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Sweep will allow flight at higher speed because the perpendicular air flow speed (over wing) is less.

But why is there trailing edge sweep?

We can have a swept leading edge and unswept trailing edge to produce elliptical lift distribution. Also, unswept trailing edge will help to improve flap effectiveness, right?

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2 Answers 2

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Both leading and trailing edge sweep help to reduce compressibility effects. For simple estimates, quarter chord sweep is used. This is more influenced by the leading edge, but the trailing edge is still involved with 25% of the total value.

If we look at the particular influence of trailing edge sweep, it is mostly found in the effectiveness of flaps and ailerons. Their effectiveness is proportional to the cosine of hinge line sweep. However, flaps at the end of a long wing chord will create a high pitching moment, requiring a big tail. Most delta wings use simpler, less effective flaps with less moment change. In supersonic aircraft trailing edge sweep helps to reduce sonic boom intensity.

In the end, trailing edge sweep is mostly an outcome of the design process, similar to wing chord. The structural engineer will want to have a deep root airfoil in order to have enough wing height for a stiff spar or the landing gear without increasing relative thickness too much, and the aerodynamicist wants to reduce wetted area, since it will create friction drag and increase wing mass. The stealth engineer will align the trailing edge with other edges or at least put substantial sweep on the trailing edge to reduce the frontal radar echo. In the end, all have to agree to a compromise.

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  • $\begingroup$ I would say that the trailing edge sweep does not have a very large influence on the compressibility effects, as the higest velocities (and thus the strongest shocks) will not occur at the trailing edge? $\endgroup$
    – ROIMaison
    Jan 14, 2015 at 13:59
  • $\begingroup$ @ROIMaison: In supersonic flight you have a shock at the trailing edge all the time. Maybe it is not strong, but then the wing does not create much lift. Remember what was planned to reduce noise emissions of supersonic passenger jets: This focused a lot on trailing edge sweep! $\endgroup$ Jan 14, 2015 at 14:21
  • $\begingroup$ Ah right, I was thinking about transonic flight. $\endgroup$
    – ROIMaison
    Jan 14, 2015 at 14:25
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For transonic flight the trailing edge sweep is not a design variable, it mainly follows from the quarter cord sweep (to control the effective Mach number) and the taper-ratio (which follows from a compromise between aerodynamics and structural weight, see this answer this answer for a more detailed explanation).

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