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Lift is a term that is used very frequently here, but I cannot find any post explaining what it is exactly. I am looking for the exact definition of lift, not an explanation of how it is generated.

It seems a very simple question, but I have not been able to find consistent answers to it. Some very well respected sources even contradict each other:

At https://en.wikipedia.org/wiki/Lift_(force) Lift is defined as:
A fluid flowing past the surface of a body exerts a force on it. Lift is the component of this force that is perpendicular to the oncoming flow direction.

But according to NASA (https://www.grc.nasa.gov/www/k-12/airplane/lift1.html):
Lift is the force that directly opposes the weight of an airplane and holds the airplane in the air.

If a plane is flying straight and level (at constant altitude) these definitions are the same, but otherwise they are not the same at all. So, which one is correct?

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  • $\begingroup$ One is a technical definition, the other (NASA site) is a layperson's description for the general public. $\endgroup$
    – JZYL
    Jan 11 '20 at 21:54
  • $\begingroup$ This question may be helpful $\endgroup$
    – Pondlife
    Jan 11 '20 at 22:06
  • $\begingroup$ It's what keeps you from going smashy-smashy. $\endgroup$
    – GdD
    Jan 11 '20 at 22:20
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    $\begingroup$ The quote from NASA is only the introductionary first sentence. The rest of the paragraph provides the actual definition: "Lift is a mechanical aerodynamic force produced by the motion of the airplane through the air. Because lift is a force, it is a vector quantity, having both a magnitude and a direction associated with it. Lift acts through the center of pressure of the object and is directed perpendicular to the flow direction. There are several factors which affect the magnitude of lift." $\endgroup$
    – Koyovis
    Jan 12 '20 at 1:10
  • $\begingroup$ @Koyovis That is true, the word perpendicular is even printed bold, even though it does contradict the first sentence. I did find definitions like the one in the first sentence in several other sites too: web.mit.edu/16.00/www/aec/flight.html, "In order for an aircraft to rise into the air, a force must be created that equals or exceeds the force of gravity. This force is called lift." and britannica.com/science/lift-physics "Lift, upward-acting force on an aircraft wing or airfoil." I figured this could be confusing to many enthusiasts so I asked this question. $\endgroup$
    – Orbit
    Jan 12 '20 at 15:27
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It's the component of the aerodynamic force perpendicular to the direction of the air flow, the aerodynamic force being the reaction of the wing to the relative wind.

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  • $\begingroup$ What if we introduce a bank angle, how would that change the direction of the lift, would it rotate with the plane or stay upward? And if we now keep this bank angle constant and also add a yaw angle, would the sideway force due to the yaw angle be included in the lift? $\endgroup$
    – Orbit
    Jan 12 '20 at 11:12
  • $\begingroup$ @Orbit Pure bank has no effect on aerodynamics. The actual definition of lift under yaw depends on whether you're defining it in stability axis or wind axis. Most define it under the stability axis. $\endgroup$
    – JZYL
    Jan 13 '20 at 13:21
  • $\begingroup$ @JZYL Bank does not influence aerodynamics, but lift does rotate with respect to gravity, which is interesting for a complete understanding. As xxavier states, lift is defined relative to the incoming flow, but even when decomposed to the aircraft coordinate system, the direction changes when yawing. $\endgroup$
    – Orbit
    Jan 13 '20 at 21:27

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