Hi Udo,

Yes, the Zuni has a vent line running from the outboard end of the
tank, back through the tank, and exiting the bottom side of the wing,
near the leading edge, ahead of the dump hole. This is not exactly as
you had asked about, but very similar. It does provide good venting of
the tank. It does have a drawback in that if the tanks are full and
the plane is slightly wing low (sitting with the wing wheel on, for
example), water will tend to siphon from the low wing out the vent
hole. This vent is about 5/16th diameter on my Zuni.

Also, the HP-18 built by my father, Robert Leonard, and now owned by my
brother, Ron Leonard, has a vent tube running the length of the wing.
It also exits the underside of the wing, but near the trailing edge,
just outboard of the root rib. I believe this vent tube is 1/2 inch
tygon tubing. His dump system comes out of the stock Schreder dump
port on the front face of the spar, goes through a 90 degree elbow to
go outboard, through a dump valve, and exits through the bottom of the
wing, one or two foam ribs outboard of the root rib. The intent of
this system was to keep the water plumbing all in/on the wing. That
way, there would be no leaks into the fuselage.

When building this HP-18, he had considered adding an extra ballast box
in the first bay ahead of the spar. His plan was to put an isolation
valve on this tank so he could either fly with half water (fill the
spar only) or full water (by openning the valve and filling both
tanks). He decided against this, and just stuck with the stock water
capacity. Is this similar to what you are planning to do, but with one
tank ahead of and one behind the spar?

But, yes. Vents back to the root do work. Oh, yes. You can set a wing
down and not worry about loosing your water! But you will fill the
vent lines and then drain at least part of that water overboard when
you level the wings for takeoff.

If you do tie all vents to a common manifold, you wil probably not be
able to be assured that you will not get some water into a tank you did
not fill. Amount would likely be vent line volume. Maybe a bit more
or less, depending on how you fill the system. My suggestioin would be
to keep each tank vent seperate.
Steve Leonard
Zuni II
HP-14
Too many others to mention ;-)