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The lift of an airplane is similar to that of a balloon. It uses the density difference of air, right? The air density on the top surface of the balloon is low and the air pressure is small. The air density on the bottom surface of the balloon is high and the air pressure is large, which produces an upward force. If the weight of the balloon is less than this force, the balloon will rise. Due to the movement of the wing, the air density on the top surface of the wing is low, the air pressure is small, and the air density and air pressure on the bottom surface of the wing are high, which produces an upward force. If the weight of the aircraft is less than this force, the aircraft will rise. Right?

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  • $\begingroup$ Haven't you ever heard of incompressible flow? $\endgroup$
    – D. Halsey
    Sep 3 at 22:52
  • $\begingroup$ @D.Halsey Any fluid is compressible. $\endgroup$
    – enbin
    Sep 3 at 23:49
  • $\begingroup$ Aircraft work by pushing the air downward. $\endgroup$ Sep 5 at 0:46
  • $\begingroup$ @MikeDunlavey The air cannot be pushed down without a change in density. $\endgroup$
    – enbin
    Sep 5 at 1:47
  • $\begingroup$ Aircraft work by Bernoulli. Balloons don't. $\endgroup$ Sep 5 at 12:11
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It uses the density difference of air, right?

No, it doesn't. At least for slower speeds (not approaching transsonic), we can treat the air as an incompressible fluid. The dynamic pressure changes from point to point, but the density of any parcel of air is approximately constant.

Yes, both buoyant and aerodynamic lift require there to be a pressure difference between the top and the bottom of the lifting body. But balloons exploit the pressure gradient already in the atmosphere, while wings create a pressure difference based on dynamic air movement.

It is not correct to point to density differences as important for subsonic wing operation.

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  • $\begingroup$ In fact, air is compressible, and once compressed, its density must change. $\endgroup$
    – enbin
    Sep 4 at 1:25
  • $\begingroup$ Yes. And in many realms that compressibility is a factor. But for subsonic flight, you can treat the air as if it is not compressible. In particular, density changes are not a significant factor in subsonic lift generation. $\endgroup$
    – BowlOfRed
    Sep 4 at 3:04
  • $\begingroup$ physics.stackexchange.com/questions/63039/… suggests that below Mach 0.3, the density of an accelerated parcel will be within 5% of the static airmass. $\endgroup$
    – BowlOfRed
    Sep 4 at 3:12
  • $\begingroup$ Usually, density changes and pressure changes are one thing. Therefore, the difference in density is also the pressure difference. If the temperature changes are not considered. $\endgroup$
    – enbin
    Sep 4 at 3:40
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    $\begingroup$ Even in water (which is far less compressible) we can have significant pressure changes. Dynamic pressure changes due to accelerations and does not require corresponding changes in density. $\endgroup$
    – BowlOfRed
    Sep 4 at 3:44

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