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