"Gary Emerson" wrote in message
m...
Hopefully thermal expansion will not become an issue....splitting a wing
open because it warmed up in the sun could ruin your day.... Your "check
valve" has the potential to allow such a situation...

I agree,
A solution would be a valve that positively closes as well as opens.
The mentioned flap type valve will stick and any pressure build up
will put undue stress on the tank.
In the case of vents being at the wing tips, a rolling type ball valve will
do the job. The ball is stainless steel and it will roll into position when
the wing is down, when horizontal the ball will unseat and roll back.
I made at prototype valve for my last project and I was only able to fly
with it once before I sold the glider. There was a minimal acceptable leak
when the wing was down. When the wing was horizontal I was able to see the
ball clear the seat. ( I used a clear housing to observe)
Regards
Udo