Aileron / flap corner drag ?

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

"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.

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.

tango4 writes:
Just how much drag difference do aileron
fences make given that they increase the wetted area by several square
inches themselves?

I had the fences on the root ends of the flaps of my ASW-20 until recently when
I discovered by accident that they were like little parachutes on my wings. A
discarded pee bag (yes I toss them, so what) burst on the wing LE and left
telltale dried evidence that the local airflow in the region of the flap fence
was out of alignment with the fence by about 20 degrees, most likely creating a
large vortex with more drag that the resulting little gap once the fence was
removed.
-
Mark Navarre
ASW-20 OD
California, USA
-

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.

"Todd Pattist" wrote in message
...
"tango4" wrote:

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 Ventus C has those breaks covered with flexible
semi-permanent tape.
Todd Pattist - "WH" Ventus C
(Remove DONTSPAMME from address to email reply.)

I'm not a Ventus owner but a Nimbus 2C owner and I'm following this closely.

Todd, are you talking about the flap and aileron hinge gaps? I think the
question was about the end to end gaps between the each flap section and
between the outer flap and aileron when the flaps are deflected.

Bill Daniels

Hi Todd

So does mine, I was thinking more of the breaks between flaps and ailerons
where there is a differential between the surfaces or between the wing and
the aileron since, at least when I'm flying, they are rarely lined up for
more than a second or two and must be creating a small drag bucket all the
time.

Ian

"Todd Pattist" wrote in message
...
"tango4" wrote:

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 Ventus C has those breaks covered with flexible
semi-permanent tape.
Todd Pattist - "WH" Ventus C
(Remove DONTSPAMME from address to email reply.)

Hi Bill

The Ventus has one break in the aileron where the trailing edge taper
changes and there is a different hingeline. The chordwise gap betweeen the
two aileron sections is closed off with fabric type tape. The two 'ailerons'
move at the same rate but its made in two pieces I think because of its
length and the differing hingelines.

Ian



"Bill Daniels" wrote in message
ink.net...

"Todd Pattist" wrote in message
...
"tango4" wrote:

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 Ventus C has those breaks covered with flexible
semi-permanent tape.
Todd Pattist - "WH" Ventus C
(Remove DONTSPAMME from address to email reply.)

I'm not a Ventus owner but a Nimbus 2C owner and I'm following this
closely.

Todd, are you talking about the flap and aileron hinge gaps? I think the
question was about the end to end gaps between the each flap section and
between the outer flap and aileron when the flaps are deflected.

Bill Daniels

"tango4" wrote in message
...
Hi Bill

The Ventus has one break in the aileron where the trailing edge taper
changes and there is a different hingeline. The chordwise gap betweeen the
two aileron sections is closed off with fabric type tape. The two
'ailerons'
move at the same rate but its made in two pieces I think because of its
length and the differing hingelines.

Ian

Ah... OK. Thanks, Ian. I was thinking of the Nimbus wing where the flap
sections and ailerons all move down, but at different rates. That leaves
places along the TE where vortices can form as air escapes between the ends
of the flap/aileron sections. I can't think of anything to do about that.

Bill Daniels

Here's the basics of my idea.

Looking down on the wing in planform.

Imagine an aileron inboard end was cut at an angle of about 30 degrees to
the chord rather than the traditional 90 and the wing box ( or adjacent
flap ) was cut back at an opposite 30 degrees. The view from above would be
of a trailing edge with a 60 degree trianglular 'bite' missing. Now take a
triangular piece of aileron section that fills the 'bite' and at its apex
install a ball joint. Into the rear of the aileron spar mount a ball socket
to match the one on the triangle. Now you have a triangle section filling
the gap and free to rotate long the ships longitudinal axis. ( gee a picture
would work wonders here !)

By installing a mylar seal along the ( almost chordwise ) edges of the
aileron and wing (or adacent flap ) such that a sliding seal is made with
the triangular piece you might get an aileron / flap joint that doesn't kick
off that vortex.

This might be advantageous at the flap root end helping to clean up the flow
at the root training edge.

It all sounds rather complex but perhaps the .01 % difference it might make
would be worth it.

Any thoughts on this?

Ian

PS: There is a variation on the triangular piece that is more elegant but is
almost impossible to visualise without a drawing!