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Variable sweep on fighter aircraft is a bit out of fashion, but seems to a worthwhile option to adjust the wing to the current speed and needed lift. All this is bought with complexity and weight of the system.

Would it be possible to build a variable sweep aircraft that goes low speed and in a dogfight into a forward swept wing configuration like the Su-47 and high speed in a rearward swept wing? enter image description here Image of Su-47 credit to Wikipedia (Leonid Faerberg)

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    $\begingroup$ What is the aircraft in the photo? $\endgroup$ – TomMcW Nov 19 '16 at 16:18
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    $\begingroup$ It's a Su-47. I edit my post accordingly. $\endgroup$ – sargas Dec 2 '16 at 13:57
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Yes it could, but don't hold your breath.

Forward sweep means that the aeroelastic forces will twist the wing at high lift into an increased angle of attack, so such a wing needs to be stiff in order to avoid divergence (PDF!). Also, aileron effectiveness is excellent, so a forward-swept configuration is a good choice for dogfighting.

Since backward sweep is mostly helpful at trans- and supersonic speed, swing wings were the right answer when very high top speeds (Mach 2+) needed to be combined with high loads at take-off and short field length requirements. When flying ever faster went out of fashion in the 1970s, the attractiveness of variable sweep disappeared.

Forward sweep helps both in air combat and in transsonic flight, so there is little advantage in a variable-sweep configuration with forward sweep. An aircraft optimised for air combat has little need to fly Mach 2+, so the moderate sweep angle is a good choice for most of its flight time. Also, the aeroelastic instability increases with sweep angle, so more than moderate forward sweep needs a very stiff and heavy wing.

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  • $\begingroup$ Aircraft with forward-swept wings also have negative yaw-roll coupling (yawing in one direction causes the aircraft to roll out of the yaw, rather than into it (as is the case with aircraft with backward-swept wings)), which could be useful in certain situations. $\endgroup$ – Sean Dec 4 '18 at 4:45
  • $\begingroup$ @Sean: That only means their optimum dihedral is higher than that of backward swept wings. Dihedral is used to adjust the overall roll response to yaw, and sweep only changes the starting condition for selecting dihedral. $\endgroup$ – Peter Kämpf Dec 4 '18 at 6:27
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Yes. Variable sweep airplanes exist and it shouldn't be a problem to design a system able to sweep the wings forward and rearwards. However, the few aerodynamic advantages of the forward swept wing at low speeds doesn't justify the added complexity, weight and loss of space inside the fuselage.

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