram I attached to my first post the correct formulas are: ##F_{L}=F_{X} \sin{\alpha}+ F_{Y} \cos{\alpha}## and ##F_{D}=F_{X} \cos{\alpha}- F_{Y} \sin{\alpha}##, right ?
 
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Looks good to me.
 
31
7
Thank you very much. Apparently there was an error in the article where I’ve found the first formula which confused me and I couldn’t figure out how to derive it.
 
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Thank you very much. Apparently there was an error in the article where I’ve found the first formula which confused me and I couldn’t figure out how to derive it.
Any time you get such an equation from a different source, the chances are good that they do not use the same coordinates and sign convention. So you must make the appropriate conversions.
 

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