Maybe slightly off-topic, but here's Mark Drelas explanation of wing sweep:
(
http://yarchive.net/air/sweep.html )
Sweeping a wing makes sense only if you are up against the Mach number limit, and want to fly faster, as with a jetliner. It doesn't make sense if you want to fly higher, as with the U-2, or if Mach is of no concern, such as with a General Aviation aircraft.
The airfoils on a swept wing behave as though they were flying at a reduced speed, reduced Mach number, and reduced dynamic pressure.
effective speed = V cos(L)
effective Mach = M cos(L)
effective q = 0.5 rho V^2 [cos(L)]^2
where L is the sweep angle, and V and M are the airplane's speed and Mach.
Imagine a straight-wing airplane flying at its maximum Mach number. As you sweep the wing in flight from 0 to L degrees, the available lift drops by a factor of [cos(L)]^2, and the Mach compressibility effects on the wing's airfoils decrease (weaker shocks, etc.). You then increase the speed by a factor of 1/cos(L), so that the effective dynamic pressure and lift are increased back to their original levels. The effective Mach is also increased back to its original level. In effect, you haven't done anything to the wing's lift or compressibility effects, but the airplane is now flying faster!
In reality, this isn't a complete freebie, since the skin friction drag has increased by a factor of [1/cos(L)]^2 -- the wing skin friction isn't affected by sweep very much, and feels the full brunt of the real dynamic pressure increase, just like the rest of the airplane. So the overall L/D will typically decrease from the sweep. An airliner depends on the higher speed to more than compensate for the lower L/D and give better overall range (the product V x L/D is what appears in the range equation). And of course flying faster gives faster revenue stream for the airlines.
Sweep doesn't make sense on slower piston and turboprop airplanes. In general, if Mach number is not a speed-limiting factor, it makes more sense to get more speed by reducing the wing area.
