what reference of airfoil is used to define lift and drag?

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

• Free stream flow. You should have a word with your instructor.
– JZYL
Commented Sep 1, 2019 at 19:29
• why are you hesitating between all the possibility? It recommend you read the airfoil section of how it flies? Commented Sep 1, 2019 at 19:51
• 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 Commented Sep 1, 2019 at 19:53
• @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.
– JZYL
Commented Sep 1, 2019 at 20:54
• @PeterKämpf Not sure what you mean. For a wind-less relationship, $\alpha=\theta - \gamma$. $\gamma$ being the flight path angle. $\alpha$ is what counts here. See en.wikipedia.org/wiki/… for illustration.
– JZYL
Commented Sep 1, 2019 at 21:30

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:

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.

• 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. Commented Sep 3, 2019 at 1:56