Unusable fuel and tank calibration

[PapuaPilot]

Any suggestions on how to determine the unusable fuel quantity so I can calibrate the fuel sight tubes? Do you just fill the header tank and hoses to the bottom of the fuel tanks and call everything in the tanks useable? How much pitch have others allow for?

When I sloshed out my tanks last week I put 2 gallons of fuel in the tanks and it didn't show in the sight tubes yet. Is that normal? Actually the wings were laying flat on sawhorses and didn't have any dihedral which would bring more fuel toward the sight tube.

Once I determine the unusable fuel I will add in 2 gallon increments and mark the sight tubes.

[t j]

I calibrated for the placard with the plane level. Three quarts was unusable in that attitude. I actually can use all the fuel in a wing by flying with that wing high for a while.

My tanks have the fuel port at the rear corner. That location can unport in a low fuel long descent situation. The ports in later tanks were moved forward a little to prevent this.

Here's the service letter about that. It includes some information on calibrating unusable fuel with those. http://kitfoxaircraft.com/support/service_letters/sl45.htm

[Dave S]

Phil,

I think your extimate that the lack of dihedral with the wings sitting on sawhorses would prove true. There are a couple sets of figerglass baffles in the wing tank; and, with the wing level - the bottom of the baffles might hold back some fuel; and, a couple gallons of fuel would spread out pretty thin given the area of the bottom of the tank - not likely it would register on the sight tubes. Once the aircraft is assembled, with dihedral - the amount of fuel that can't be seen on the sight gauge would be a whole lot less...On our S7 (which has the earlier design tanks offered by Kitfox), I found the amount of fuel that would not drain to the header tank (with the aircraft in a level position) to be about a quart in each tank.

Even after assembly, there is going to be some fuel in the bottom of the tank that can drain to the header tank; but, will not show up on the sight gauge.

The service letter which was Identified by TJ is very useful......although, I did take things a bit more extreme in testing my fuel system because I wanted to understand it better...including testing with a 10 degree nose down - not likely an attitude a person would assume in practice - just wanted to know.

Sincerely,

Dave S
KF 7 Trigear
912ULS Warp Drive

[PapuaPilot]

Thanks guys the SB answered the questions I had.