Moving fuel tank necks inboard.

[schu]

I took the time to remove the factory fuel necks and install the threaded bung from here: https://www.aircraftspruce.com/catal...fuelFlange.php But now I'm thinking about installing a second set of bungs on the inboard side then just machining up a plug on the lathe to cap the outboard side.

The reason is because I really want to inspect the strainers at annual and because I'll be running bushwheels and it would be nice to be able to fill the tanks without a ladder. I estimate around 3 hours of work for this change which isn't really the end of the world. The question is how much fuel would I loose?

Are others inspecting the strainers? Can you get a camera through the baffles of the fuel tank?

Thanks,
schu

[Glenn Patterson]

I would think that it may affect capacity in that it can only be filled to the bottom of the fill neck leaving a large void. If the airplane is sitting unlevel then when the lower side will take more fuel then it could slop fuel when the plane levels out. The high side is more limited as a more volume is taken up with air. If it is filled on the level it may slop fuel in the early stages of flight in turns. Perhaps one way to stop spillage would be to install a vent on the high end of the tanks that goes out side the wing. It is hard to estimate the fuel capacity lost. We have a wooden ladder like set up steps that have skids on it that stabilize the ladder that makes access to the tanks easy. We use it both on the the field and on the ice. It also is useful for maintenance on the engine. All our fuel goes in with a fine filter in the funnel and if filling at the water base the fuel goes through a large filter before the fuel base. I am not sure how one checks it short of draining all the fuel and if the filler is over one outlet strainer then it would be hard to see the other in the tank.
Just my thought.
Glenn

[jaredyates]

I'm comfortable with not inspecting the strainers. If the sump check yielded anything suspicious, then I'd investigate further. I would think your only capacity loss would be from dihedral. Slope parking and uncoordinated flying can impact the as-designed system and seems like that would be a wash. The capacity loss would be easy enough to test on the bench with a digital angle finder and 25 gallons of liquid.

Intuitively, pitch effects seem more significant than dihedral, and I worry about the operational threat of fueling in the 3-point attitude. If you are in a 3-point attitude, how easy is it to overfill, such that when you pitch level, the fill point becomes submerged? Put another way, in the 3-point attitude only the front part of the tank is higher than the fill point. Once level in pitch, almost all of the tank has some volume higher than the fill point.

[Glenn Patterson]

Well said Jared. The fill point fueled to the neck would submerge in level flight. It would create a mess over time and poses a risk of fire. Fuel running over the paint surfaces and maybe inside the wing discolor the surfaces. The disadvantages outweigh the advantage of fueling convenience. The great thing about the Forum is a person can bounce an idea and get reasonable feedback. We check the gascolator screen at annuals and don't worry about strainers. If funnel with a fine screen is used it should catch anything before it gets into the tank.
Glenn

[500AGL]

My fuel fills are inboard for one reason. They are (will be) reachable to refuel from my tires. With 52+ gallons of capacity in a Patrol, losing a few gallons or so to air space is a non-issue. Being unable to fuel without a ladder is a major issue. Every Cub is built this way. Skywagons are a bit further out but can be done with the step pad on the strut.

[kestrel]

I finally figured out where the tank drawings are.... old beartracks, and plugged the dimensions into cad to get the volume. Without the baffles, the tank volume is 6151.64ci or 26.63 gallons. When you tilt the tank 1 degree (I thought it was 2, but the plans confirm 1 degree), the tank effectively shrinks to 5822.18ci or 25.20 gallons. So, the difference is only about 3 gallons total.

You may find it is less than that. Check my thinking:

Did you consider the orientation of the tank in a 3 point attitude, or did you base it on the tank being "level"? When 3 point, the aft portion of the tank will have no air. Only a small portion of the front outboard will have air.

Because filler necks tend to reach down into the tank a little, is a very hard to get a tank truly full and I suspect that few address that when fueling. ..and it is impossible to prove that you have. The stock system would need a hole or slot in the filler neck to let air that is above the bottom of the filler neck out.

If you make a tall filler neck and have vents from the outboard front corner of the tank, you can fill the entire tank from an inboard filler. ...though it will still be a challenge to confirm that all air is out without some mechanism for "looking".

NOTE: I am modifying the fuel system. I am a test pilot. Don't do what I'm doing.

[whee]

Short guy problems.

[500AGL]

LOL! I resemble that remark.

[Battson]

I took the time to remove the factory fuel necks and install the threaded bung from here: https://www.aircraftspruce.com/catal...fuelFlange.php But now I'm thinking about installing a second set of bungs on the inboard side then just machining up a plug on the lathe to cap the outboard side.

The reason is because I really want to inspect the strainers at annual and because I'll be running bushwheels and it would be nice to be able to fill the tanks without a ladder. I estimate around 3 hours of work for this change which isn't really the end of the world. The question is how much fuel would I loose?

Are others inspecting the strainers? Can you get a camera through the baffles of the fuel tank?

Thanks,
schu

Bushwheels make it easier to fill tanks, not harder. Stand on the tire and one knee on the strut, if required.

Borescope would be my suggestion. The strainers probably need an inspection once after the first 10 hours, but that'll probably do it for the life of the plane.
I would put the borescope through the fuel port into the strainer, or through the drain port.

[kestrel]

Bushwheels make it easier to fill tanks, not harder. Stand on the tire and one knee on the strut, if required.

Have you done this? Maybe I gave up too easily, but I tried, gave up and found a ladder.

[Battson]

It's those skintight jeans you wear Nev :P Hard to get a knee up!

[Nev]

Not sure Jon. I also tried with my favourite Lycra one-piece and it didn't seem to make a difference :/

[kestrel]

Ummm.... yeah.... ;-)

[AZBearhawk272]

Potentially a dumb question.
Why not add two additional outlets and then just swap the tanks side for side…….?

Kevin D
# 272

[500AGL]

Its outlets, sight gauges, and sump fitting. So 5 ports.
And, then you have to hope the hole aligns.
Better to move the filler neck.