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I think that statement is true, but in the book "Fundamental of aerodynamic", page 35: the aerodynamic forces R don't depend on the skin material, 5 variables are mentioned but none of them is relate to skin material of wings. So why is that??? enter image description here

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  • $\begingroup$ Not on the material, but on the surface roughness and flexibility. $\endgroup$ – mins Jun 11 '17 at 17:13
  • $\begingroup$ yeah, I mean that, surface roughness but the question is still the same. why are 5 variables not related to the surface roughness ? $\endgroup$ – Dat Jun 11 '17 at 17:32
  • $\begingroup$ Effect of Surface Roughness on Characteristics of Aerofoils N.A.C.A. 0012 and R.A.F. 34, written 81 years ago. $\endgroup$ – mins Jun 11 '17 at 18:55
  • $\begingroup$ don't simply post an image full of text, tape the sentences that are needed to understand the question and eventually give a link to the book. $\endgroup$ – Federico Jun 13 '17 at 7:28
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Generally, skin materials will impact the local deformation of the surface of the airfoil (think parachute). They also can impact the local boundary layer, with things like surface texture (think Reynolds number changes). Generally speaking these effects are tertiary, and not a prime mover in R.

If you are looking for something more comprehensive, then I will have to break open the book.

Addendum #1 Understood the question is redefined. In this case, I agree with @mins and his commented reference to the Jones and Williams paper. You can find it readily on the net. It addresses the surface texture and corresponding Reynolds number impact. There are some recent works which take that further and model it nicely, but that is beyond the scope of the question.

In general, there are two regions of Reynolds, laminar and turbulent, and how they impact drag is different. Perhaps the question should be reformulated.

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  • $\begingroup$ sorry, I mean surface roughness, forget about the deformation things. I think drag depend much on surface roughness but the page did not mention that. that confuses me $\endgroup$ – Dat Jun 11 '17 at 17:38
  • $\begingroup$ There is induced drag and parasitic drag. Parasitic drag is impacted more by surface roughness. $\endgroup$ – mongo Jun 12 '17 at 4:41
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Surface roughness has a minor impact, except in a range of Reynolds numbers mostly encountered in General Aviation. Only when you use an airfoil that has been designed for laminar flow but experiences an early transition will the performance be impacted significantly.

The book you quote assumes a certain minimum standard of build quality: Flush riveting, smooth surface and a solid, stiff structure. Once you move to more advanced topics, more variables will need to be added, such as elastic deformation in flight or substandard surface quality. At the introduction level some simplifications are justified. The lessons derived from this reasoning are valid and valuable. When you progress, they will be expanded with more complex concepts. Trust me.

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