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I am studying about the lift generated by a fluid flowing on a surface. In the case of an airfoil I have seen that there are various ways to explain it, you can use for example:

  • the third law of newton: the wing exerts a downward force on the air and therefore the air exerts an equal force but upwards;

  • the preservation of the mass: after the point of the wing in which the flow separates, we will have flow lines (streamtubes) closer above the wing and more spaced under the wing; assuming that the air is not compressible, since the flow rate is constant, this translates into higher air velocity above the wing and lower under the wing, and therefore lower pressure on the upper surface and higher pressure on the lower surface (Bernoulli's principle).

In the latter approach, however, I did not understand why the streamtubes above/below the wing change size. In essence, none of the explanations found seem to clarify the real reason for the change of air speed above (speed increases) and below (speed decreases) the wing.

Since the lift is generated even if the wing has a symmetrical profile, it seems to me that having a higher profile more curved than the lower profile is not the only characteristic at the base of the lift but serves to increase its efficiency.

So, taking the case of a symmetrical airfoil with a certain "positive" angle of attack (i.e. the leading edge is higher than the trailing edge), I think that the most important thing to understand the lift is that the point where the flow separates into two flow lines (one that passes over the wing, one that passes under the wing) does not coincide with the leading edge. But I do not understand the implications of this.

Summing up what I do not understand is what the difference in the air speed above and below the wing is due to. Rather than in the explanation by formulas I am more interested in a physical explanation (what pratically happens to the fluid).

Qmechanic
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5 Answers5

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Since the lift is generated even if the wing has a symmetrical profile, it seems to me that having a higher profile more curved than the lower profile is not the only characteristic at the base of the lift but serves to increase its efficiency.

...

Summing up what I do not understand is what the difference in the air speed above and below the wing is due to. Rather than in the explanation by formulas I am more interested in a physical explanation (what pratically happens to the fluid).

See: https://aviation.stackexchange.com/questions/39146/how-do-symmetrical-airfoils-generate-lift and Bernoulli and Newton.

An oversimplified and short explanation is that the upper surface has a longer path whether the airfoil is symmetrical or not, that provides the lift. If the angle of attack was zero, like symmetrical rocket fins, there would be no lift; only straight travel, accounting for weight and force / vector.

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Rob
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  • Differences in path length are not necessary to get lift. For example, an infinitely thin cambered foil at its ideal angle of attack has the same path length on upper & lower surfaces, and yet it can have considerable lift. (Note - at the ideal angle of attack, the flow impinges smoothly on the leading edge, without a stagnation point on either surface.) – D. Halsey Jun 05 '18 at 00:33
  • A cambered foil with thickness can also have an ideal angle of attack. With the stagnation point at the leading edge, the path lengths are equal & lift is still produced. – D. Halsey Jun 05 '18 at 00:44
  • See here, #2: https://www.coursehero.com/file/p44v03a/DRAW-1b-add-force-vectors-in-above-drawing-indicate-lift-and-drag-and-define/ - camber *or* angle alone increases the path length. – Rob Jun 05 '18 at 01:59
  • Except at the ideal angle. I also disagree with #3, where he attributes the stall to transition from laminar to turbulent flow. Maybe that's not a good reference to use. – D. Halsey Jun 05 '18 at 02:11
  • See http://wrightstories.com/einsteins-wing-flops/ for a discussion of the fallacy of the path-length explanation. – D. Halsey Jun 05 '18 at 02:17
  • @Rob: Careful. You say "short explanation is that the upper surface has a longer path". That implies air parcels separated at the leading edge must meet at the trailing edge. That's the "equal time fallacy". If it were true, wings could not lift because they could not generate the bound vortex. Check it out. – Mike Dunlavey Jun 05 '18 at 11:07
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Read this. Basically, the wing pulls the air down into a vortex centered below the wing that spans along the length of the wing. So the air above the wing has to move faster. At the tips of the wings, the vortex turns to point backward, forming the wake. (The tail of the plane, by the way, does the opposite. It also lifts, but downward.)

Mike Dunlavey
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When a body travels through a medium such as air, the air flows around it in such a way that the pressure gradient around it is balanced. In the case of an aerofoil, the gradient is designed to cause a differential because upper surface area is greater than the lower surface in relation to the angle of attack, therfore the velocity of the airflow over the top is increased comparatively to the lower surface that when the air has passed over the surfaces as it maintains the same atmospheric presure as the surrounding air. The reasons for this are the relative compression of the air under the wing and the dynamic transmission of force due to pressure through the wing then causes the wing to generate lift and, the depressurised region behind the parallel tangent on the top surface of the wing causes turbulence and therefore lift and drag. The latter is important to understand because the vortices generated by a wing cause a delay of atmospheric pressure return to disturbed air and also reduce density of air in that region. This is why there is a minimum time delay between large aircraft taking off into the same airspace successively and was the cause of the Air France Flight 4590 concorde disaster at the turn of the millenium.

Rhodie
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The simplest explanation for the lift is that it is the reaction force to the downward change of momentum of the air flowing over it.

Its magnitude can be calculated from either the momentum changes in the flow or from the pressure differences between the solid surfaces of the wing.

Because of the downward deflection of the streamlines, the flow below the wing resembles the classical solution for the flow into a concave corner, which has a stagnation point (higher pressure) at the corner; while the flow above the wing resembles the solution for the flow around a convex corner, which has a strong peak in the flow speed (lower pressure).

D. Halsey
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Suppose the wing moves horizontally in the atmosphere. At the top of the wing, the leading edge of the wing makes the atmosphere move upward, so after the leading edge, the atmosphere tends to move away from the wing in the normal direction (the leading edge of the wing acts as throwing up the atmosphere), so the top of the wing produces low pressure. At the bottom of the wing, the atmosphere tends to approach the wing in the normal direction, so the wing forces the atmosphere to move downward, so high pressure is generated at the bottom of the wing. Low pressure at the top, high pressure at the bottom, which creates wing lift.

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enbin
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