REFLECTOR: FUEL FEED

Sat Feb 19 19:34:07 CST 2005

Way back when I had my engine on a test stand I used the standard sump for a fuel tank. At first I used the fuel pump in full speed mode all the time.  When operating at levels that didn't require too much fuel the bypassed volume was obviously much greater. The bypassed fuel was fed back into my sump tank. I saw through the clear fuel lines I was using at the time that LOTS of bubbles were coming out of the fuel regulator. I thought at first I had a leak. The phenomena was more like opening a carbonated drink.  Those bubbles filled my sump tank almost half full.  I enlarged the sump tank vent to allow the bubbles to get past the fuel and now run the main fuel pump at a slower speed until I reach higher power levels.
If I had a low level switch back then it would have surely have been flashing.
Wayne
----- Original Message ----- 
From: "Scott Derrick" <scott at tnstaafl.net>
To: "Velocity Aircraft Owners and Builders list" <reflector at tvbf.org>
Sent: Saturday, February 19, 2005 7:19 PM
Subject: Re: REFLECTOR: FUEL FEED


> 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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>>
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