REFLECTOR: FUEL FEED
Scott Derrick
scott at tnstaafl.net
Sat Feb 19 18:19:59 CST 2005
Al,
Very well put. I have the stock factory system except a one way valve
on the sump vent line(Was this a required change I missed?).
I had uneven flow until I got my plane rigged correctly. 400 hours
later, nary a fuel problem. I'm sure there are numerous V's flying with
a whole lot more hours on them than I have with no problems..
I think Jim must have a blockage in one of the fuel lines to the sump or
something really odd.
Al Gietzen wrote:
> This issue of fuel flow and vents troubles me because it is difficult
> for me to believe that there is simpler, more reliable fuel system
> setup than gravity feed from two tanks to a sump tank, and a sample
> drain at the bottom of the sump. Gravity never fails. If the system
> isn't working; we're doing something wrong. Here are my thoughts on
> conditions that must be met:
>
>
>
> 1) Main tank vents from the highest point in the tank, continuous
> upward slope to the manifold point that is well above the upper level
> of the tanks. You do not want fuel in the vent lines.
>
>
>
> 2) Feed from the bottom of the tank that slopes continuously
> downward to the sump tank, with no restrictions, or screens finer than
> a screen door screen with at least 2-3 sq. In of area. The feed from
> the two tanks should be close to symmetrical; same size, same length,
> so the flow characteristics are the same.
>
>
>
> 3) Fuel caps that don't leak.
>
>
>
> 4) Airplane trimmed to fly straight and level.
>
>
>
> 5) Vent system exit in ambient (or higher) pressure area. The exit
> of the vent should probably extend at least 1 1/2 inches out the
> bottom of the fuselage to get through some of the boundary layer, and
> be bent into the air stream to be sure of positive pressure. A 1/8"
> hole at the back of the bend can help alleviate the concern of
> plugging, but if the check valve idea works, go for it.
>
>
>
> Further thoughts: Except in the case of a cap leaks, a 1/4" vent line
> to the tanks is fine - the makeup air flow required for the fuel flow
> rates we're talking about will easily flow through a 1/4" line. Tank
> outlet of 3/8" will handle something close 20 gph from each tank. I
> know this because during engine tests I did static runups on my engine
> at WOT running fuel from one tank (with only 5 gal in the tank), and
> the low fuel warning light never came on. I have 1/4" vent line to the
> tank. (Dyno tests at same conditions showed 19.2 gph).
>
>
>
> About the sump tank vent - 1.) the tank must be vented to allow it to
> fill, 2.) because of the low vapor pressure of fuel (particularly auto
> fuel) you do not want to depend on "sucking" fuel anywhere because of
> the possibility of forming vapor at the pump (vapor lock); 3.) if you
> have fuel injection with recycle flow back to the sump, you want a
> sump vent for air or vapor bubbles from the return line.
>
>
>
> Further, it seems to me; if you vent the sump for it to fill, and then
> close the vent, you eliminate gravity flow from the mains because you
> have fixed the level in the sump (somewhere up the vent line) at
> whatever the level was in the mains at that time, and you now are
> depending on the slight negative pressure created by the fuel pump to
> draw the fuel from the tank. Not the best situation.
>
>
>
> If keeping the caps leak tight is so difficult, we must be using the
> wrong caps.
>
>
>
> I can think of no explanation for the problems the Jim S. experienced
> because he tried every fix, but there is something wrong somewhere.
> Possibly different flow characteristics from the two lines; and/or
> less than ambient pressure at the vent exit.
>
>
>
> The blinking of the low level warning float switch at high altitude is
> a mystery to me. For those of you who have observed this; is it more
> likely with high, or low, levels in the main tanks.
>
>
>
> As near as I can figure; it seems that if the stock system is properly
> executed, and it is not working properly, something is wrong and we
> should find out what it is.
>
>
>
> FWIW,
>
>
>
> Al
>
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