Many airliners have a distinct unswept part at the wing root at the trailing edge. See for example this image from Wikimedia of a B737-400.

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Other examples include the B777, A320, Embrear 145, and many more. Virtually all airliners have at least some degree of less sweep at the wing root.

Why is there a part of less sweep at the wing root (to facilitate engine mount; Aerodynamic reasons)? Why does a completely unswept part seem to be a preferred choice for this part of less sweep, as opposed to any other sweep angle than zero?

  • $\begingroup$ Related, or perhaps duplicate: aviation.stackexchange.com/questions/11704/… $\endgroup$ – ROIMaison Jul 11 '16 at 8:46
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    $\begingroup$ Definitely related, @ROIMaison, but not really duplicate. Once I read this Q & Jordy's A I could make a duplicate out of the other, but without his answer, the other Q&A didn't really address this specific question. At least, not to me. $\endgroup$ – FreeMan Jul 11 '16 at 11:50

There are two main reasons:

  • Swept wings suffer from interference at the center of the wing. If wing taper and sweep remain constant over the full span, the lift distribution will show a distinct drop at the center. This is called the "Mitteneffekt". The remedy is to locally increase the wing chord near the root and to decrease the wing's sweep angle. This "fills up" the dimple in the lift distribution and reduces induced drag.
  • Large aircraft need heavy wing spars, and increasing chord near the root gives the spar more height while the relative thickness of the airfoil can remain unchanged. A higher spar is structurally more efficient, so increasing its height at the root saves weight. Since the relative thickness determines the maximum operating Mach number, it cannot be increased locally without hurting the whole wing. The better alternative is to disproportionally increase wing chord at the root.

The unswept trailing edge near the root is a consequence of Boeing's flap track mechanism. Boeing Fowler flaps move perpendicular to the hinge line, and an unswept hinge line avoids lateral movement of the flap. Another consequence is a gap between the inner and outer flaps in order to allow both to move back without colliding. Boeing fills the gap with a high-speed aileron.

Airbus flaps move in flight direction and leave the sweep angle of the trailing edge free. Also, they don't need a gap between the flap segments left and right of a sweep change of the hinge line.

  • $\begingroup$ These are more important reasons indeed. I'll change my answer to 'minor reasons' $\endgroup$ – Jordy Jul 15 '16 at 8:12

Two other (minor) reasons for this:

Landing gear storage

The main landing gear is positioned just behind the aircraft's center of gravity. For most aircraft with wing-mounted engines this is near the trailing edge of the wing root. In order to have enough space for the landing gear when it is retracted, and to have a part of the wing structure that is dedicated to support the landing gear, this part of the wing has a larger chord, and thus a smaller trailing edge sweep angle.

Flap effectiveness

Another benefit of a low trailing edge sweep angle is that the effectiveness of the inboard trailing edge flap is higher. The maximum $C_L$ of the flaps decreases with increasing sweep angle. (http://adg.stanford.edu/aa241/highlift/clmaxest.html)

  • $\begingroup$ I'm guessing that flap effectiveness was more a driving factor in this kind of wing platform as you see a similar arrangement on Soviet-era airliners whose gear retracted into mid wing pods. $\endgroup$ – Carlo Felicione Jul 12 '16 at 4:38
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    $\begingroup$ These are relatively minor factors compared to what Peter (both Peters, actually) mentioned later. $\endgroup$ – Zeus Jul 15 '16 at 5:52

I think it is also about arranging the wing spars. Due to the swept wings they meet at an angle in the middle and must handle a high torque. This can be reduced by either having an A connecting piece or leading the spar in the unswept part.

  • $\begingroup$ I think this is not the case. As far as I know, the wing spars are either located parallel to the air flow, or they change direction at somewhere in the wing. Anyway, the root trailing edge is not a part of the wing box. example: an A320 wing $\endgroup$ – Jordy Jul 15 '16 at 8:10

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