strange vent line routing

KayS · #1 11-28-2010, 09:01 PM

Hey i'm just curious, maybe somebody knows the answer...

couple of months ago i installed my rv-7 vent lines. i just did what the manual/drawing ordered me to do and switched my brain off during the built (i learned that this is the best way to follow orders when i served 13 years in the german air force).

today i was looking at them and asked myself "heck, why do they have such a strange routing?". they go into the fuse, then upwards, then forward and downwards again, pointing out at the bottom skin just behind the firewall. it was a struggle to bend them. there should be a more easy- less labour-solution, or? does somebody know the reason for this set up? maybe because the forward bottom skin is a high pressure area or something like that...?

Kay

DakotaHawk · #2 11-28-2010, 09:13 PM

It's all an effort to prevent siphoning your fuel tank in all attitudes of flight! Since the drain point is lower than the fuel tank, once you get a siphon started, you'll be out of fuel in a matter of minutes!

Greg Arehart · #3 11-28-2010, 09:18 PM

I actually modified my routing and went all the way forward, then up and back down. That made more sense to me than the routing on the plans. Serves the same purpose of keeping the fuel from siphoning out the drain.

cheers,
greg

terrye · #4 11-28-2010, 10:02 PM

The fuel tank vent line needs to do two main things:
1. Act as an overflow when the aircraft is parked and the tanks are full. Any expansion of the fuel in the tank due to high ambient temperatures needs to be relieved to keep from overpressuring the tank. The routing to a high point inside the fuselage allows the fuel to expand inside the vent line and will only relieve (drain) when there is enough fuel to push the level up to the top of the drain line inside the fuselage. In addition, if the airplane is parked on a sloped ramp the high vent line routing in the RV resists draining. You will see many airplanes parked at the ramp where they have insufficient vent line height either for thermal expansion or sloped ramp and these airplanes drip fuel constantly.
2. Act as a positive air pressure source for the fuel tanks during all in flight manoevers to allow air to replace the fuel pumped to the engine and to prevent siphoning.

Incidentally, unless there is a secondary source of air into the tanks, for example from a vented fuel cap (which RVs don't have), I don't think the 1/4" vent line would support fuel siphoning. But during aerobatic manoevers, there is undoubtably some fuel slosh in the tanks some of which may end up in the vent line and may be vented overboard due to inertia/G forces.

LarryT · #5 11-28-2010, 10:12 PM

Some Rocket builders were using a coiled vent line. It wrapped 540 degrees and exited through the bottom inboard wing skin. Can anyone comment how this has worked out over time.

LarryT

Rockyjs · #6 11-28-2010, 10:35 PM

If you search on fuel vents you'll turn up several threads. I adopted the Rocket coil vent lines on my RV-7 which are located in the wing root area. With this system there are no inside the cockpit area vent lines. After two years they seem to be working just fine. An additional benefit is that a small hole .040 can be drilled in the top of the last loop to act as a backup to the main vent in the event of a clog. I used the prepunched holes for the vents and installed a marker and transponder antenna.

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Janekom · #7 11-28-2010, 10:40 PM

In my mind it was done in order to prevent siphoning as described. I would rather call it percolating because that is really what is happening. Fill your RV with cool fuel from the under ground storage and let it stand in the hot summer sun for a few minutes and it will percolate out some fuel.

What I have done on the RV7 and the RV10, is to make about three loops of tubing at the wing root and then run it out on the bottom of the wing root fairing. So far no problems.

Rocky was posting while I was busy as well - It is basically the same as shown in his picture ( upside down picture nearly caught me out ) , but without the straight section.

KayS · #8 11-29-2010, 08:12 AM

thank you guys for giving me a better understanding of this issue!

Kay

trib · #9 11-29-2010, 09:32 AM

I installed the rocket type coil vent in my 6A. I've never seen any fuel issue or leave any streaks, even with sloshing type aerobatic maneuvers. Beats the cumbersome arrangement inside the fuselage and is one less penetration!

Jamie · #10 11-29-2010, 10:19 AM

Quote:

Originally Posted by DakotaHawk

It's all an effort to prevent siphoning your fuel tank in all attitudes of flight! Since the drain point is lower than the fuel tank, once you get a siphon started, you'll be out of fuel in a matter of minutes!

Well, not quite. You will lose some, but it can't drain the tank. Remember that the vent line in the tank is at the top-most point of the tank. You would only drain fuel until the open end of the vent line is sucking air.