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[FUT to lugnet.space since there is no lugnet.air <G>]
In lugnet.build.contests, Bram Lambrecht writes:
> Trevor Pruden writes:
> > Although I think that it was said by somelse before, I don't know if
> > that forward swept wing is that air worthy.
>
> I believe I've read somewhere that forward swept wings can work. And I
> think they look more unique, but I can see why someone would prefer that
> they sweep the other way. It's a bit hard to change that now :)
I just wanted to elaborate on what Amy Hughes has already posted
in this thread.
In subsonic flight (below mach 1), sweeping a wing *aft* increases
the aircraft's directional stability. This means that if the plane
is rolled slightly to one side, the swept wing gives it an inherent
natural tendency to level the wings again. The greater the sweep
angle of the wing, the greater this stability. In fact, a large
sweep angle can make an aircraft so stable that it becomes hard
to turn---the plane is fighting you because it has such a strong
natural tendency to fly straight-and-level.
The opposite occurs with a forward swept wing. A forward sweep
angle decreases directional stability. If the plane is rolled
slightly to one side, it will actually want to roll even further
to that side. There is no natural tendency to level out again.
So the pilot will have to fight against the plane to bring it
back to level. With larger forward sweep angles, this instability
worsens, and the plane becomes uncontrollable..., by a human.
So by adding forward sweep to the wing, the aircraft is more
interested in turning than in flying straight. As a result it
is very responsive to pilot controls and highly manoeuvrable.
But too much of a good thing means the aircraft responds faster
than the pilot can and the aircraft is uncontrollable. You then
pile on the flight-control computers which allow the pilot to
fly an uncontrollable aircraft without screwing it into the
ground.
From a structural point of view, forward-swept wings aren't good.
Almost any aircraft has a critical speed at which the wings want
to bend so much that they may fail (as in rip off). Sweeping
a wing forward means this occurs at a much lower speed. By
using clever construction lay-up tecniques with composite
materials this effect can be reduced.
The air flowing over the wings is always going faster than
the plane itself. If the air ever gets to sonic speed (mach 1)
pressure shocks will form. At high subsonic aircraft speeds (say mach
0.8 to 1), the air over the wings may be sonic (mach 1 or greater).
This means there will be shocks on the wings, which will
destroy the lift that keeps the plane flying and controllable.
Sweeping the wings either forward or aft will lower the air
speed over the wings. This means the aircraft can fly a little
faster before the shocks form on the wings, which is the major
reason many aircraft designs have swept wings to begin with.
Going into the supersonic flight regime (over mach 1) you usually
want an aft swept wing. This keeps the wing behind the mach cone
formed by the aircraft's nose. Ahead of the cone is supersonic
air, and behind it is subsonic, so the wing will still be in
subsonic air. If you sweep the wings
forward, the wing tips may be ahead of the mach cone from the
plane's nose, and will actually form mach cones of their own.
You end up with a wing that is partly in supersonic air (ahead
of the cones) and partly in subsonic air (behind the cones).
There will be shocks on the wing between these zones. This
is A Bad Thing. The result is lost lift and controllability
problems.
As well a forward-swept wing in supersonic flight will not generate
as much lift from the area inboard near the fuselage. The aft
swept wing will do better, and therefore the aft-swept wing uses it's
planform area much more efficiently.
Personally I still think the FSW looks the coolest! :] But if
you're dealing with atmospheric flight at supersonic speeds,
you're going to want an aft-swept wing. But if you're looking
for high manoevrability at subsonic speed, throw on the
forward-swept wings and have you pilots prepare to party!
KDJ
_______________________________________
LUGNETer #203, Windsor, Ontario, Canada
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