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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 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 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 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 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. $\endgroup$ – theunamedguy May 26 at 12:56
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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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There are two ways to improve pitch stability, make the tail larger or make the wing wider. In the case of the F15, they most likely chose to "fill out" the area behind the swept wing (making it a sort of hybrid swept wing delta). This may have also created more space for fuel and/or helped balance the aircraft a bit better.

Note the F 22 Raptor has a reverse swept trailing edge, but also thrust vectoring.

As airliners dutifully followed the computer controlled horizontal stabilizer of the F16, we await the use of thrust vectoring to help control pitch as well.

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  • $\begingroup$ Do you have a source for this? $\endgroup$ – theunamedguy May 27 at 19:00
  • $\begingroup$ @theunamedguy trailing edges have gone through a number of iterations over the years. Amazingly, some very futuristic designs can be seen in the vertical stabilizers of aircraft from the pre-supersonic era. I'm not sure if they flattened the sweep near the fuselage or added some near the wing tips but this is certainly within the range of workable designs. The wing of the F22 Raptor very closely resembles ... the vertical stabilizer of the Bf 109! So an interesting aviation fetish indeed. $\endgroup$ – Robert DiGiovanni May 27 at 23:15
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    $\begingroup$ What you say makes sense, but I feel like it could still be improved with a reference. $\endgroup$ – theunamedguy May 28 at 15:43
  • $\begingroup$ @theunamedguy just based on building experience. The 109 vertical comparison is based on the very advanced understanding of aerodynamics achieved by its designers. $\endgroup$ – Robert DiGiovanni May 28 at 16:30

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