4-place Unusable Fuel

[whee]

I’m wondering what you guys have found to be your unusable fuel. Based on some conversions with other BHers we figured worst case unusable fuel would be at max climb so that’s were we did the flow testing and found we had 1.5gal of unusable for each tank. Apparently this was a bad assumption. In our airplane we have 5gal of unusable in level flight.

10gal total of unusable is not acceptable and I’d like to determine what is causing it.

Anyone else know what their level flight unusable fuel is?

[Archer39J]

I've come across several examples where folks have raised fuel lines at the wing root, this of course then increases the unusable fuel in each tank. Hopefully this isn't what's wrong here but thought I'd mention it.

[Mark Goldberg]

My thinking was that less than 1 gallon per tank is unusable. 5 is very strange.

Two things come to mind. First, your fuel outlets have to be as low as possible. Second, if you have your tanks installed in a way that the inboard side is higher than the outboard side - that would do it. But with the 1 degree dihedral - that would seem hard to do.

I have seen a situation that you could be seeing also. If the lines are empty of fuel - as Dave said above - if the fuel lines from the tanks into the fuselage go UP instead of going downhill, sometimes the fuel will not flow even though the fuel level is higher than the outlet. It would seem that the fuel would push the air out of the way. But in my 4 place it takes about 1/3 tank of fuel to make enough pressure to push the fuel down and start flowing. Obviously, I do not run my tanks dry. Mark

[Bcone1381]

Is there any way that a fuel pump could be sucking air when one port becomes uncovered with fuel?

[svyolo]

Whee;
How did you find out about the 10 gallons unusable? Hopefully the easy way. Not gliding in for a landing.

The easy problem is geometry. I am going to go the easy route and assume you are technically proficient enough to know that. New airplane, slow build. Gunk in the filter/gascolator? Blocked vent, or partially blocked?

At higher fuel levels, head pressure overcomes the partial blockage. Lower fuel and head pressure. Not so much?

Just a thought.

[whee]

Thanks for the ideas guys. My fuel lines come straight out of the tanks and run downhill. I checked the tank position with a level and the inboard portion of the tank is the lowest part of the tank. I've checked the vents and the gascolator and both are clean.

My thinking is inline with Brooks; one of the outlets in unporting and the fuel pump is sucking air. But there are a few things that I'm scratching my head about: 1) In that situation the engine should only suffer a partial power loss instead of totally shutting down. 2) With 5 gallons in the tank neither fuel outlet should unport in level flight. 3) Tests showed that the rear fuel lines (1/2" on my plane) flow enough fuel with gravity to keep the fuel pump flooded with fuel so no "sucking" should be occurring as long as the rear tank outlet remains covered in fuel.

These points are why I'm asking if anyone here has ran a tank dry and refilled it to determine their in flight unusable fuel. This is how I determined I have 5gal unusable in each tank. Today or tomorrow I'll run a tank dry and turn on the boost pump to see if that make any difference.

Thanks guys.

[svyolo]

How about partially (mostly) clogged finger strainers? They will probably clog from the bottom up, so maybe fuel is feeding from only the top portion of the strainer, leaving the fuel below the upper portion of the strainer as unusable?

[kestrel]

I'm not proud to say that I once pulled up to the pumps and put 46.x gallons into the tanks. Its been on my list to someday run one tank dry at altitude while over a long runway. ...then do the same thing on a different flight to the other tank. I did that with our RV-4.

[swpilot3]

I did my fuel flow test with 2.5 gal in each wing. It flowed fine during the test, so I just considered 2 gal per side my unusable, knowing that it was probably something lower. However, I've never actually tested what the real number is.

I think running a tank dry would be very enlightening. I've always used 10gal total as my "minimum landing fuel". Now I feel obligated to do a real world test.

[Chewie]

I'm not sure if this is aviation-kosher but it would solve all these issues:

https://www.holley.com/products/fuel_systems/hydramat/

[whee]

How about partially (mostly) clogged finger strainers? They will probably clog from the bottom up, so maybe fuel is feeding from only the top portion of the strainer, leaving the fuel below the upper portion of the strainer as unusable?

It's possible but not likely that both tanks have the strainers blocked in equal fashion. Also, We cleaned the tanks right before installing because we drilled and welded return ports. Only swarf that has been found is metal filings from drilling the tanks. Checking the finger strainers will require pulling the tanks so that will be the very last thing checked.

I'm not proud to say that I once pulled up to the pumps and put 46.x gallons into the tanks. Its been on my list to someday run one tank dry at altitude while over a long runway. ...then do the same thing on a different flight to the other tank. I did that with our RV-4.

Sounds like you have less than 2 gallons unusable per tank depending on how that 46.x gallons was distributed. That's useful info, thanks!

I did my fuel flow test with 2.5 gal in each wing. It flowed fine during the test, so I just considered 2 gal per side my unusable, knowing that it was probably something lower. However, I've never actually tested what the real number is.

I think running a tank dry would be very enlightening. I've always used 10gal total as my "minimum landing fuel". Now I feel obligated to do a real world test.

I too put 2.5 gallons in each tank for my flow testing. At 19deg nose high it was obvious that the front outlet was unported but the flow test went fine and the pump didn't suck air till there was less than 1.5 gallons remaining in each tank.

I look forward to hearing your real world results.

I'm not sure if this is aviation-kosher but it would solve all these issues:

https://www.holley.com/products/fuel_systems/hydramat/

That's really cool! I'm not sure it has a aviation application but it would work awesome in the fuel tank I'm currently making for my boat. I'll have to see if they have a way to secure them in aluminum tanks.

[zkelley2]

Whee, do you have a header tank with the continental fuel injection?

[tf104]

I think you need a header tank. I think what you are experiencing is the same reason a low wing plane does not have a both selection on the fuel valve

[whee]

No header tank and no “Both” on the fuel valve.

To be completely honest guys, I’m not asking for ideas or thoughts on what you think the issue is. The single purpose of this thread is to find out what people’s actual unusable fuel is as determined by running on one tank till the engine quite. It had never crossed my mind that no one here had done it.

[bearhawk2015]

I have 2.5 gallons of unusable in each side. Verified by draining in the level attitude and then refueling in level attitude.