Fuel Flow Discussion, Moved from Float Mounting

[tailwind]

If this is to be a permanent install I would use aluminum tube over most of the install. If you want to observe what is happening, put the clear tube in an area where you can observe the activity but where the tube will be easily replaced. The clear tubes I have found become hard, shrink and become discolored over time.

[svyolo]

There is only a degree of dihedral. I doubt it would be possible to have the line run slightly uphill to the center of the fuselage. There will probably be a slug of fuel most of the time in the middle, but I think it still works. I wonder what Cessna's implementation looks like.

[alaskabearhawk]

Spoke with my mechanic friend again about the vent. Tentative plan is to weld bungs on the top/fwd/outside of each fuel tank. Put in a barbed fitting and connect the two tanks with clear tubing and watch how that goes. There is plenty of room for the fitting. The only potential negative is the tubing would have to go down a bit to fit through the front spar lightening hole. After that it's a straight shot to the other side. If it doesn't work just plug both tank ports and call it good.

[Nev]

This Safety Bulletin shows surprisingly similar symptoms to the Cessna AD from 1969, although they prescribe different fixes.
Murphy term it an “air locked” fuel line and suspect air somehow entering the fuel lines. Cessna suspected vapor lock from fuel essentially boiling in the lines at bends under specific atmospheric conditions.

[JJohnston]

That's not the same problem as we've been talking about, either. They think their problem was from having the fuel feed at the bottom of the sight gauges, and their solution is to move the fuel feed to its own fitting. Bearhawk tanks are already that way.

[Battson]

That is interesting, thanks for sharing!
Again, they have been unable to come up with hard test results to prove what was happening, and were forced to assume it was the sight gauge causing the issue. I have not thought about the problem hard enough to understand how interconnected sight gauges could cause the problem they were having - air bubbles in the main fuel lines. Seems very counter-intuitive, and I am getting a sore head already!

[MattS]

Below is the bulletin. There is a previous one that talks about the tanks needing to be interconnected. I can't find that one.

[MattS]

http://www.pattersonaerosales.com/Aircraft/Rebel/TechSpt/FuelSystemMods.pdf

[svyolo]

It might be tough to put a fitting on the outboard edge, at least on a QB kit. To give myself space for fittings on the inboard side, I put the tank fairly far outboard in the bay. It might fit better to put it on the forward side of the tank. I think I had about 1.7 inches or so from the front of the tank, to the spar. That also makes installing the whole tank a bit of fun. I think it would have to be a flexible line. The flexible line would have to be secured so that it doesn't chafe on the tank straps.

[Battson]

On Murphy Rebels, they used to have the interconnecting vent connected through the sight gauges. After some problems, they issued a bulletin to move it to a separate port.

It would be good to know what the issue was.

[Battson]

Jon - you have to admit, it's a big assumption everyone here is making. We have no firsthand sight or knowledge of the investigation and determination of the cause. It's only natural for people to question it. Nothing personal.

[robcaldwell]

How will the vent line be routed through the cabin? I guess you could place a bulk head fitting on each inboard wing rib? Then join the line on both sides after the wings are installed? With the vent line in the outboard forward location should prevent cross feeding in a wing high parking position unless the tanks were totally slap full, even then not much should transfer.

[AKKen07]

Thanks, I haven’t uncrated the wings yet, so lots of time to make whatever changes seem prudent.

[yateselden]

AKKen07 definitely better tapping into outboard tank ends if your still assembling the wings. I would need to remove the tanks, work in the closed up wing. But I don't like the idea of connecting the sight gages or having a valve either. Welding in bungs in tanks that have had fuel in them doesn't excite me at all. All things considered I think I'll let it be.

[MattS]

On Murphy Rebels, they used to have the interconnecting vent connected through the sight gauges. After some problems, they issued a bulletin to move it to a separate port.