There are different types of fences. Boundary layer fences usually
start near the leading edge, and can be used to control the spanwise
flow component (normal airflow is chordwise). They can control the
propagation of the stall, are often placed inboard of the ailerons to
keep them working at high angles of attack.

Aileron and flap fences close the gap between the movable surface and
the fixed trailing edge of the wing. They are used to prevent higher
pressure air from below the wing sneaking through the gap to the lower
pressure above.

"BTIZ" wrote in message news:yFRob.119670$La.49811@fed1read02...
if I remember correctly... fences are added to reduce the propagation of the
stall from wing root outward over the ailerons, keeping the ailerons flying
just a bit longer in stall regimes.. normally found on STOL equipped
aircraft.

BT

"Doug Haluza" wrote in message
om...
"tango4" wrote in message
...
On an aircraft like the ventus there are several sections of control
surface
along the T/E of the wing resulting in several 'breaks' in the trailing
edge. My question is - Are the breaks or corners of a deflected control
surface the source of their own little vortices and therefore additional
(
very small) drag? I know that the inboard end of flaps is an area that
causes a lot of problems, my ventus has fences fitted there and some
people
fit fences to the aileron joints. Just how much drag difference do
aileron
fences make given that they increase the wetted area by several square
inches themselves?

Ian

The outboard end of a deflected flap can produce a very strong vortex.
You can see this from the ground (or the back of the cabin) when
alomst any transport category aircraft is on approach to land on a
humid day.

As you correctly point out, the addition of a fence is a tradeoff.
Since the flap is not always deflected, there is an additional duty
cycle tradeoff, because the fence is always in the flow.

There is also a safety tradeoff, since the fence can fail and jam the
control surface. A jammed flap is manageable, but a jammed aileron is
not.

My general understanding is that aileron fences are usually added to
enhance aileron authority, not to reduce drag directly. There may be
an incidental reduction in drag by using a smaller control surface
with fences, however.