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what is The usage of thin aerofoils for high-speed flight

I tried those answers:

 They are perfect for controlling the local angle of attack, for instance in turbomachinery and flaps.

 At high speeds such as supersonic and transonic, because conventional airfoils cause wave drag.

 They are used at high-speed aircrafts such as Military aircraft.

but am not sure if they are correct!....or if they are correct someone can add more answers.

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    $\begingroup$ The third is circular reasoning. You're saying the usage of X for high-speed flight is that X is used for high-speed flight. (I'm not commenting on the correctness of the statement.) $\endgroup$
    – user
    Oct 31, 2018 at 15:24
  • $\begingroup$ This sounds like a homework question. $\endgroup$
    – zymhan
    Nov 2, 2018 at 16:57

1 Answer 1

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The wording of the question doesn't really make sense. What is the usage of thin airfoils for high speed flight? Well, strictly speaking, what their usage is... is defining the streamwise profile of the wing.

Based on the answers you've provided, I'll assume the question is actually asking:

Why are thin airfoils used for high speed flight?

Lets look at your 3 answers:

They are perfect for controlling the local angle of attack, for instance in turbomachinery and flaps.

Perfect is a strong word. Thin airfoils certainly could be well-suited for those applications, seeing as you'd want to avoid having really thick airfoils due to possible stall and blockage. I'm not sure what is meant by "Controlling the local angle of attack". In the case of both a flap and a compressor blade (turbomachinery), the airfoil doesn't determine what angle of attack the airfoil sees. That depends on how it's mounted or actuated. For a compressor blade, the angle of attack it sees would be influenced by the preceding stator. Perhaps the answer is trying to imply you can better control the flow angle coming off a thin airfoil than off a thick one? But the usage of angle of attack implies that somehow changing the airfoil will change the local angle of attack. This is untrue because the local angle of attack is defined between the chord line and the freestream. Simply changing the airfoil thickness while holding all else constant will not change the local angle of attack.

At high speeds such as supersonic and transonic, because conventional airfoils cause wave drag.

This is true. Wave drag rises more slowly for thin airfoils than thick ones. Onset of shock is delayed leading to lower wave drag.

They are used at high-speed aircrafts such as Military aircraft.

This is true, but it doesn't really answer your question (if I am interpreting it correctly). It is describing a use case for thin airfoils, not why they are used for high speed flight.

I had a very difficult time parsing your question because it's difficult to tell if It's asking about when thin airfoils are used or why they are used.

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