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I always thought that flaps are trailing edge high lift devices and slats are leading edge high lift devices. This is also confirmed by the accepted answer for What is the difference between flaps and slats?.

Inspired by this question, I learned that Krueger flaps are actually leading edge devices, despite being named flaps. The definition on SKYbrary is:

Krueger flaps are high lift devices that are fitted to all or part of the leading edge of the wings of some aircraft types. The aerodynamic effect of Krueger flaps is similar to that of slats; however, they are deployed differently. Krueger flaps are mounted on the bottom surface of the wing and are hinged at their leading edges. Actuators extend the flap down and forwards from the under surface of the wing thus increasing the wing camber which, in turn, increases lift.

Krueger flaps and slats
(image sources: Wikimedia and SKYbrary)

I understand that the deployment mechanism is different, but the Krueger flaps are still deployed differently than other (trailing edge) flaps:

Flaps
(image source: SKYbrary)

So why are they called flaps and not Krueger slats instead?

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A flap increases airfoil camber, and can be on the leading edge or trailing edge. The Krueger Flap is a Leading Edge Flap, the actual generic term.

Krueger Fap

The Krueger dude developed the LE flap used on some airliners so his name is attached.

A slot increases stalling AOA, without necessarily increasing camber, as in the fixed slots used on a lot of light aircraft in the 40s to improve aileron control at the stall instead of wing washout, like Stinsons and Globe Swifts, etc.

enter image description here

Combine the slot function of injecting slightly higher velocity air along the top to keep flow attached at higher AOA, and the flap function of increasing wing chord, together on the leading edge, and you get SLAT.

Or you could say more correctly, Slotted Leading Edge Flap.

Without the slot part though, it's just a flap, as in Krueger Flap.

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    $\begingroup$ So if it’s a SLot and a flAP it should be called a SLAP! $\endgroup$ – TomMcW Oct 11 at 21:11
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    $\begingroup$ I was going to mention that but thought better of it lol. There must've been a committee that met somewhere and mulled it over and concluded SLAP was a little too out there. Also, there's the fact that the early versions look like blind slats as Jpe61 mentions in his answer. $\endgroup$ – John K Oct 11 at 21:37
  • $\begingroup$ Leading-edge droops increase the camber of an airfoil, but aren't considered flaps... $\endgroup$ – Sean Oct 12 at 1:11
  • $\begingroup$ No because it's the entire leading edge being lowered, not a hinged panel being extended. $\endgroup$ – John K Oct 12 at 1:15
  • $\begingroup$ @JohnK: The same is true for most types of flaps (just replace "leading" with "trailing"). $\endgroup$ – Sean Oct 12 at 22:58
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For a leading edge high-lift device to be called a slat, it has to have a slot behind it. Otherwise, it is a leading edge flap (or, if it's of a specific design, a Krueger flap).

Because:

By definition, the word slat means a thin, narrow strip. Its etymological roots lay in Middle English word slate, first recorded use in the aforementioned meaning dates back to 1764: Merriam-Webster: slat.

Contemporary use of the word slat would include the vertical parts of window blinds, which pretty well describes the ideology of the words adoption into aviation terminology, as a leading edge high lift device with a slot behind it: Wikipedia: High lift devices, slats and slots

The Krueger flaps, when deployed, do not form a slot behind them. The same applies to other hinged leading edge high-lift devices found for example on fighter jets such as F-16, on which they are called leading edge flaps, or LEF's: F-16C/D flight manual (see page 133) and airliners such as Airbus A380 and A350, on which the are called droops or droop noses: Wikipedia: droop (aeronautics)

These days one will, of course, find terms slat, flap, slot, droop etc. used quite liberally in common communications, verbal or written...

Here is some further reading on leading edge flaps: Science direct: Leading Edge Flap (an excerpt from the book General Aviation Aircraft Design, Applied Methods and Procedures

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    $\begingroup$ Makes sense that the Krueger flaps are not slats without a slot behind them, but this raises a whole new question: why are slotted flaps still called flaps? $\endgroup$ – Bianfable Oct 11 at 15:12
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    $\begingroup$ That's a good one @Bianfable. $\endgroup$ – Jpe61 Oct 11 at 16:34
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    $\begingroup$ Because naming is hard, @Bianfable... $\endgroup$ – FreeMan Oct 11 at 18:07

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