Aux tanks w/o transfer pumps?

[Westward_Flyer]

Hey all,
just a random question that came to mind. Has anyone put in aux tanks without transfer pumps? it seems that if you plumbed the line low in the tank that the tanks should equalize, and during straight and level flight due to the dihedral there should be some slight difference in elevation between the two. Just figuring it would make for a little bit simpler of a system and more lightweight You wouldn’t have to worry about transferring during flight. What are the downsides to this? The main one I can think of would be in a sustained turn fuel would run from the main to the auxiliary tank, but I don’t think this would cause too big of an issue.

[AKKen07]

I think there are significant problems. If the wings aren’t perfectly level you would lose fuel from the main tank to the aux’s. Also since the dihedral isn’t that dramatic the aux tanks aren’t much higher than the mains, so even in level flight if the mains are full the fuel may go the wrong direction to an extent - until things equalize. You would have no certainty at any point just where your fuel was and whether or not it would be where you need it when you need it there.

[Jim Herd]

I have some relevant experiences on this topic.

When I was still learning my Bearhawk Patrol a couple of years ago, I taxied back one day and my mechanic was alarmed cos one wing was lower than the other. I had the fuel selector set on BOTH, and I had been flying a little uncoordinated. (I have since fixed the rigging.) So fuel had flown from one wing to the other wing. I now always place the fuel selector to one side, with a half hour reminder to switch over, set in my Garmin Aera 760.

Also, I have calibrated the tanks quite accurately, and they both hold the stated 27.5 gallons. One day I was flying along with a known quantity of about 5 gallons in each tank. I had planned to fly for about 20 minutes before refueling. Well, I was on the right tank and the engine quit! I was at about 1,000 feet agl over decent fields, so I didn’t panic. I swapped tanks and the engine fired up. All good. But what happened? After filling fuel, that right tank delivered fuel to the carb again.

My conclusion was that the tanks are almost flat – what is it, one degree of dihedral? So maybe I was not flying fully coordinated and the fuel in the right tank had sloshed away from the exit pipe location, thereby starving the carburetor. Is this plausible? Has anyone else experienced similar? What else could it be? What should be considered “unusable fuel”? So my change in SOP was to avoid either tank getting below about 8 gallons, but that significantly restricts range & endurance.

As for a sustained turn, if it is COORDINATED with ball in the middle, the fuel should not slosh to the low side.

And one more thing that seems relevant. I have ordered a Carbon Cub UL with the Rotax 916iS engine. The standard tanks are 12 g each side, with auxiliary tanks of 10g each side & mounted just outboard of the main wing tanks. The pair of tanks are connected by a short hose, with just one fill place on top of the wing, and no added pump. The sight gauges are obviously calibrated differently. So this seems to suggest that a similar configuration would be feasible in a Bearhawk. I don’t know what baffling is in any of these tanks. I think it would be good to have baffles to stabilize the fuel load.

BTW – with 55 gallon fuel capacity in my standard Patrol fuel tanks, I don’t see a need for more fuel.

I hope to learn from this group. ?

From Jim Herd

[rkennell]

I wonder if a passive system for the Aux tanks would work better with a check valve between the main and Aux. That would keep the fuel in the main but still allow the Aux to feed the main.

[Westward_Flyer]

very good points so far, thank you! I hadn’t thought of the risk of draining the main tank into the aux and causing fuel starvation, however rkennell brings up a good point, a check valve might solve that problem.

I wonder if a passive system for the Aux tanks would work better with a check valve between the main and Aux. That would keep the fuel in the main but still allow the Aux to feed the main.

[jaredyates]

I doubt you will find a check valve that has low enough forward pressure to allow gravity flow.

[Westward_Flyer]

I doubt you will find a check valve that has low enough forward pressure to allow gravity flow.

If it were possible though do you think it’s a feasible plan?

I found some with very low “cracking” pressures, this one is 0.04 PSI.

using my very basic physics knowledge, the change in pressure in a column of gasoline is about 0.03 psi per inch of elevation change and the wing tip is about 3 inches above the wing root so if the two tanks are
More than a 3rd of a wing apart that alone is enough to crack open the valve and allow flow assuming the aux tank has fuel and the main is empty. Of course in a moving plane I’m sure there will be much more PSI than that going both directions as the fuel sloshes around.


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