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Why is there such a small Krueger flap just inboard of the engine pylon on the Boeing 777-300, when the rest of the wing (including the further-inboard section of the wing) has leading edge slats?

I don't see an equivalent Krueger flap outboard of the pylon, unless there is one but can't be seen in this video There isn't a Krueger flap outboard of the pylon, according to this image from Wikipedia.

Refer to this video (flap extension beginning at 2:35).

Screenshot from the linked video Screenshot from the linked video with Krueger flap circled.

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Due to wing sweep the engine nacelle blocks airflow that would feed a slot in that location, so the Kreuger flap is used instead. Outboard the pylon there is no such problem.

enter image description here

ref

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It has to do with the slat–strut sealing; a photo from the other side shows the issue better (images below).

A normal slat would create a hole since the 777 engines are "close coupled"; this does not affect the lift (since there's a chine), but it affects the drag.

Boeing's solution to this is that tiny Krueger, which they call a "Seal Krueger" as it seals the hole, and with three engine manufacturers for the first generation 777, each manufacturer got a unique Krueger for a perfect fit.


Source:
Horse's mouth (Boeing's engineers):
- Nield, B. N. "An overview of the Boeing 777 high lift aerodynamic design." The Aeronautical Journal 99.989 (1995): 361-371. p. 367. https://doi.org/10.1017/S0001924000028670


Images:

hole
Nield, B. N.

other side
Jerry Pang

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  • $\begingroup$ The part about "each [engine] manufacturer had a unique Krueger [flap]" - interesting! (But surely that adds complexity to the design and manufacture of this aircraft.) $\endgroup$
    – pr1268
    Mar 9 at 21:04

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