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Thanks Charlie.
That would be a real simple solution.
Could actually make the loop inside the tank against the most outboard rib, with the 90 degree fitting coming out through the bottom skin.
The only downside (other than loop not extending well above the tank) would be the disturbed airflow (drag) from the fitting being out on the wing rather than in the already disturbed airflow from lower cowling (and perhaps hot air from the engine preventing icing).

So is the simplicity worth the extra drag and the risk of having fuel running/siphoned out if plane is parked in one wing low position (and possible icing over the vent inlet)?

Finn

On 3/23/2018 11:19 AM, Charlie England ceengland7@gmail.com wrote:

On 3/23/2018 9:41 AM, Finn Lassen finn.lassen@verizon.net wrote:

This looks very tempting.

http://www.vansairforce.com/community/showthread.php?t=1062

But can anyone explain the physics of it?

RV vent lines run from the upper tip of the tanks (next to filler hole) and then normally into cockpit, up cockpit wall, loops down to floor and into 90 degree fitting down through floor and pointing forward.

(http://www.vansairforce.com/community/showthread.php?t=1062&page=4)

There is a mention of RV-10 vent lines, that gives the idea they just come out of the tanks and down and out, but no pictures.

Finn

(Still figuring on where to put vent, transfer and return fittings in right tank)

The good thing about the 'stock' design is that it absolutely will work. :-)

The -10 is non-acro; you can get away with less care in venting.

The 'physics' is just that the fuel has to run around the coil a few times to get out instead of up to the top longeron. The idea is that if some is trapped in a loop due to expansion, it'll get sucked back in as you burn off fuel.

The coiled version is commonly called the 'Rocket mod'; apparently it's the go-to design for Rocket builders. I intend to do that with my aux tank vents, but the vent port is on the outboard (high) end of the tank. As someone in that thread pointed out, when you do it with the port on the root end of the tank, you can't get the coil above the actual vent 'pickup' point in the tank. If I elected to do the mod on a -4's main tanks, I'd move the vent penetration to the outboard end of the tank & make my coil there. However, you then have the issue of managing the vent's exit to the air, since getting the tank on/off with the vent penetrating the leading edge skin outboard of the tank. Perhaps you could use a bulkhead fitting in the bottom tank skin at the outboard end, with a coiled section of tubing (inside the tank) up to the same area as the stock vent line.

Charlie

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