New Vent!

The non-technical write-up at DG on the Mandl air extractor
experiments backs up your position (http://www.dg-flugzeugbau.de/mandl-absaugung-e.html
). But graph 7a in Prof. Boermans' paper (http://frotor.fs.cvut.cz/doc/37.pdf
) suggests that the tail position is on a par with the pressure in
back of the wing.

Well I wish he would have told my glider that news, cause it
apparently didn't get the memo My tail vent does not work -at all.
Like most gliders with tail vents... My forward located vent does,
like a champ. It also works on other gliders, and the difference is
not subtle.

Even if the pressures were equal, the forward spot still wins by a
long shot since the air doesn't have to travel such a long distance,
past obstacles, down a tapering duct (ducting adds significant static
resistance, a tapered one even more so, and bulkheads with holes in
them are dealbreakers...), and out an orifice that even if coupled to
the same differential pressure is not a shape that is as conducive to
generating low pressure (not efficiently anyhow...).

The low pressures being generated by these shapes are small. The
penalties of inefficient routing like the same old standard tail
configuration adds however are not. But as I understand it, the
pressure at the tail is kinda on a ship by ship basis. Some ships even
suck water up the tailpipe when blowing ballast, which is a pretty
good indicator of a poor choice for a 'low pressure' location. Also,
whenever I've looked at color coded pressure distribution charts I've
never seen it as hot at the base of the rudder as it is on the dorsal.
When I look at the Beorman graph however, I see a spike in pressure at
the tailvent location, up to +.175 or so... the numbers on the left
get smaller moving up... There *is an unexpected low pressure knee
back there, but it is well before the end of the tail where vents
would be and the same ducting/obstacle penalties still apply.

-Paul

Paul,

Thanks! I knew you'd been experimenting, so I knew you agreed with
DG's findings. Sorry to say that I mis-read Boermans' graph, I
"assumed" the pressure was lower at the lower end of the Y axis.
slaps forehead So, if we agree that the tail vent would be located
at x=0.95 (not at 1.0, that's the end of the rudder), it should have a
pressure similar to the aft wing position, say, x=0.35.

My vent works pretty well on my LS8, but on my ASW-19 it was abysmal.
I was putting it down to the ASW having fairly small openings around
the rudder horns, as well as having those goofy NACA ducts under the
wing that reversed the incoming air before reversing it again at the
inside air vent. The LS has a nose intake, plus hefty openings around
the rudder, and it produces a nice air stream. Still, DG claims much
better venting by using the Mandl air extractor, as well a performance
gains.

-John

On Dec 7, 12:45 pm, sisu1a wrote:
Well I wish he would have told my glider that news, cause it
apparently didn't get the memo My tail vent does not work -at all.
Like most gliders with tail vents... My forward located vent does,
like a champ. It also works on other gliders, and the difference is
not subtle.

Even if the pressures were equal, the forward spot still wins by a
long shot since the air doesn't have to travel such a long distance,
past obstacles, down a tapering duct (ducting adds significant static
resistance, a tapered one even more so, and bulkheads with holes in
them are dealbreakers...), and out an orifice that even if coupled to
the same differential pressure is not a shape that is as conducive to
generating low pressure (not efficiently anyhow...).

The low pressures being generated by these shapes are small. The
penalties of inefficient routing like the same old standard tail
configuration adds however are not. But as I understand it, the
pressure at the tail is kinda on a ship by ship basis. Some ships even
suck water up the tailpipe when blowing ballast, which is a pretty
good indicator of a poor choice for a 'low pressure' location. Also,
whenever I've looked at color coded pressure distribution charts I've
never seen it as hot at the base of the rudder as it is on the dorsal.
When I look at the Beorman graph however, I see a spike in pressure at
the tailvent location, up to +.175 or so... the numbers on the left
get smaller moving up... There *is an unexpected low pressure knee
back there, but it is well before the end of the tail where vents
would be and the same ducting/obstacle penalties still apply.

-Paul

Well, Paul isn't tooting his own horn with the experimentation he's
done, but I can attest to how well the extractor works that he has
built. He built a prototype for my ASW-20 that replaces the control
hookup hatch. The difference is incredible. I am able to fly with
the side window vents closed most of the time now, even when it's
pretty hot and we aren't getting that high. Not that heat has been a
problem for the last month or so. The whistling and air noise is so
significantly reduced that I found myself initially thermalling at 5+
knots higher airspeed than before, just because of the reduced audio
feedback. It took a bit to recalibrate to the reduced noise.

It's good enough that I was already considering modifying my Duo.
Especially if you live somewhere hot, the comfort is well worth it
regardless of if the performance impacts.

Morgan

from Paul (sisu1a):
..... *But as I understand it, the
pressure at the tail is kinda on a ship by ship basis. Some ships even
suck water up the tailpipe when blowing ballast, which is a pretty
good indicator of a poor choice for a 'low pressure' location.
...

Paul (ALL)

I have this problem with my Genesis 2 (both water and "other" fluids).

Does this come from a "venturi" effect of air leaking from Horz/Vert
Stab Junction or poor Rudder Sealing? It amazes me how much gets
sucked back into fuselage. (Genesis has VERY short fuselage)

Another Genesis owner (DK) has experimented with an exit duct out the
fuse hatch over wings. ... comments Don???

Curt - 95

On Dec 8, 12:46*pm, CLewis95 wrote:
from Paul (sisu1a):

..... *But as I understand it, the
pressure at the tail is kinda on a ship by ship basis. Some ships even
suck water up the tailpipe when blowing ballast, which is a pretty
good indicator of a poor choice for a 'low pressure' location.
...

Paul (ALL)

I have this problem with my Genesis 2 (both water and "other" fluids).

Does this come from a "venturi" effect of air leaking from Horz/Vert
Stab Junction or poor Rudder Sealing? *It amazes me how much gets
sucked back into fuselage. *(Genesis has VERY short fuselage)

Another Genesis owner (DK) has experimented with an exit duct out the
fuse hatch over wings. ... comments Don???

Curt - 95
Likely due to pressure recoveing over the very short length of
fuselage and large reduction in cross section
over that length. High location closer to centerline would probably
better in order to avoid effects on root fillet
area.
Just speculating- It's cold here.
UH

I have this problem with my Genesis 2 (both water and "other" fluids).

Does this come from a "venturi" effect of air leaking from Horz/Vert
Stab Junction or poor Rudder Sealing? *It amazes me how much gets
sucked back into fuselage. *(Genesis has VERY short fuselage)

Another Genesis owner (DK) has experimented with an exit duct out the
fuse hatch over wings. ... comments Don???

Curt - 95


I have had an air outlet on my Genesis2 for several years. It was
modeled after the outlet on DB's ASW-22. It seems to work very well.
Curt is correct that on a Genesis 2, dumped water ballast will enter
the fuselage thru the fairings over the rudder horns. These fairings
are suppose to be the air outlets. I have taken photos of that area
with tufts taped around the fairings. And indeed the tufts turn and
are sucked into the "outlets".

Don (DK)