Re: Fuel Vent Line

David J. Gall

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. Anybody remember the excellent manometer tests that the late Larry Koutz did on his cowl outlets? It might be beneficial if someone did similar tests on the pressure inside the cowl near the carb vs. near the various fuel tank vent locations. In fact, the pressure in the float bowl could be measured against the pressure in the fuel tank directly, on the ground with the engine running to give some indication of the expected in-flight pressure performance of the fuel system....

Back to lurking (and hacking foam),

David J. Gall
P.S. Who was looking for Onan engine mount parts? I might have some in the garage; email me off list.

----- Original Message -----
From: French <LJFrench@...>
To: Q-LIST@...
Subject: Re: [Q-LIST] Re: Fuel Vent Line
Date: Tue, 8 Aug 2006 20:04:17 -0500

Remember, what really matters - even with a float bowl carburator - is the
difference in pressure between the carb bowl and the tank pressure. After
all, all the vent pressure in the world coming from ram air will not do any
good if the float bowl is seeing the same pressure. Same goes for the head
pressure of the fuel supply above the carb. A header tank 10 feet above the
carb doesn't help if the float bowl pressure overcomes the added head
pressure. If you measure 100 GPH fuel flow from your fuel system on the
ground at the carb, it doesn't mean in any way that you have a system
capable of delivering any where near that amount of fuel flow in flight.

Anybody know what pressure is in the float bowl under various flight
conditions (a clue is in the mixure control...anybody need the MA-3SPA

Anybody know what pressure is being provided by the ram air vent under
various flight conditions??

Woudn't it be awsome if there was some enhancement to the vent system of the
plans Q200 that would avoid fuel starvation if you accidently flooded the
vent system by overflowing the main tank from the auxillary tank, or if the
gas cap accidently got a crack in it that didn't get detected, or any of the
many other "accidents" that have happened with our systems. Now that would
really be cool.......a root cause fix.

Lynn French

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