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The F-15's wing has an curious feature: a swept-back trailing edge near the wingtips, along with an unswept inboard portion of the wing.

F-15 with wingtips circled

(Modified from https://en.wikipedia.org/wiki/McDonnell_Douglas_F-15_Eagle)

This feature is not present on comparable teen-series fighters (e.g. the F-16):

F-16

(https://en.wikipedia.org/wiki/General_Dynamics_F-16_Fighting_Falcon)

Lots of Soviet-made aircraft (e.g. MiG-25, -29, and Su-27 families) appear to use a swept trailing edge. However, these fighters all have a constant sweep across the wing--not two sections of different sweep like the F-15.

Su-27 (https://en.wikipedia.org/wiki/Sukhoi_Su-27)

Why then does the F-15 have such a seemingly unique feature (i.e. a trailing edge with two sections of differing sweep)? Clearly its designers must have thought it served some purpose--I presume they would not have added it for mere decoration.

EDIT: to clarify, my question seeks the physical reason(s) for why the trailing edge of the wing tip has a rearward sweep--not why the wings are cropped at an oblique angle, or a citation of historical precedent.

EDIT 2: I'd also like a well-sourced answer, if possible.

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    $\begingroup$ I've always noticed this but never thought about it long enough to actually ask why, so thanks for posting this $\endgroup$
    – zymhan
    May 22, 2019 at 22:24
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    $\begingroup$ It looks like an attempt to to increase aspect ratio a little (compared with delta). F16 is a slower airplane and has less sweep angle, so the aspect ratio is a inherently lower. Flow separation is strong at high AoA so you don't want wing root to be too long (or aspect ratio too low) on a fighter, unless there's a vertex generator like canard wing strakes (which F16 has and F15 not), (not sure if the addition of vertex controller is reason between SU-27 and SU-57's trailing edge angle, maybe?) $\endgroup$
    – user3528438
    May 23, 2019 at 1:06
  • $\begingroup$ @theunamedguy Those and the dogteeth on the Hstab appear to be an effort to improve high AOA performance. Notice it also changed angle of aileron control surface slightly, perhaps in an effort to deflect turbulence away from the tail. A longer wing tip chord would increase stall AOA. $\endgroup$
    – Robert DiGiovanni
    May 25, 2019 at 1:32
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    $\begingroup$ Lots of planes have wings with swept trailing edges. You might just as well ask why the inboard portion of the trailing edge of the F-15's wing is NOT swept, given that the outboard portion of the trailing edge of the F-15's wing IS swept. Granted, trailing-edge sweep may be less common in a wing with a delta shape like the F-15's wing, but still-- Look at the planform view of the wing of the F-4 here. I see a swept trailing-edge -- en.wikipedia.org/wiki/McDonnell_Douglas_F-4_Phantom_II#/media/… $\endgroup$
    – quiet flyer
    May 26, 2019 at 2:33
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    $\begingroup$ @quietflyer That is a good point and is actually what I was looking for. I suppose my question is better stated as: why does the F-15's wing have a varying trailing-edge sweep through its span? I'll edit it to reflect this. $\endgroup$
    – built1n
    May 26, 2019 at 12:56

3 Answers 3

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The F-15 wingtip is a "raked" tip configuration, which increases lift and decreases drag.1 [comparison of swept wing with raked wingtip]2

This is the design decided by the USAF after discovery of several design deficiencies during initial flight test which included aeroelastic deficiencies (wing bending) and aerodynamic effects (buffeting). "In early June 1974, the Air Force flew the “raked” tip configuration and were completely satisfied with the configuration’s buffet characteristics." The Saga of the F-15 Wingtip

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The actual answer is straightforward. The F-15 wing was originally just a swept wing with a given taper ratio which resulted in a slightly swept trailing edge. The inboard portion was straightened just to simplify the flap. Because the flap hinge was now perpendicular to the fuselage the end of the flap was effectively sealed against the "booms" of the fuselage. This increased the flap effectiveness significantly.

The wing tips were raked because the tips were developing more lift than expected and causing a slight overload on the wings. The rake reduced the wing area slightly and because of the "conical camber" built in the effective washout was increased as well which solved a airflow separation issue under high load conditions. So 2 birds with one stone.

The above was from early books (1970's) published about the F-15.

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  • $\begingroup$ "The rake reduced the wing area slightly and because of the "conical camber" built in the effective washout was increased as well "-- it would be great if it could be explained just a little more what "conical camber" is and how it relates to "effective washout". Even just a link or two would help. Appears to be great answer even just as is though. $\endgroup$
    – quiet flyer
    Feb 4, 2021 at 21:50
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Not sure about the hard core aerodynamics/physics you're trying to explain, but simply put, the curved wingtip doesn't "bleed" as much energy/airspeed in high AOA flight/turning fight, like the pure sharp Delta tip. Explained to me by F-15 test pilot.

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    $\begingroup$ You may develop your answer, at least by adding some brief explanation of why this phenomena is observed, and quite better by adding links to further detailed reading $\endgroup$
    – Manu H
    Oct 17, 2019 at 9:23

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