Fuselage Vents

Dear Uncle
If your altimeter changes indications when you open the vent, that
indicates your altimeter is not connected to STATIC. Best to check
you plumbing.

Guy Acheson, "DDS"



On Jul 30, 9:16�am, Uncle Fuzzy <james.d.wynh...@saic.com> wrote:

> Sounds plausible to me, but what do I know? �I DO know that at 70
> knots, opening the vent in my Speed Astir (AKA the 'Grob Solar Sauna)
> will drive the altimeter down 100' instantly, so there is positive
> pressure

On Jul 31, 9:40 am, Jim White <j...@boffins.co.uk> wrote:
> >Uncle Fuzzy decided that he'd like his ALTIMETER to read the same
> >pressure as his FLIGHT RECORDER when the pneumatic switch is in the
> >CABIN position on his line to the Altimeter.
>
> Now there is a thought.... Open the air vent if you are close to airspace
> to make the logger think you are lower, and fit an extraction vent if you
> want that height record.
>
> jim

I'd think the relatively sudden increase (or decrease) in the delta
between the GPS altitude and the Baro altitude would raise suspicions.
I know I'd look at it if I saw a significant jump, and I'm not exactly
a rocket scientist.

>
> I posted pictures of 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 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

On Aug 19, 7:34 pm, Uncle Fuzzy <james.d.wynh...@saic.com> 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
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, 10:35 am, toad <toads...@mac.com> 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 <john.cochr...@gsb.uchicago.edu> wrote:
> > I posted pictures of 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 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 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 <pmsc.mem...@> wrote:
> On Aug 20, 10:35 am, toad <toads...@mac.com> 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