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I was watching this video and it shows that adding fins on the wing helps the air get turbulent on the upper part of the wing, which forces the air to stay longer, and ultimately this helps with lift.

To my understanding, if we didn't have those fins, the air wouldn't stick so long on the upper wing, leaving an "empty" space, with low pressure. So this should help even more with lift (lift = pressure from the bottom - pressure from above).

Is that correct? In other words, the fins help adding back "horizontal" pressure, and thus reduce pressure drag, although it makes the airplane lose some lift (which is ok because there is plenty)

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Putting small, tilted fins on the upper surface of a wing has the effect of stirring up the boundary layer of air flowing over the wing- mixing high velocity air from the outer portions of the boundary layer with the low velocity air that is right at the surface of the wing.

This energization of the boundary layer delays the onset of flow separation of the air flowing over the wing, a condition known as a stalled wing, which maximizes the wing's effectiveness in generating lift. This is particularly true when the wing is operating at high angles of attack.

By delaying the onset of a stall, the fins help the wing keep generating lift and the pilot's flight controls maintain their effectiveness at high AoA conditions.

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  • $\begingroup$ thanks but I still don't understand. If you delay the flow separation, there is more (vertical down) pressure on the upper wing, so how does it help with lift? $\endgroup$ – Thomas Aug 23 at 4:04
  • $\begingroup$ No- if you delay flow separation to a point further along the chord length of the wing you maintain the wing's effectiveness at generating lift, because any portion of the wing on which you have flow separation is an area that is producing lots less lift. $\endgroup$ – niels nielsen Aug 23 at 4:12
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The fins cause the air molecules on the top of the wing to act in a certain way, meaning those molecules will not act in another way. This is a key understanding. Their random motion might push them down against the wing if they would be left alone, whereas the fins force their motion in a certain direction which means the down component becomes smaller, because a molecule can't be going two ways at the same time.

The molecules at the bottom of the wing are left more by themselves. They will naturally end up carrying upwards momentum to the wing because they move more randomly. So, it's not really just about the time the molecules spend on top of the wing, it's about controlling the randomness of the motions. I also dislike using "pressure" as this is an emergent phenomenon, and kind of complicates thinking about lift imo.

This is all a big simplification btw, there's several optimisations in place wrt shape of the wing, that do ultimately have to do with the time it takes for certain aerodynamics to take effect and remain, at certain speeds.

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  • $\begingroup$ I understand that the fins would force the horizontal motion, but not act on the down component? (so that on average the down component stays the same?) $\endgroup$ – Thomas Aug 23 at 4:00
  • $\begingroup$ You’re neglecting the effect the molecules have on each other! $\endgroup$ – buddhabrot Aug 23 at 16:25

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