Fuel Flow Discussion, Moved from Float Mounting

[jaredyates]

I would like to understand the science here.. for me it seems that when the valve is on "both" the left tank can drain to the right tank, and vice versa. Does that not allow the fuel to cross over?

[svyolo]

I believe I read the requirement in AC43. I will try to look it up again. I was planning on return lines to the mains for my EFI so I had 1/8 NPT ports welded to the top center of the inboard edge of each tank. I am probably going to use it to cross vent the tanks.

[JJohnston]

The only failure scenario I can think of is to have one cap vent hole plugged. If that happens the fuel won’t drain out of the tank with the plugged cap and an alternate air source would be necessary.

If that were the case, would a fuel crossover serve to equalize the pressure? Do the kit tanks have a provision for a vent crossover?

[Mark Goldberg]

One thing to keep in mind is that Matt's BH has a IO360 CONTINENTAL. Not a Lycoming. I am not sure if the fuel injection systems are the same. But worth noting. Mark

[Archer39J]

Only thing I'm finding has to do with Cessna 182s, 185s and their interesting venting scheme, maybe an AD for vented caps? What's the FAR that requires a cross vent between independently vented tanks?

[alaskabearhawk]

Ok, zkelley2 I admit, I don’t get it. If the fuel tank is vented through the tank caps, what would a vent line connecting both tanks do for you that the cap doesn’t already do? The only failure scenario I can think of is to have one cap vent hole plugged. If that happens the fuel won’t drain out of the tank with the plugged cap and an alternate air source would be necessary.

[zkelley2]

It's more or less exactly what he said. If you have a fuel pump and have a both setting on your fuel selector, you need to either a) never use it in both b) have a vent line running between the two tanks towards the top of the tank to equalize the pressure in the air above the fuel. I could do some math with it later to show how that happens, but that's what's happening. You can have one tank empty and the other tank completely full and not be able to get any fuel to the engine.

Every certified aircraft has this if they have a both selector. They are required to by FAR.

[jaredyates]

Sounds like a great Beartracks article.

[JimParker256]

Matt, add me to the list of folks who would like more details on the issue you encountered. Sounds like something the entire Bearhawk community needs to better understand.

[MitchG]

Very interested as well. I'm at a bit of a loss to understand this cause. Thanks!

[Nev]

Same here, I’d appreciate any discussion to learn more about this.

[svyolo]

I am interested to learn a lot more about this, if you don't mind.

[zkelley2]

short story; Running on "Both" without having interconnecting tanks. Pressure differential between the tanks caused fuel flow stoppage.

If you dont mind its probably worth doing a post on this. It's an issue that is addressed in certified aircraft and rarely in experimental.

And since fuel related issues are the #1 experimental problem, it's worth discussing.

[MattS]

short story; Running on "Both" without having interconnecting tanks. Pressure differential between the tanks caused fuel flow stoppage.

[JJohnston]

Did you determine the cause of the engine failure?