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 !