| Bill,
I happen to be putting these valves in now. I talked to the lady at Andair and got an answer that approximates what you have below. ....However, it would be comforting to have some actual data. Did you happen to do any testing?
e.g. in a 2000 fpm climb you need to lose about 1.5" of mercury per minute to avoid pressurizing. The worst case would be nearly empty tanks because more volume of air would have to escape to maintain nearly neutral pressure.
On May 18, 2011, at 8:42 AM, Bill Harrelson wrote:
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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