Fuel system delivery testing and results

Spent Saturday, March 1 2025 testing the fuel system.

Bottom line:  Good new, and news.  The news is yet to be determined if good or bad.

Needs:

• Everything is going to be in Liters.  It’s just easier.
• Then UL520T needs 68 Liters / hours (based on 0.3 Liters / hour / HP)
• Using a 50% safety factor.  Gravity system needs to deliver 102 liters / hours

Test setup

• Fuel system emptied of all fuel, except anything unusable (specifically header tank).
• 20 liters added to each main wing tank
• Check valves installed in header tank vent, with directional arrows pointing away from the header

Header fill

• Took just shy of 6 min to fill header tank.

Test 1

• Drained 20 Liters into Jerry can at firewall, same level as exit.
• Fuel selector on BOTH
• Gravity only.  Tailwheel on the ground.
• Time:  8.8 min, which equals 136 Liters / hours.
• Pass (102 Liters / hour needed)

Test 2

• Drained 20 Liters into Jerry can at firewall, same as test 1
• 20 Liters added to left tank
• Fuel selector on LEFT.
• Gravity only, Tailwheel on the ground
• Time:  11.2 min.  107 liters per hour.
• Pass

Test 3

• Completely drain useable fuel from header
• Add fuel to both tanks.
• Test fuel consumption with header low
• Upon setting up for this test, with fuel selector OFF, header tank would not drain.  This is due to the check valves blocking the vent path.
• Test fail.  
• Not sure if this is a bad thing or not.

Test 4

• Repeat of test 1
• 8.7 min, 138 liters / hours
• Pass

Test 5

• Fuel selector on both
• Fuel pumps connected to firewall bulkhead connector
• Fuel line from engine disconnected.  Fuel pumped from gascolator into Jerry can.
• 20 liters pumped
• 4.92 minutes, 244 liters / hour
• Pass
• This is an unreasonable test.  The header low warning lights came on… the first one at 2/3rds came on after 1.76 min.  The 2nd one came on after 3.73 min.  There is no scenario where this would be needed… I wanted to see where the header tank could not keep up with the demand, and I found it.
• It took 6.6 min for the header to fill back up after the pump was shut off.

So what did I learn from all this:

• The check valves on the header tank solve the problem of fuel coming in via the vents.  This also solves the cross feed problem.
• The header tank fills slower than I would like.
• In no reasonable scenario tested did the header ever show a warning.
• Even on 1 tank, there is enough fuel delivery for full power fuel consumption plus a 50% safety factor.
• All my tests were with a maximum of 20 liters / 5 gallons in each tank.  This test needs to be performed with FULL main tanks (60 liters per side).  This should add significantly more head pressure.

More testing needed, but so far the system works.  Need to change some variables and test again.