New Vent!

On Dec 6, 7:37*pm, sisu1a wrote:
http://tinyurl.com/337bok7* *...I'm sure it works like a champ. *Nice
work John/Hank/Dick!

Looks nice but where do I put all the stuff that now sits in the over-
spar storage area? I don't think I want to discard my landout kit so
I can fit a vent system that has an unknown (to me) performance
advantage.

For me to be interested in a kit for the ASW-28 it would have to
extend the storage area aft so that, with the vent kit installed, at
least the same storage volume was still available, preferably more.
There is lots of room to extend backwards as is done in the 27's.
Unlike shorter pilots there in no room behind my seat and everything I
carry has to go just forward of, or over the spar.

Where can I find a picture of the installation looking aft into the
spar area?

Andy

On Dec 7, 7:56*am, Andy wrote:
On Dec 6, 7:37*pm, sisu1a wrote:

http://tinyurl.com/337bok7**...I'm sure it works like a champ. *Nice
work John/Hank/Dick!

Looks nice but where do I put all the stuff that now sits in the over-
spar storage area? *I don't think I want to discard my landout kit so
I can fit a vent system that has an unknown (to me) performance
advantage.

For me to be interested in a kit for the ASW-28 it would have to
extend the storage area aft so that, with the vent kit installed, at
least the same storage volume was still available, preferably more.
There is lots of room to extend backwards as is done in the 27's.
Unlike shorter pilots there in no room behind my seat and everything I
carry has to go just forward of, or over the spar.

Where can I find a picture of the installation looking aft into the
spar area?

Andy

An interesting gadget. However, wouldn't it be more logical to vent
nearer the rear of the fuselage, say in the lower tail?

Mike

An interesting gadget. *However, wouldn't it be more logical to vent
nearer the rear of the fuselage, say in the lower tail?


Absolutely not... and this one of the major contributing factors to
why most glider's vent do not work. That's where designers have been
sticking them ever since Wil Shuman put one on his Libelle.

-Paul

Paul,

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.

-John

On Dec 7, 11:24 am, sisu1a wrote:
An interesting gadget. However, wouldn't it be more logical to vent

nearer the rear of the fuselage, say in the lower tail?

Absolutely not... and this one of the major contributing factors to
why most glider's vent do not work. That's where designers have been
sticking them ever since Wil Shuman put one on his Libelle.

-Paul

Isn't there a lot of drag in a long, narrow tail boom that will restrict
the amount of air flowing to the tail? Also, how big of an exhaust
outlet can you put back there?

Also, I don't think the Boersmans graph shows pressure on the top of the
fuselage, where these outlets are located.



On 12/7/2010 8:40 AM, jcarlyle wrote:
Paul,

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.

-John

On Dec 7, 11:24 am, wrote:
An interesting gadget. However, wouldn't it be more logical to vent

nearer the rear of the fuselage, say in the lower tail?

Absolutely not... and this one of the major contributing factors to
why most glider's vent do not work. That's where designers have been
sticking them ever since Wil Shuman put one on his Libelle.

-Paul

Greg,

I think the major effect on air exiting at the tail would be the total
area of tail vent. My LS8 tail has a lot of vent area (can't measure
it right now, but it's got to be 4 times the area of the inlet at the
nose). Even at that size, the interior diameter of the tail cone at
its narrowest has a much bigger area.

As for Boermans' graph, it's the total pressure around a "rotationally
symmetric body". I really have no idea from this graph what the
pressure is on the top or bottom of the fuselage, but somewhere (can't
find it now when I want it) I've seen a 3D colored pressure
distribution on a sailplane fuselage that clearly showed the pressure.
Unfortunately, I can't say where it showed the least pressure
appearing.

-John

On Dec 7, 11:52 am, Greg Arnold wrote:
Isn't there a lot of drag in a long, narrow tail boom that will restrict
the amount of air flowing to the tail? Also, how big of an exhaust
outlet can you put back there?

Also, I don't think the Boersmans graph shows pressure on the top of the
fuselage, where these outlets are located.

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