Fuel tank vent routing

[peter anson]

I believe the villain was a mud wasp. You could argue that he should have checked the vent during the daily, but I guess that vents under the aircraft are hard to check.

[mike.smith]

My vent goes vertical out the cowl at the filler area, then does a 180 degree turn downward and comes out the bottom of the cowl, from the same opening as the exhaust pipes. The tube is bent/curved facing forward. It gets a cover with a "Remove Before Flight" red flag on it so it's always protected on the ground, and obvious for removal before flight. 4 years and 315 hours with no issues, and no fuel ever, anywhere near the canopy. I do lots of aerobatics, so if I vented out the top my windscreen would be a mess. In fact, I started out that way but after two aerobatic flights with gas coming out the vent I had to make the change before I did any more aerobatics.

http://www.mykitlog.com/users/display_l ... 363&row=24
In these photos you can see the vent routing. Originally this was my overflow pan drain, but I later connected the vent to it, and added a drain line going vertically down the firewall.

http://www.mykitlog.com/users/display_l ... 214&row=21
The vent looks like this, though I made another one after this one, to get the right exit point to connect to the vent line in the previous link, above.

The vent goes up first so that I can fill the tank with 17 gallons and not have anything go overboard (I have a tail dragger, so that allows some more space in the tank for fuel when sitting at the pumps).

[WaiexN143NM]

Hi mike,
Thanks for sharing. yes another good idea to put a cover and red banner on the tube to keep out the villians. You may want to drill a small hole on the backside of the tube and up a little from the bend , just for the one in a million chance something would hit and clog up the tube on the entry opening. bird, bugs etc.

waiexN143NM
michael

[builderflyer]

We don't have fuel at my home airport so I routinely top off the tank at another airport and fly the 18 minutes back home. On descent into my home field, there is no fuel spilled from the vent tube. Why that would be different from what others experience is a mystery to me.

Michael, when I first built my Sonex I drilled a small hole in the back of the fuel tank vent tube as you have suggested as that was something I had seen on a standard catagory aircraft. In doing the fuel flow testing with the main vent opening blocked and the small hole open, the flow rate dropped a couple of gph over having an unblocked main vent opening, and that was still very satisfactory. But that was with the aircraft sitting on the ground and I always wondered what that same test would have shown during flight with the reduced pressure at the small hole due to the relative wind. Have you done that test in the air (main vent opening blocked with small hole open)?........I never have had the courage to try that. I imagine that one using a leaf blower could simulate that flight experience on the ground but I have not tried that either. What have you done to assure yourself that the small secondary vent hole performs as expected in flight?

Art,,,,,,,,,,,,,,,,,,,,Sonex taildragger #95,,,,,,,,,,,,,,,,,,,Jabiru 3300 #261

[WaiexN143NM]

Hi art, all,
thatsa good question. I too have seen these extra small relief holes on factory certified aircraft, for air if the main entry is blocked. i too have thought about the wind blowing around the tube in flight. i would assume tho that ambient air pressure would still go in the tube.
Weve had our tube configured like this before we flew it. so far so good. i would hope that the aeronautical engineers knew what they were doing, so its a copy of what they did.
we also added carb heat box for our rotec tbi. carb ice in a tbi? ive seen a few ntsb reports attributing engine failure/power loss due to carb ice with a tbi.
im also thinking of running a ‘y’ out of the firewall , two fuel lines wrapped together, ending up at the rotec with a ‘y’ fitting. one line will have a facet pump, the other with just gravity feed. will have to have one way check valves. we did flow tests with and without the facet pump in line, wasnt happy with the restriction the pump caused when turned off and just gravity feed. weve never had any problem yet in flight, even in the heat, with vapor lock. a few times the engine stumbled when taxing back from the fuel pit, jab 3300, rotec tbi, gravity feed. 100LL. we always open the fuel and oil doors when pushed back into hangar to vent out some heat.
the plane has been flying about four years now. fun stuff to experiment , the fuel tanks in the wings will be the next mod.

WaiexN143NM
Michael