REFLECTOR:Low fuel light

[KeithHallsten]

John,
The point I've been trying to make (not very successfully) is that the
valveless manifold vent system will either pressurize (or not) BOTH the
strake tanks and the sump tank.  A little (relative) air pressure in the
strake tanks is a good thing,  and helps push the fuel to the sump tank.
Conversely, a little pressure in the sump tank RESISTS the flow of fuel into
the sump tank from the strakes.  With the valveless manifold, these effects
SHOULD cancel each other out.  It's not clear whether the net effect is
really absolutely zero.  If the vent to the sump tank is eliminated (or shut
off), the small positive pressure from a forward-facing vent absolutely WILL
assist in the flow of fuel to the sump.  I have yet to see any evidence or
convincing logic that the sump tank vent is needed.  Pat Shea has proven
experimentally that the sump tank will not trap air if the sump vent is
closed off.

The vent system is pressurizing (or not) the entire system of tanks now, and
isolating the sump tank will not induce any higher pressure in the strake
tanks than already exists, so I don't see any increased risk on that aspect.
The only thing we should be careful about is assuring that the vent does not
suck on the strake tanks; a little forward bend on the free end of the vent
should take care of that!

Can anyone make a physics-based case that the sump tank vent should be used?
It would make my day to find out that I've overlooked something!

Keith


----- Original Message ----- 
From: "John Dibble" <aminetech@dixie-net.com>
To: <reflector@tvbf.org>
Sent: Saturday, July 26, 2003 12:19 PM
Subject: Re: REFLECTOR:Low fuel light


<SNIP>
> I didn't give it much thought, figuring that if there is fuel in both
tanks, the
> sump has to be full.  But the idea of vacuum on the tank from a leaky
> cap makes me wonder.  What about the tank vent which points forward?
> Doesn't this tend to pressurize the tank with a ram air effect?  It
> seems that any cap leak would have to be larger than the size of the
> vent opening in order for a vacuum to occur and I can't imagine such a
> big leak unless the cap is completely off.  It seems like the tanks
> would have a slight pressure from the vent.
<SNIP>
> John