Re: Fuel Vent Line

David J. Gall


Dang it, I hate it when I mess up like that! Yes, you're right. I was calculating in psf and writing psi. Then I went from there to inches of mercury and really screwed it up. So much for rushed emails. So much for conclusions, too. Anyway, if water is 8.35 lbs per gallon and fuel is 6 lbs per gallon, then 10 inches of water is about 14 inches of fuel... check my work for me please... so if I get something like 14 inches of fuel into the vertical part of the vent line while in cruising flight (due to turbulence or something(?)), there's a chance that the airspeed (ram pressure) will be enough to keep it from draining out and the weight of the fuel will be enough to prevent it from being pushed back up into the tank by ram pressure, and it'll act like a plug in the vent line....

David J. Gall

----- Original Message -----
From: "Bob Farnam" <>
Subject: RE: [Q-LIST] Re: Fuel Vent Line
Date: Wed, 9 Aug 2006 11:01:00 -0700

-----Original Message-----
From: []On Behalf Of
David Gall
Sent: Tuesday, August 08, 2006 7:15 PM
Subject: Re: [Q-LIST] Re: Fuel Vent Line

Good points, Lynn

Fuel weighs 6 lbs. per gallon. One gallon holds 231 cubic inches. If we
stack all those cubic inches atop one another, we get all 231 cubic inches
pressing down on one square inch of surface area at the bottom of the
container. Therefore, there is 6 lbs per square inch of fuel pressure at the
bottom of a column of fuel 231 inches tall. Dividing 6 lbs by 231 inches
yields a usable value of head pressure of 0.026 lbs per inch of height. So
if your header tank bottom (figuring a nearly empty tank) is 20 inches above
the carb needle valve, then the fuel pressure at the carb is .52 psi or
about ONE inch of mercury. Not a lot, and easily overcome by misguided ram
air pressure. At 60 mph the dynamic air pressure is about 9 psi or about 18
inches of mercury.

[Bob Farnam] Actually, at 60 mph the dynamic pressure of air at standard
conditions is about 1.5 inches of water. At typical Q200 cruise speed, it's
about 10 inches of water.

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