Unusable fuel in XL

[Bob Wood]

I was stall testing yesterday after a 2.4 hour flight and got a cough after I dropped the nose and sucked some air with all the fuel in front of the tank.

Did some testing ......


I have determined thru ground testing that I have 1.5 gallons unusable fuel in my plastic minimax tank. So my max flight  needs to leave 2 gallons in the tank. I burn 1.8/ hour in my twin carb 37ho mosler. So 2 hours possible which is fine.

But it would be nice to cut this unusable down. Anybody fool around with a front and rear fuel outlet with the ram air tube on the cap. Seems like it may work. Search back and saw some discussion on header tanks, would rather avoid that if possible.

Thanks for input!!
Bob

[Tom H]

Bob, I installed a front and rear tank outlet on DE Stubby, and have the ram air on the top filler caps.  Of course, the DEs have two tanks.  Each tank has the front and rear outlet at the bottom inside edge, with strainers.  The two tubes from those outlets run forward and down, and join lower and forward of the forward outlet into a single tube.  Same on the other tank.  Then, the two single tubes go to a small header tank, maybe a cup in volume, which is located at the upper part of the windscreen area.  The header tank also has a vent tube on top which goes to the top vents in the gas tanks.  This vent tube is clear, and the upper part serves as a fuel tank level indicator, showing the overall level of the two tanks.  There is an outlet from the bottom of this small header tank where an on/off valve is located, then a tube goes down and forward to the gascolator.

Hopefully the picture that I attempted to attach shows up and it will make sense.

Of course, the LEs and LEUs don't need this complexity, but the front/aft outlet with header tank works just fine.  Never had hiccup during any flying, including stalls.

[Bob Wood]

Thanks Tom,

I wonder if the 2 outlets & ram air on cap (no header) would reduce the chances of sucking air at lower fuel levels?( Both outlets merging into the main line to the 2 carbs.) I suppose a small plastic header would solve it all, and not weigh much how much fuel does your header hold? I could go to a lighter tank and add this and net out about the same.  The mimimax tanks are 10 lbs I think.....

I came across a avid flyer/kitfox aluminum tank with explosafe mesh in it that I am thinking may be better anyway as the explosafe may baffle the sloshing.  It is lighter and this just may work.

See you at Oshkosh next week, we can discuss

Thanks,
Bob

[Tom H]

If you have one tank, a ram air on top, two outlets, one fore, one aft, joined together, no header tank, I think the key is to use relatively large tubing from the tank outlets to the engine.  The fuel flow will have a higher velocity if using small diameter tubing possibly carrying air bubbles down with the fuel flow.  If using larger diameter tubing the flow velocity will be lower, and will allow any bubbles to rise up, back into the tank, and not be carried down to the engine.  Also, larger tubing will contain a larger volume of gas, essentially serving as a small header tank.  All of this assuming that the fuel tubing/path is downhill, with no air traps in the routing.

Our header tank holds maybe a cup, but probably less.  Our tubing is 3/8" OD.  If the on/off fuel valve (which is after the header) is not opened, there is enough fuel in the tubing to make a short taxi, but not enough to make a take-off roll, which is a good reminder to open the valve!

I plan to be as Osh Mon-Wed.  See you there.

[Bob Wood]

Thanks Tom,
All makes good sense and seems to apply to what  I am seeing. Right now I have the larger 3/8 tubing from the Y down to the carbs, the tank outlets are the smaller 5/16 tube because the tank barbs I got with the tank were the rubber grommet set up and had to be that size.. The 3/8 tubes are much nicer as you can squeeze bubbles out much easier etc.  I will look into converting the whole thing to the large tubing.. Much easier to work put bubbles in the large tubing. You can look it over next week, see you there.
Bob

[stevejahr]

Note that the two feeds with lines all the way to the common join point logically become part of the tank and will have fuel/air at the same level as the tank.  So think of your climb attitude and the line of the surface of fuel in the tank.  Extend that line to the front feed tube and you know where the air/fuel level in the line is.

All means the joint point height determines how immune you are to changes of flight attitude.  And also that moving the join point back to the middle of the tank makes it more effective with less drop.

With respect to tube size, I would think using a seperate vent tube one could acheive fuel/air seperation through a vertical chamber of sifficient size rather than large tubing?  Think tubular header tank with feeds in the middle and vent out end with header mounted vertically.