Colyn,
The ES has tiny NACA scoops in the wingtips like the IV with the
thought that they would slightly pressurize the fuel tanks to help the fuel feed
to the engine. Sort of makes sense to me. I have no idea if it works
at all. I've never found a bug in a vent line, but the vent lines are
definitely part of my preflight inspection. I would guess that vent lines
don't get clogged during flight (maybe ice?). They only get foreign matter
in the inlet on the ground.
I can only think of two times that you need to have air flowing out of a
fuel tank. One is when ambient temperatures rise and the air expands in the
tank. The other is during a climb when you're feeding off the other
tank. Fuel also expands with heat, but not nearly as much as air.
We've all had fuel spill out a vent from heat expansion when we've topped of our
tanks and left the plane on the ramp. None of these is a particularly high
flow rate so the two small holes in the Andair check valve should be able to
handle that amount of back flow.
My bigger concern is how much the Andair valve restricts the air flow into
the tank when you're using fuel from that tank. Andair lists a "cracking
pressure" of 0.4 to 0.7 psi. I'm not capable or qualified to analyze how
much that would effect fuel flow in a low wing aircraft. But the
difference between a small NACA scoop slightly pressurizing the tank and a
check valve that creates a slightly negative pressure in the fuel tank might be
enough for me to leave check valves off my plane. The
number of installed check valves probably proves this isn't really an issue,
however.
You can't inspect them during a preflight either.
You might be able to make the argument that check valves ad a potential
failure point that only eliminates a slight inconvenience of a little fuel
spilling out while taxiing.
Mike Easley
Colorado Springs
That
would be 1.4 miles per hour. Wonder what a naca vent that gets the air
to slow down smoothly from 250 mph down to 1.4 looks like?
I had to drain a tank on a IV to repair a fuel leak. I plugged
the vent while I removed the drain valve from the bottom of the wing, with
the hope that it would slow the flow of fuel down my arm. I only got
about a half gallon from the tank until the flow slowed to a drip. I
don't think the engine driven pump or boost pump could overcome the vacuum
caused by a clogged vent line. So a clogged vent would stop the engine
I'm guessing, not cause it to surge.
I did the calculation one time to determine the air flow that has to
come in the vents to replace the fuel going into the engine. I have
3/8" vent lines on my ES and that air is moving through that vent line
a couple feet per second if I remember correctly.
Mike Easley
Colorado Springs
In a message dated 5/18/2011 6:42:57 A.M. Mountain Daylight Time,
n5zq@verizon.net writes:
Good point, Bill. The check valves supplied by Lancair for the vent
system on my IV are free flowing inward and “somewhat” restricted flowing
outward. This prevents venting a lot of fuel overboard while taxiing with
full wing tanks (no winglets) yet allows air to escape at a relatively
slow rate to avoid the problem that you describe. Certainly, a normal one
way valve (free flow one direction – no flow in the other) should NOT be
used on a vent system.
Bill Harrelson
N5ZQ 320 1,900 hrs
N6ZQ IV under construction
Sent: Wednesday, May 18, 2011 7:43 AM
Subject: [LML] Fuel restriction or air?
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{Did you verify the fuel vents are working
properly? ....and that the check valves on the vents are
oriented correctly?} There should be no check valves on vent lines. If
you take off at sea level with a wing tank at atmospheric pressure,
14.7 psia trapped by a check valve, and climb to 18,000 feet, static
pressure 7.35 psia, the differential pressure on the tank will be
7.35 psi, 1,058 pounds per square foot trying to rip the skins off
the spar and ribs. Vent lines must be free flowing both
directions.
Regards,
Bill Hannahan
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