Fuselage Vents

On Aug 20, 4:13 am, PMSC Member wrote:
On Aug 19, 9:15 pm, Uncle Fuzzy wrote:



On Aug 2, 3:25 am, Derek Copeland

wrote:
In that case, I suppose you would have to go onto oxygen a little bit lower
down with thefuselageextractionventfitted!

At 16:40 31 July 2008, Jim White wrote:

Now there is a thought.... Open the airventif you are close to
airspace
to make the logger think you are lower, and fit an extractionventif
you
want that height record.

jim- Hide quoted text -

- Show quoted text -

I made this vent to replace the access hatch for connecting my
controlshttp://griderpirate.smugmug.com/gallery/3702283_9hEKD#355235771_pv8zu
Next weekend I will fly with a manometer to compare static to cabin
pressure with the 'normal' hatch, then the 'Mandl Extractor' hatch. I
can report subjectively that I was MUCH more comfortable in the
cockpit.

Nice work.

Now DB is a pretty smart guy, so chances are he has this figured out.
But I sure don't. I can't see how exhausting air into the low
pressure area that's responsible for producing aerodynamic lift is a
good thing. Perhaps the mass flow is trivial and doesn't matter as
long as the flow isn't spoiled. But we sure go to a lot of effort to
eliminate air leaks in this area and sealing surely does help....

Ahhh, another victim of the all too common "equal path length theory".
Although a wing does generate a certain amount of reduced pressure on
it's upper surface and that does contribute to the lift, the
dominating force of lift rather comes from the thrust generated by the
air being flung down off the trailing edge for the wing. To account
for the actual physics of flight, one must also rely on Newtonian
physics coupled with Coanda Effect (http://en.wikipedia.org/wiki/Coand
%C4%83_effect ), since if you rely on Bernoulli alone a plane can not
fly inverted! See: http://www.eskimo.com/%7Ebillb/wing/airfoil.html
for much better explanation.

-Paul

On Aug 20, 10:12*am, sisu1a wrote:

Ahhh, another victim of the all too common "equal path length theory".
Although a wing does generate a certain amount of reduced pressure on
it's upper surface and that does contribute to the lift, *the
dominating force of lift rather comes from the thrust generated by the
air being flung down off the trailing edge for the wing. To account
for the actual physics of flight, one must also rely on Newtonian
physics coupled with Coanda Effect (http://en.wikipedia.org/wiki/Coand
%C4%83_effect ), since if you rely on *Bernoulli alone a plane can not
fly inverted! See:http://www.eskimo.com/%7Ebillb/wing/airfoil.html
for much better explanation.

-Paul

Ahhh, another victim of the equating "Bernoulli" with "equal path
length theory" fallacy :-)

The "thrust generated by the air being flung down off the trailing
edge" that you refer to is simply the total difference between the
air pressure on the top surface and the bottom surface.

The top surface pressure is MUCH lower that ambient and the bottom
surface pressure is only a little higher that ambient, so most of the
lift is caused by the reduction of pressure on the top of the wing.

If you aren't using Kutta–Joukowski, then you are just making noise
about lift. See these articles. They will allow you to actually
calculate the lift !

http://en.wikipedia.org/wiki/Kutta_condition
http://en.wikipedia.org/wiki/Kutta%E...kowski_theorem

Todd Smith

Pilots should not be allowed to discuss aerodynamics without a
license !

On Aug 20, 10:12 am, sisu1a wrote:
On Aug 20, 4:13 am, PMSC Member wrote:



On Aug 19, 9:15 pm, Uncle Fuzzy wrote:

On Aug 2, 3:25 am, Derek Copeland

wrote:
In that case, I suppose you would have to go onto oxygen a little bit lower
down with thefuselageextractionventfitted!

At 16:40 31 July 2008, Jim White wrote:

Now there is a thought.... Open the airventif you are close to
airspace
to make the logger think you are lower, and fit an extractionventif
you
want that height record.

jim- Hide quoted text -

- Show quoted text -

I made this vent to replace the access hatch for connecting my
controlshttp://griderpirate.smugmug.com/gallery/3702283_9hEKD#355235771_pv8zu
Next weekend I will fly with a manometer to compare static to cabin
pressure with the 'normal' hatch, then the 'Mandl Extractor' hatch. I
can report subjectively that I was MUCH more comfortable in the
cockpit.

Nice work.

Now DB is a pretty smart guy, so chances are he has this figured out.
But I sure don't. I can't see how exhausting air into the low
pressure area that's responsible for producing aerodynamic lift is a
good thing. Perhaps the mass flow is trivial and doesn't matter as
long as the flow isn't spoiled. But we sure go to a lot of effort to
eliminate air leaks in this area and sealing surely does help....

Ahhh, another victim of the all too common "equal path length theory".
Although a wing does generate a certain amount of reduced pressure on
it's upper surface and that does contribute to the lift, the
dominating force of lift rather comes from the thrust generated by the
air being flung down off the trailing edge for the wing. To account
for the actual physics of flight, one must also rely on Newtonian
physics coupled with Coanda Effect (http://en.wikipedia.org/wiki/Coand
%C4%83_effect ), since if you rely on Bernoulli alone a plane can not
fly inverted! See:http://www.eskimo.com/%7Ebillb/wing/airfoil.html
for much better explanation.

-Paul

I've got the basic Newtonian physics pretty well understood, actually,
but that is a good point. I think the popular misconception you refer
to is actually "equal transit time" not "equal path length" btw. And
if it turns out that I can be convinced that a turtle deck exhaust is
okay aerodynamically, I am absolutely going to steal your idea for my
ASW-20. Imitation, after all, is the sincerest form of flattery.

Intuitively it would seem that other low pressure areas on the
fuselage, typically behind the gear doors (DG) or near the tail (Wil
Scheumann, many others) would be a better pick. However, intuition
often leads astray in aerodynamics.

There's a lot of ways to look at this - and they all sort of work.

Putting aside for a moment the problem of designing a low drag air exit,
it's easy to see that creating a low pressure inside the glider is a good
idea. Anywhere there is a surface discontinuity (like the canopy or hinge
line) there is a likelihood of tripping the flow to turbulent. If air is
flowing out of this discontinuity it's the equivalent of a blowhole
turbulator. If you can reverse the flow with low internal pressure, it
becomes suction boundary layer control. If the low pressure extends to the
entire glider, you are sucking air in through control hinges. The potential
performance gain from this is significant.

Then, the "no free lunch" principle kicks in. Maintaining pressure
differentials in a leaky glider takes energy. Cleverly designing a low
energy cost air exit that takes advantage of low pressure areas is the key -
but it's not easy. I remember a professor saying that the main thing you
need to know about air scoops is they're ugly and usually don't work.

I expect to see many gliders festooned with ugly reverse scoops - some
functional, most not. Putting them on the lower fuselage just aft of the
thickest point has the advantage that you can't see them.

Then there is a 70 year old idea from Dr. Gus Raspet - suction fans. Thin
film solar cells are slowly creeping up in efficiency and down in cost.
Tiny, very efficient fans are found everywhere on electronics. Put a pan
fan on that access hatch powered with a stick-on solar panel. At a minimum
it'll help keep the cockpit cool on the ground.





"sisu1a" wrote in message
...
On Aug 20, 4:13 am, PMSC Member wrote:
On Aug 19, 9:15 pm, Uncle Fuzzy wrote:



On Aug 2, 3:25 am, Derek Copeland

wrote:
In that case, I suppose you would have to go onto oxygen a little bit
lower
down with thefuselageextractionventfitted!

At 16:40 31 July 2008, Jim White wrote:

Now there is a thought.... Open the airventif you are close to
airspace
to make the logger think you are lower, and fit an extractionventif
you
want that height record.

jim- Hide quoted text -

- Show quoted text -

I made this vent to replace the access hatch for connecting my
controlshttp://griderpirate.smugmug.com/gallery/3702283_9hEKD#355235771_pv8zu
Next weekend I will fly with a manometer to compare static to cabin
pressure with the 'normal' hatch, then the 'Mandl Extractor' hatch. I
can report subjectively that I was MUCH more comfortable in the
cockpit.

Nice work.

Now DB is a pretty smart guy, so chances are he has this figured out.
But I sure don't. I can't see how exhausting air into the low
pressure area that's responsible for producing aerodynamic lift is a
good thing. Perhaps the mass flow is trivial and doesn't matter as
long as the flow isn't spoiled. But we sure go to a lot of effort to
eliminate air leaks in this area and sealing surely does help....

Ahhh, another victim of the all too common "equal path length theory".
Although a wing does generate a certain amount of reduced pressure on
it's upper surface and that does contribute to the lift, the
dominating force of lift rather comes from the thrust generated by the
air being flung down off the trailing edge for the wing. To account
for the actual physics of flight, one must also rely on Newtonian
physics coupled with Coanda Effect (http://en.wikipedia.org/wiki/Coand
%C4%83_effect ), since if you rely on Bernoulli alone a plane can not
fly inverted! See: http://www.eskimo.com/%7Ebillb/wing/airfoil.html
for much better explanation.

-Paul

On Aug 19, 7:34*pm, Uncle Fuzzy wrote:

Speaking of thanks.... in characteristic style, I neglected to than
Bob Kuykendall for his advice regarding mold making, and DG for
publishing their findings in the first place. *Does any other glider
manufacturer 'share' as much? This is an honest question. *I've spent
quite a lot of time on the DG site, very little on any others.

You're quite welcome, of course. Nice looking piece, I think you
nailed the shape! Interesting to see how your idea follows that of
Dick Butler--exhausting the vent air on top instead of on the bottom.

Now you get to experience the joys of sanding, filling, sanding, and
painting a 3D shape... Some folks ask me why I still mold on gelcoat,
why do I do that when others have made a practice of molding bare and
painting afterwards? My general answer is so that I can put the orange
peel and pinholes on the inside where nobody will ever know they're
there.

Thanks, Bob K.

On Aug 20, 7:59*am, Bob Kuykendall wrote:
On Aug 19, 7:34*pm, Uncle Fuzzy wrote:

Speaking of thanks.... in characteristic style, I neglected to than
Bob Kuykendall for his advice regarding mold making, and DG for
publishing their findings in the first place. *Does any other glider
manufacturer 'share' as much? This is an honest question. *I've spent
quite a lot of time on the DG site, very little on any others.

You're quite welcome, of course. Nice looking piece, I think you
nailed the shape! Interesting to see how your idea follows that of
Dick Butler--exhausting the vent air on top instead of on the bottom.

Now you get to experience the joys of sanding, filling, sanding, and
painting a 3D shape... Some folks ask me why I still mold on gelcoat,
why do I do that when others have made a practice of molding bare and
painting afterwards? My general answer is so that I can put the orange
peel and pinholes on the inside where nobody will ever know they're
there.

Thanks, Bob K.

LOL! The bodywork is tedious but simple. I can do that. Next time
I'm going to get REAL tooling resin! BTW, I am getting senile. AFTER
all the pain of making a fiberglass mold from the hatch, I realized I
could have pulled one on my vacuum form in minutes. I even have
several flavors of theromplastics on hand. D'oh! Mold rigidity is
achieved by filling the back of the mold with plaster of paris, which
I ALSO had on hand. Maybe an hour of hands-on work to produce the mold.

On Aug 20, 10:35 am, toad wrote:


If you aren't using Kutta–Joukowski, then you are just making noise
about lift. See these articles. They will allow you to actually
calculate the lift !

Naw, they'll allow me to calculate the 2-D circulation, which I really
don't care about. And even DB has a finite wingspan :-).

Pilots should not be allowed to discuss aerodynamics without a
license !

We just do it to annoy :-).

How come you're not at R3?

-T8

On Aug 20, 8:19 am, BB wrote:
I posted pictures of Dick Butler's air exhaust in my Uvalde blog.
You need to accelerate the air smoothly inside the fuselage
then direct it to exit parallel the exterior flow; these photos
show how Dick did it.

OK, I need help to understand this from real aerodynamics geeks. I've
heard that these exits are placed in "areas of low pressure", and the
turtledeck sure looks like an obvious candidate for such an area. But
if you exhaust air there, surely you get separated or at least
disturbed airflow downstream of it. Just past "low pressure" is where
there should be "pressure recovery", and I would think venting air to
the low pressure point destroys the pressure recovery. For example,
the lowest pressure place possible would be right on the top of the
wing in the middle. But venting air there is obviously suicide. If
anything you want to suck air at that point to keep the boundary layer
attached. OK, if you've found a low pressure area where airflow is
already separated behind it, that would seem ok, which is what the
vents at the bottom of the rudder or through control horns do. But
otherwise, aren't you creating more drag than you reduce, by causing
separated flow downstream of the vent?

John Cochrane BB

Careful ! A few points...

Do not confuse "separated" with "turbulent".
Separation is BAD.
Turbulent (non-laminar) is OK and less prone to separation in areas
aft of canopy for example. Modern gliders like yours and Antares
use turbulent (non-laminar) airfoils near the fuselage.

Lot's of "obvious low pressure" areas are not, for example
underneath fuselage (you have to look at the pressure analysis
for the particular fuselage).

What Dick did was add turbulent flow parallel to already
turbulent flow. The acceleration funnel is necessary to
avoid turbulence due to big speed mismatch.

Hope that helps...
Best Regards, Dave "YO electric"

PS: Great to see you in Uvalde !

On Aug 20, 11:41*am, PMSC Member wrote:
On Aug 20, 10:35 am, toad wrote:

If you aren't using Kutta–Joukowski, then you are just making noise
about lift. *See these articles. *They will allow you to actually
calculate the lift !

Naw, they'll allow me to calculate the 2-D circulation, which I really
don't care about. *And even DB has a finite wingspan :-).

You can't calculate the lift without the circulation :-)


Pilots should not be allowed to discuss aerodynamics without a
license !

We just do it to annoy :-).

That works !


How come you're not at R3?

Currently I am too poor.


-T8

Todd

On Aug 20, 1:37 pm, toad wrote:
On Aug 20, 11:41 am, PMSC Member wrote:

On Aug 20, 10:35 am, toad wrote:

If you aren't using Kutta–Joukowski, then you are just making noise
about lift. See these articles. They will allow you to actually
calculate the lift !

Naw, they'll allow me to calculate the 2-D circulation, which I really
don't care about. And even DB has a finite wingspan :-).

You can't calculate the lift without the circulation :-)

Based on the mass of my glider, I know the lift!


Pilots should not be allowed to discuss aerodynamics without a
license !



How come you're not at R3?

Currently I am too poor.

Which just goes to show what we as pilots knew all along: it's the
circulation of *money* that creates lift :-).

Here's wishing you more of that.

Thanks for the Wiki links btw.

-T8