Why aren't the fuel tanks vented straight down?

I think I have gotten used to having some fuel in the low point of the vent lines. Now I don't fill my tanks as much so that the vent lines only bleed air pressure (or vapor pressure). I am just wondering why they are not vented down to allow fuel to escape instead of block the vent when the fuel heats.

My understanding is that the pressure in the tank is not able to to force the fuel in the line up over the fuse entry point ( I think that is the high point).

A second question is what happens to the fuel that might make it into the vent line? It doesn't seem to be able to blow back into the tank. Is the use of the fuel in the tank creating some vacuum that eventually suck the line clean again?

Thanks!

If the vent is pointed down so flow is across the hole there will be low pressure and it will at worst suck the fuel out or at least make it harder for the pump to pull fuel to the engine.

AndyRV7 said:

My understanding is that the pressure in the tank is not able to to force the fuel in the line up over the fuse entry point ( I think that is the high point).

A second question is what happens to the fuel that might make it into the vent line? It doesn't seem to be able to blow back into the tank. Is the use of the fuel in the tank creating some vacuum that eventually suck the line clean again?

Thanks!

Click to expand...

It is actually the pressure outside the tank that will move any fuel in the vent line back into the tank.

The whole purpose of the vent is to allow air into the tank to replace the fuel that is being removed. If that did not happen, outside air pressure would compress/collapse the tank as fuel was pumped out (that is why a plugged tank vent is such a big deal).

Try this....
Grab one of the flimsy clear plastic bottles of drinking water and start drinking from it without breaking the seal on your mouth (allowing air in).

They come inside the fuselage so that they can loop higher than the tanks, thereby preventing excess fuel spilling if you are parked and a hill with one wing high. High wing airplanes have no option and if left topped off on a hill they drain a bit out the vent.

A little fuel trapped in the line will eventually move out, either gets pushed over the loop ( by cold fuel warming) or sucked back into the tank ( from warm fuel cooling). No harm either way.

Wow, great information. Thanks!!

Another option, which I did to my 7 and my neighbor did to his Rocket, is to not take the vent into the fuselage. Simply make a 3 - 4 wrap coil at the wing root and take it straight down from there. Makes it much easier to mount the wing and much less plumbing!

carrollcw said:

Another option, which I did to my 7 and my neighbor did to his Rocket, is to not take the vent into the fuselage. Simply make a 3 - 4 wrap coil at the wing root and take it straight down from there. Makes it much easier to mount the wing and much less plumbing!

Click to expand...

That's what I did, and I used the Bonaco vent fitting on the bottom of the wing that has a 45-degree forward facing cut, with bug screen pressed into it.

curious

What function is the coil of tubing in the wing root supposed to perform?

The coils allow some fuel to be "stored" rather than pour out on the ground when the fuel and air in the tank heat up and push fuel out.

I experimented with 3 coils of 3/8" tubing vented out the bottom of the wing root. It worked with no problems and although I am sure it would "breath" much easier than that longer length of 1/4" bent up tubing that goes through the inside of the fuselage, in the end I decided that thousands of RVs using the system as designed is a pretty good indication that the original design works fine.

Try blowing your breath through that length of 1/4" tubing. You will be amazed at how much pressure it takes to get the air through it. Doing the same with the 3 coils of 3/8" is super easy in comparison. This made me think that the 3 coils version might not require as much suction to get the fuel through the system and thus less prone to boiling fuel.

I went through this exercise as I had some fuel pressure issues at altitude, or so I thought. It turned out to be an intermittent failure of the Dynon fuel pressure transducer.

Randall in Sedona