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I've seen mixed sources on this subject. Some say leading edge devices allow the wing to obtain a higher AoA, and others say it flat out increases lift. For me, the latter makes most sense, because you're forcing more air to the upper surface of the wing, making lower pressure, therefore increasing lift?

Examples of different sources saying different things : "When deployed, slats allow the wings to operate at a higher angle of attack before stalling. With slats deployed an aircraft can fly at slower speeds, allowing it to take off and land in shorter distances." -Wikipedia

"The leading edge slats play an essential role in landing and in takeoff which tend to increase coefficient of lift and the stall angle. They are especially useful in takeoff which increase the lift production at a low drag penalty" -ScienceDirect.

((I believe that slats increase stall AoA, but this whole question was mainly about if you compared the lift of 2 planes, one with slats deployed, and the other with them retracted. All at the same AoA and airspeed.))

(Also, these sources kind of say the same thing in a way, but the Wikipedia one doesn't talk about them increasing lift at the same AoA and airspeed (for what I saw). I'm not sure if the ScienceDirect one does either, but that's what I interpreted it as.)

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Leading edge slats mainly modify the pressure distribution over the forward and upper part of the airfoil which is, at least at high AoA, the most stall-sensitive part of the airfoil. In particular, the leading edge slats forces the airflow to remain attached until higher AoA in respect to the same wing with no slats extended. So the AoA can be increased further without stall therefore obtaining higher lift as consequence.

The following figure (source) represents in a good way the net effect of a slats deployment:

lift coefficient with slats deployed

Note that, being the slats (at least locally) at basically a negative AoA, it is locally creating a negative lift and therefore it's global contribution to the lift is also negative: this can be also seen in the previous plot where $C_l$ is slightly smaller at the same AoA. Anyway the delay of the stall region more than compensate for this local phenomenon.

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    $\begingroup$ @Pilothead: I personally interpret "flat out increase of lift" as "extend the linear portion of liff"... but I agree that the wording in the original question isn't that unambiguous... I'm going to delete that reference and give a more general explanation. $\endgroup$
    – sophit
    Feb 21 at 15:51
  • $\begingroup$ Please keep the original graphs are well. The effect of the slat depends a lot on wing design and Reynolds number. Full scale aircraft generate lift most efficiently with the top of the wing (subsonicly). Lower Re models must bounce it off the bottom to generate lift (but they can be made "scale" because of their lower weight to surface area ratio). Since this site deals with full scale (mostly) those graphs are good. $\endgroup$ Feb 21 at 16:25
  • $\begingroup$ @sophit this new graph is better l. Also since we don't actually disagree, perhaps you should remove the downvote. $\endgroup$ Feb 21 at 16:44
  • $\begingroup$ I see, that makes sense, thanks. I'm going to copy and paste a question I had in the comments of a different answer to maybe get your view on it : One question about leading edge flaps : because they’re at an angle relative to the chord line, wouldn’t they force air upward, promoting separation? Basically the air is coming to the main airfoil at an angle (because of the LE flaps giving it an upward velocity vector) so it has to work harder to follow the contour of the wing. $\endgroup$
    – Wyatt
    Feb 21 at 18:40
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Leading edge slats and droops decrease the lift at low AoA due to the downturned nose generating negative lift.

So if your "constant AoA" is lower than the angle of flow separation onset, lift will decrease.

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    $\begingroup$ Cambered wings generate lift at even negative AoA. Slats increase the camber of the wing and lower local AoA. But aircraft AoA "weathervanes" can be located on another part of aircraft, such as the fuselage. $\endgroup$ Feb 21 at 15:23
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    $\begingroup$ This is true according to pg 420 of Sophit's Anderson reference at archive.org/details/FundamentalsOfAerodynamics5thEdition/page/… $\endgroup$
    – Pilothead
    Feb 21 at 15:26
  • $\begingroup$ One can also lower the flaps a little too (when slats are deployed). This increases camber even more, while maintaining (chordwise) AoA. $\endgroup$ Feb 21 at 20:11
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Adding the constraint "...All at the same AoA and airspeed ... " defeats the value of any comparison and makes the statement meaningless. The point of adding slats is to allow the aircraft to generate enough lift to remain in flight at a slower airspeed, in order to reduce takeoff and landing distance.

And the amount that AOA increases as a result of deploying slats or flaps depends on how you measure the AOA. Adding slats and flaps changes the chord line of the wing, and as AOA is arbitrarily defined as the angle between the chord line and the flight path, changing the chord line makes the change in AOA totally dependent on how the chord line has changed, and therefore meaningless by itself as any indicator of performance changes. If you deploy slats across the entire leading edge of the wing, and do not deploy flaps (the F-4 Phantom could do this), then because the chord line rotates leading edge down, the AOA decreases as a result. The maximum lift that the wing could generate, nevertheless increased, and the minimum airspeed required to generate any specific amount of Lift decreases.

This is because AOA is measured from an arbitrarily defined zero point (by the chord line) and comparisons in AOA are only meaningful when comparing measured AOA that are relative to the same base point. It would be like concluding that the temperature in Europe is always hotter than in America (based on the reported temperatures in the media there), without considering that they measure in Centigrade not Fahrenheit.

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    $\begingroup$ I think I see where you’re coming from, thanks. One question about leading edge flaps : because they’re at an angle relative to the chord line, wouldn’t they force air upward, promoting separation? Basically the air is coming to the main airfoil at an angle (because of the LE flaps) so it has to work harder to follow the contour of the wing. $\endgroup$
    – Wyatt
    Feb 21 at 3:59
  • $\begingroup$ Leading edge slats are themselves shaped airfoils, with a leading edge that points downward, and grabs air from below the leading edge of the rest of the wing, and lifts it and turns it to point backwards along the top of the wing, flowing parallel to the top of the wing. It re-energizes the airflow across the top of the wing. with the air that flowed under the slat between the slat and the leading edge of the rest of the wing. $\endgroup$ Feb 23 at 5:00
  • $\begingroup$ Consider the effect of deploying the wing lift mechanism on the F-8 Crusader. Would you say that the AOA increases, because you only see the direct absolute effect of raising the leading edge of the wing up from it's current position, as it would if you did it while parked on the ramp? No, because what really happens is that the fuselage is lowered. In the air, the wing itself is the reference point from which meaningful comparisons should be referenced. $\endgroup$ Feb 28 at 13:50

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