The upper forward hole is for the cross vent, which is necessary to equalize
pressure in the tanks, such as if one external vent (see below) is plugged
by bugs, for example. Imagine what happens if one tank's external vent is
plugged and there's no cross vent. The tank develops negative pressure --
and the fuel stops flowing down. That may result in fuel starvation and ruin
your day -- whether you inline valves for each tank, or just one valve at
or after the junction of the fuel lines from the two tanks. Regardless, the
cross vent is mandatory. I have some pictures of mine & will try to locate
them.
Of course, you also need the external vents, which could be snorkels coming
up from somewhere in the cross vent, or snorkels in the gas caps. Both
locations will put some positive air pressure into the tanks, which is a
good thing. Putting snorkels in the gas caps may be a little better, because
you don't run the risk of a blockage in the cross vent line between
snorkel(s) and tank(s), and snorkels in the gas caps are easily tested with
a blow when you add fuel, etc.
Not sure about your descriptions of the other holes. The bottom-most hole in
the root rib, about 7-8 inches forward of the torque tube, is for fuel line
output. The hole in the bottom of the wing is for a fuel tester drain.
If you use capacitance probe gauges (or some float gauge type -- as far as
that goes) you'll need to cut a hole for them, per their installation
instructions. See the photo I linked to before, where the capacitance probes
are shown as a roughly 2.5" dia. black "hockey puck" circle near the upper
forward part of the rib, about 2 inches forward of the cross vent
connection.
Ron
On Sat, Apr 19, 2008 at 10:20 AM, Craig Walls <
snowyrvr@mtaonline.net>
wrote:
So, if I get this, the upper hole at the front of the root rib would be a
vent to the other tank, the lower hole at the front of the rib would be
for
the fuel gage, and the lower hole at the rear would be for the fuel
pick-up.
In my manual they show a cross vent between the two tanks with a metal
tube
that extends through the roof to get a ram air effect. Not sure if
that's
necessary, could probably do the same thing with those fuel caps with the
ram-air tubes built in.
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