posted on Jul, 11 2006 @ 01:21 PM
This is in my undergrad aero notes, which are in the lab, but off the top of my head:
The complicated explanation [skip to simpler one at bottom if yez want]
The desire for swept wings was led by the possibility of optimising the wing sweep through the flight profile.
There is an equation for aerodynamic efficiency, well, induced drag IIRC, I think the factors in it include the wing volume, flight mach number,
aspect ratio and lift coefficient. It reads something like [I'm very rusty here, I think there are other variables]
Cdi = CL*(1+X)
X is just a factor dependant on wing volume, aspect ratio, Mach number etc - using variable geometry wings allows you to keep X at 0, meaning you pay
theoretically a 0 penalty for supersonic drag rise.
* I'll check my notes next week and correct this if anyone is that keen.*
The air speed at 90 degrees to the wing leading edge is what creates the lift and drag of the wing, ideally you want to run the aircraft as much in
its design zone as possible [for sake of argument lets say thats 150 -> 400 mph at sea-level].
So, if we have a wing swept at 60 degrees, your optimal aircraft speed is actually 300 -> 800 mph.
Obviously 300 mph is too quick for a good take-off speed, but by sweeping the wing forward to 0 degree sweep, aircraft optimal speed is back to 150mph
up to 400, which is much better.
As you increase up the speed range, and transonic drag rises start to move the aerofoil/wing out of its best operating zone, you sweep the wings back
to lower the "wing speed", taking it back into optimal zone.
[edit on 11-7-2006 by kilcoo316]