Fuel Flow Discussion, Moved from Float Mounting

[TimTall]

I didn't run the low tank all the way dry. There was nothing showing on the gauge in the three point attitude so there wasn't much in the tank, but enough to keep it running. Wings level or empty tank high didn't make a difference. The engine ran with no hiccups. It wasn't until i went wing low on the low tank that I got it to quit, and then only after several seconds.

With such little fuel in the tank it wouldn't take much to unport. I did a few shorter sloshes and there was no issues. The lines would have definitely unported for a few seconds but I guess the fuel replaced the air quickly enough that the engine was never starved. I do have a mechanical pump with a vapour return circuit so that may have got rid of the air instead of sending it on to the fuel distributor.

It seems like there's at least 15 seconds in the lines at low power settings. In fact the math supports it. 5' of AN6 line at 5 gal/hr should give about 12 seconds of fuel. At idle at 2 gal/hr that's 30 seconds. There's about 5' of fuel line above the level of my engine mounted mechanical fuel pump in flight attitude so it seems this is probably about accurate.

[Nev]

Very interesting. Incidentally, when I was doing fuel flow testing on the ground we discovered something that has potential to make someone's day more memorable than you'd otherwise want. Starting with an empty tank and pouring 20 liters (5 Gallons) into the tank, nothing came out where we had disconnected the fuel line at the firewall. We stood there scratching our heads, checked that the fuel selector was on the tank we had added the fuel to. But nothing. Eventually we discovered that by moving the fuel selector, fuel would start flowing. It appeared to be the action of moving the selector that somehow restarted the fuel flow. It didn't matter if it was from BOTH to the tank with fuel in it or VV, or to any other position then back to the tank with fuel in it. At the time we reasoned that it was an improbable thing to occur inflight. However the scenario that you have tested may have emptied the fuel line with the tank unported and fuel sitting at the end of the tank (ball out to the right). Then when it "re-ports" it's in the situation we discovered during flow testing. It's possibly the situation that Brooks described above.