Roger Halstead wrote in message . ..
On 10 Nov 2003 17:04:49 -0800, (Dan
Thomas) wrote:

Fuel tank venting can be tricky. If you have two tanks, and
there is a "both" position on the fuel selector valve, any difference
in vent pressure will cause one tank to flow faster than the other. If
the difference is large enough, it could actually prevent fuel from
flowing from the lower-pressure tank.
This arrangement has caused accidents in homebuilts in the past,
and it's the reason that certified airplanes having interconnected
tanks (the "both" position) must also, by law, have interconnected
tank venting to keep vent pressures equal.

I think you will find even certified planes like Bonanzas have an
either, but not both.

As I recall the Cherokee was the same way, but didn't have aux tanks.

Ram pressure won't be worth the effort, if you're counting on

It probably wouldn't be enough to push the gas half the height of the
tank unless you were really moving.

Roger Halstead (K8RI EN73 & ARRL Life Member)
www.rogerhalstead.com
N833R World's oldest Debonair? (S# CD-2)

pressure to make up for failed pumps. The fuel pressure, and therefore
flow, would be very inadequate and the engine would quit anyway.
The area of the scoop will have no effect on ram pressure.

Install a generic automotive electric fuel pump. Not expensive.

Dan

High-wing airplanes typically have a "both" position and
often rely on gravity flow. Low-wingers can't, as running one tank dry
could result in the pump being quite happy sucking air instead of
fuel.
We finished a Glastar (high-wing) that had only an on-off valve
and two wing tanks. The tank plumbing was teed together just upstream
of the valve. The tanks had separate vents, and one had a bit more
pressure than the other. That tank would drain first, but we didn't
fly it long enough to see if there was enough pressure to prevent the
full tank's fuel from flowing. We modified the vent system to generate
similar pressures, but it still needs to have the tank vent systems
plumbed together.
My quick calculations give me a static head pressure for
gasoline of about 0.32 psi per foot, so a 1 psi vent pressure could
lift the fuel three feet, more than the typical high-wing tank-to-carb
distance on a gravity feed system. There is a real danger here, I
think, though the difference in vent pressures would have to be pretty
serious. A plugged vent on one side would do it.
Cessna has their tanks vented from a single underwing vent
behind the left strut. This line goes into the top of the left tank,
and another line runs from the inboard top end of that tank across the
cabin roof to the top of the right tank. In addition, the fuel caps
have vents with one-way check valves to allow air in (but not fuel
out) in case of vent icing.

Dan