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On aerofoil section, the force of lift acts perpendicular to, and the force drags acts parallel to the :

A: chord line B: flight path C: aerofoil section upper surface D: longitudinal axis

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    $\begingroup$ Free stream flow. You should have a word with your instructor. $\endgroup$ – JZYL Sep 1 at 19:29
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    $\begingroup$ why are you hesitating between all the possibility? It recommend you read the airfoil section of how it flies? $\endgroup$ – Manu H Sep 1 at 19:51
  • $\begingroup$ I change the title of your question so that it looks like a question. feel free to edit again if it does not reflect what you wanted to know $\endgroup$ – Manu H Sep 1 at 19:53
  • $\begingroup$ @Jimmy The instructor expects B (flight path) as the right answer. Which is correct if there is no wind. $\endgroup$ – Peter Kämpf Sep 1 at 20:39
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    $\begingroup$ @PeterKampf Flight path is inertial. It has no bearing on aerodynamics. Even if we are talking about a wind tunnel case where the section is constrained, flight path is very poor wording; pitch attitude would be marginally better in that case. $\endgroup$ – JZYL Sep 1 at 20:54
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This is a basic question, and I give you the answer from the initial pages of a aerodynamic learning textbook.

The is presented in next figure:

Resultant aerodynamic force and the components into which it splits

Where:

  • L: lift: component of R (resultant force) perpendicular to air stream,
  • D: drag: component of R parallel with the air stream,
  • N: Normal Force: component of R perpendicular to chord,
  • A: Axial Force: component of R parallel with chord,

For further information you can refer to Anderson's Aerodynamic book.

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    $\begingroup$ Good diagram. This is very interesting as it presents a realistic representation of the forward component of lift (relative to perpendicular of wing) created by airflow over the leading edge curve at a higher AOA. This was noticed in drag studies, producing a negative drag coefficient! This is also a great illustration of areodynamic center through one point as AOA (potentially) changes. Correct answer - B. $\endgroup$ – Robert DiGiovanni Sep 3 at 1:56

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