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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