Fences on new aircrafts?

[Tom Sanderson]

> I was wondering why fences are hardly or never used in newer airliners and
> fighters, whereas most of older sweep wing aircraft had fences to redirect
> spanwise flow.

They're not really there to redirect the spanwise flow. They use the
spanwise flow to generate a vortex to help energize the boundary layer and
flow attached. They also help keep the separation points steady at very
high angles of attack so that you don't generate wierd rolling moments as
you approach stall.

Fences (and leading edge notches, which do the same thing) are easy and
relatively "free" fixes for aerodynamic problems elsewhere on the wing.
With modern CFD codes, lots of experience, and really good wind tunnel
testing you can design the wing so that it doesn't need them. That's why
you don't see them very often on newer designs.

Tom.

[FatKat]

> I was wondering why fences are hardly or never used in newer airliners and
> fighters, whereas most of older sweep wing aircraft had fences to redirect
> spanwise flow.

I figured they were out on fighters because they might enhance
radar-cross section; as for airliners, don't newer airliners use wings
with less sweep?

[FatKat]

> I was wondering why fences are hardly or never used in newer airliners and
> fighters, whereas most of older sweep wing aircraft had fences to redirect
> spanwise flow.

I haven't paid that much attention, but aren't the wings on most newer
airliners of lesser sweep than the first generation of jetliners?
Also, when were fences ever used on any jetliners? I can't recall
seeing them on, say, the 707. My understanding was that fences were
needed to minimize spanwise flow along wings swept back farther than
what you'd expect on anything in Pan Am livery.