Always more  than willing to hear another view pointsone fuel systems - can't know too much about how they function and possible problem areas in my opinon- given there somewhat critical function{:>)

 Its always interesting to view our beliefs and assumptions in the context of someone else's experience or knowledge.  I do fly (300+ hours)  with an unvented, small header tank that is approx 1 1/4 pint (36 cubic inches)capacity (or if I want to make it sound large - it has a capacity of 590ccs {:>)).

All my fuel from the engine is returned there via the pressure regulator discharge.  My theory was that the smaller volume of fuel gets used very quickly and replenished (especially at high power settings) with cooler fuel from the tanks.  Indeed this does seem to be the case with my setup.  I view my setup perhaps functionally similar (but not identical) to the No-return fuel systems being seen more frequently in fuel injected automobiles.

I did find early on when using auto gas that on a hot day,  and with low engine power settings (returning more hot fuel and using less thereby causing less cool fuel to be pulled into the tank),the fuel pressure could start to fluctuate.  Turning on the boost pump (approx 6 psi) pressurizing the header tank would cause the fluctuations to end.   I have since place a fiberglass box with aluminum foil around my header tank and fuel pumps and have a blast tube routing air to the box.  I am also using 100LL rather than auto gas.  In any case, I have had not further fluctuations in the pressure or any other indication of a fuel flow problem even on hot days down in Louisiana this summer.  .

I suspect that as in many cases, its the details that makes one system function well vs a similar - but not identical installation which may have problems.  So "copying" a "good" installation should be done with utmost care, small - seemingly insignificant - changes to the design may have large unintended consequences.  The one  change I would make to my own system would be to move this header tank out from under the hot cowling area say to a wing root or anyplace protected and cooler.

 I considered EFI pumps in the wing tanks but dislike the long lines of highly pressurized fuel running through the cockpit area.  Certainly many different design approaches have been made to work - but none are without some risks in some area.


FWIW

Ed A

.
> It could be an issue, however, if the fuel is
>> returned
>> to a smaller header tank depending on how the header tank is
>> constructed and
>> vented.
>
> I suspect you want it vented. Unvented opens you to new problems. Like
> fuel sys that won't self prime.
>
>
>>
>> If the fuel is recirculated long enough, and if the bubbles are
>> really air;
>> the dissolved air should be eventually nearly eliminated from the
>> system,
>> but I didn't run the pump long enough to see if this is true.  I
>> haven't
>> tried the tests with avgas, either.
>>
>> My apologies to the list if this is common knowledge.
>
> Don't apologize. This is valuable info that deals with one of the biggest
> risk items our conversion have. Thanks for providing some science.
>
>
> -al wick
> Artificial intelligence in cockpit, Cozy IV powered by stock Subaru 2.5
> N9032U 200+ hours on engine/airframe from Portland, Oregon
> Prop construct, Subaru install, Risk assessment, Glass panel design info:
> http://www.maddyhome.com/canardpages/pages/alwick/index.html
>
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