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Slotted flap is an improved version of the plain flap, generating more lift (a fact I don't challenge). However the principle is air passing between the wing and the flap.

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enter image description here Source

This seems counterproductive as this airstream could also prevent the upper stream to follow the airfoils shape, and lead to air being less rotated downward than with a plain flap.

How does the slotted flap happen to be superior?

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    $\begingroup$ Related: What are the aerodynamic differences between the different types of slotted flaps? $\endgroup$ – fooot Aug 24 '16 at 19:13
  • $\begingroup$ @fooot: This other answer states that the air going through the slot delays boundary layer separation to a higher AoA. However it doesn't explain how this happens, that's what I'm interested in. I'm not able to understand this point in the existing answer either. $\endgroup$ – mins Aug 24 '16 at 21:13
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The slot helps to prevent early separation, so the flap stays effective for larger deflection angles. What that means in reality can be read in this old NACA report which compares two NACA23012 sections, one with a plain flap and the other with a slotted flap. I have copied the pressure distributions below from that PDF:

Pressure distributions of plain and slotted flaps

Now I need to explain a little how to read the pressure distribution plots. The solid line shows the local pressure on the upper surface while the dashed line shows the pressure on the lower surface of the airfoil. The X-axis represents ambient pressure, and the Y-axis represents suction. The area between the dashed and the solid lines is proportional to the lift coefficient, which is helpfully shown below each plot.

Now to the curvature: As long as the lines show a gradient, flow is attached. Separated flow can be recognized by a horizontal line slightly above the X-axis. As you can see, we have already fully separated flow on the upper plain flap surface at 30° flap angle and 8° angle of attack. At the same angles the slotted flap still shows fully attached flow on both surfaces. Attached flow creates a suction peak at the nose of the flap, which in turn allows a higher suction value at the end of the fixed part of the airfoil. This in turn allows a much higher suction peak at the nose of the airfoil, so that the suction force over the whole chord of the airfoil with the slotted flap is much higher than on the airfoil with the plain flap.

Only at 50° flap deflection will the slotted flap also show flow separation over the rear part of the flap, but there is still a suction peak which helps to stabilize the flow over the fixed part and increases lift. Unfortunately, the plain flap was not measured at the same angle, but from the small change between the 30° and 45° flap angles you can see already that moving the flap to 50° will change little.

The reason for this difference is simple: While the plain flap has only the old, thick and tired boundary layer coming from the fixed part of the airfoil to work with, the slot allows the slotted flap to create a new, thin and vigorous boundary layer which can tolerate much higher pressure gradients. Another benefit is the unchanging direction of the local flow behind the fixed airfoil: This allows to use a thin, highly cambered flap airfoil which maximizes lift production. The old NACA airfoil did not take advantage of this, but modern airliner flaps certainly do.

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The answer lays into the pressure gradient.

The air that flows along the top surface comes from an area of (extreme) low pressure and has to travel down the wing, a direction where the gradient is "adverse": the pressure is increasing and the air would like to stop. The higher the lift being produced by the same wing, the higher this adverse gradient is.

When do we use flaps? When we need lots of lift at low speeds: it is a situation where the upper flow of air has the least incentive in remaining attached to the wing. The flow is slow and the gradient is as strong as it gets.

Enter the picture the cross flow of air passing through the slot. This is a flow coming from a high pressure region. It will encounter a low pressure region at the beginning of the flap surface, yes, but the remaining gradient will be completely negligible, and the flow will remain attached to the wing for longer, making a stall less likely.

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  • $\begingroup$ Thanks Federico. "but the remaining gradient will be completely negligible": Do you mean the slot air will add speed to the up boundary layer, and (maybe by Bernoulli's principle) decrease its pressure, making it better sticking to the top face of the airfoil? $\endgroup$ – mins Aug 24 '16 at 22:03
  • $\begingroup$ @ymb1, the 4 words following: "the pressure is increasing". $\endgroup$ – Federico Aug 25 '16 at 5:45
  • $\begingroup$ @mins I mean that the remaining difference in pressure, over the remaining length determine a gradient that is negligible for that flow. $\endgroup$ – Federico Aug 25 '16 at 5:46
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... and lead to air being less rotated downward than with a plain flap.

The air doesn't have to follow the full curvature to generate lift.

As the illustration below shows, the normal air flow above the wing follows a longer path, but not the added curvature.

enter image description here

Snap from an old NACA video.

The way I like to imagine it, is the slotted flap acts like an extra wing.

The high pressure flow through the slot, will be accelerated by the gap/nozzle, the acceleration will lower its pressure (potential for kinetic), and it'll get stuck to the boundary layer, and create a laminar low pressure area above the flap—i.e. lift.

Boundary layers flow over my head, so don't quote me just yet.

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It is only counter-intuitive until you realise that by Bernoulli's principle a faster flow has lower pressure. So while the slot adds some air, that air has high speed (being accelerated by the pressure difference between lower and upper surface) and the increase in speed will cause overall decrease of pressure and thus increase in lift.

Precise description of the boundary layers is more complicated, but this should be enough to give you correct intuition.

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