cruise if the fuel caps are sealed. MAM's caps are not sealed and I
certainly don't need that additional head anyway. So with the MAM fuel
caps, I'd suggest that snorkels may not be a great idea for everyone as
the caps will continuously vent that ram air pressure. I have one
snorkel in my cross vent and the cross vent through the cabin is
transparent. It always has some fuel trapped in it. In flight there is a
significant flow of air through that cross vent and out the fuel cap. I
used to think this was just air replacing fuel but it occurs even when
not feeding from that tank. Not just bubbles but continuous and vigorous
burps. I have not drilled the filler necks, so it is going out the cap.
Anyway in addition to the expense of venting fuel vapour and the
pollution, it is also keeping a combustible air mixture in the tanks.
Without this airflow, the vapor in the tank would generally be too rich
to burn. Perhaps not significant, but I can just barely imagine slightly
more risk from static during refueling and perhaps a tiny bit more risk
with in tank pumps or in tank electric quantity sensors as well.
Ken
Ron Shannon wrote:
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
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