Jim,

You're right, there is always a low pressure on the top side of the wing.
This is why a tank will drain so quickly if a fuel cap is left off or lose.
That is why the cap is vented in one direction. You want a vacuum is pull
air into the tank but don't want air pulled from the tank.

Bill


This is fantastic! And it makes so much sense. An instructor friend gave
me an
article with detailed drawings on how to position the vent. Mine was
correct
but I didn't understand (until now) why it was so critical. I would
imagine
that as the angle of attack would increase, as you say at high altitude,
there
would be a tendency for a lower pressure area to form above the right cap
as
well. What do you think?

Jim