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Reading this about slat cove filler meant to reduce aeroacoustic noise, it seems there is no big difference in lift if the cove filler is deployed or not.

I understand the shape of a retractable slat has to somehow fit the leading edge of the main wing when retracted, therefore its lower surface has a cove and a pointy part / cusp.

But if not meant to retract, why do fixed slats also have this specific shape, with a cusp and a cove with recirculation zone, if not needed in the first place?

Are there fixed slats with no cust and a permanently filled cove?

enter image description here (source)

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Fixed slats are called slots. They are not in the position shown in your picture, as this produces high drag at low angles of attack.

Slots are normally aligned with the wing so that drag is lower in cruise. The gap is kept fairly constant through the slot so the shape approximates the airfoil leading edge, and there is no recirculation zone.

enter image description here

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  • $\begingroup$ to me the slot is the gap between the slat and the wing's leading edge, there also are pictures of Fi-156 displaying very different camber and angle of attack for this device, for instance this one or this one where gap is wider first then shrinks towards slat's trailing edge $\endgroup$
    – user21228
    Sep 22, 2021 at 6:42
  • $\begingroup$ @qqjkztd There are different designs. A narrowing gap is intended to accelerate the airflow but there is no recirculation zone as mentioned in your question and I would still say the gap remains fairly constant. The lower edge is more rounded which exaggerates the apparent width of the slot opening. $\endgroup$
    – Pilothead
    Sep 22, 2021 at 18:30

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