SonexBuilders.net

I'm building a Onex and would like some input on best practices to route the vent coming out of the top of the fuel tank. Pictures of what you guys have done would be great.
Thanks

I've yet to install the fuel vent on my Onex but from what I've learned the vent line is best routed through the cockpit and not inside the engine compartment. Vent the line along the top of the glareshield, through a hole drilled through a longeron, down along the side of the fuselage, then have it exit through a hole drilled through the floor near the firewall. Simple plastic or braided tubing will work. Let it protrude about an inch below the belly and cover the end with screen to keep the bugs out.

I also plan on cutting the end of the tube at a 45 degree angle or bending the tube directly into the airstream to give some positive pressure back into the tank. I don't know if this is good practice but it seems like it would help with fuel flow on the gravity feed system and still allow excess fuel to vent. That's just my guess but I don't really know. I know the vent line on my 152 was positioned forward but behind the strut and out of the direct airstream so there was probably a good reason for that.

Sonex - the builder of my plane had it pop up next to the filler and then changed his mind and turned it around and down through the engine compartment to exit by the cooling slot at the bottom of the cowl.

I HATE this from a fire safety standpoint - a tank possibly venting over a hot exhaust - and it MIGHT happen in the air too because...

When the tank is full and you pick the plane up by the tail to move it - the vent turns into a syphon and pi$$es away a gallon or more of fuel before the vacuum in the tank "wins" and it ceases. Till then - once it starts the only way to stop it is to pop the fuel cap to break the syphon. And I can imagine this happening in a dive with a nearly full tank - spraying fuel over the hot exhaust.

I will be putting this back to the upright tube on the top of the tank asap.

I had thought about installing a syphon break valve - but it's a small check valve easily blocked and so wouldn't serve its purpose - and there is still the issue of the fuel spraying around in the area of the exhaust.

That would be a bad problem for the tail draggers. I did take mine out through the fire wall and down the front right side corner. Something to think about also, some of our members are doing doing aerobatics with negative G's. Not a good idea without a check valve in the vent line.

Peter Anson sells a nifty little vent line check valve

http://www.ansoneng.com/wp-content/uplo ... -valve.jpg

Made a unique fuel tank vent for my A Sonex TD that incorporates a siphon break, forward facing vent on top and drain below. Have never had any problems with it with full tank. Mostly made from light brass tubing from local model supply store. Do not know how applicable this might be to an Onex, but PM me with your direct e-mail if interested, have photos and drawing.

David A.

GraemeSmith wrote:Sonex - the builder of my plane had it pop up next to the filler and then changed his mind and turned it around and down through the engine compartment to exit by the cooling slot at the bottom of the cowl.

I HATE this from a fire safety standpoint - a tank possibly venting over a hot exhaust - and it MIGHT happen in the air too because...

When the tank is full and you pick the plane up by the tail to move it - the vent turns into a syphon and pi$$es away a gallon or more of fuel before the vacuum in the tank "wins" and it ceases. Till then - once it starts the only way to stop it is to pop the fuel cap to break the syphon. And I can imagine this happening in a dive with a nearly full tank - spraying fuel over the hot exhaust.

I will be putting this back to the upright tube on the top of the tank asap.

I had thought about installing a syphon break valve - but it's a small check valve easily blocked and so wouldn't serve its purpose - and there is still the issue of the fuel spraying around in the area of the exhaust.

Graeme:

I would NOT move the vent. I've had mine venting out the bottom for over 5 years. With the vent against the firewall you're nowhere near the exhaust. And the end of the exhaust doesn't get that hot. Certainly not enough to ignite fuel. Put the vent at the top and you WILL get fuel on the windscreen. That material will haze and crack with exposure to fuel. I have filled my tank to the brim many times and never, ever had any fuel siphon off. Never, ever. If you look at my vent it goes UP a few inches above the fuel cap before it goes down the firewall. I think you're making a mountain out of a non-existent mole hill.

I have the top vent on my Sonex and before I fitted a check valve I did see fuel come out the vent. In fact fuel would come out the vent on quite gentle descents even after about 40 minutes of flying when the tank would have been about 3/4 full. Since fitting a check valve I have NEVER seen fuel come out the vent. If I am traveling anywhere I fill the tank to the brim. Here's a diagram of the check valves that I sell but there's nothing stopping you from making your own. The little 3/16" ball is made of teflon. I bought some on Amazon. The body is just AN816-4D drilled out to 6mm and cross-drilled to accept the retaining wire. These things work!

If you don't have some sort of check valve fitted your Sonex is probably dribbling fuel out the vent much of the time as it sloshes with the aircraft's movement. If your vent comes out the bottom of the aircraft it's leaking fuel but you just can't see it.

Peter
Sonex 894
414 hours

mike.smith wrote:I think you're making a mountain out of a non-existent mole hill.

Well - I'll respectfully disagree. Even if it is a very small molehill - I'd rather flatten it - especially as the vent I acquired will pi$$ away a gallon in a heartbeat once it syphons. Which is does - quite spectacularly!

If there is a potential for ignition - a rich start could put some flame out the exhaust - and some fuel might be dripping - I'm going to eliminate that mole!

But I do take your point about the plexi getting marred. I'll figure something. I have some small check valves on order for starters - having rethought that part a bit. Especially the potential to be throwing fuel away in a dive.

Respectful disagreement is all in the spirit of constructive debate - no flaming intended (but the pun WAS intended). :-)

How do you put that check valve in a Onex. It appears to me that the valve has to be vertical with the ball down to prevent it from closing off the vent and preventing expanding gas pressure from venting. You could do it on a Sonex because it vents straight up, but the Onex? I guess I never have had a problem because in don't fill my tank to the top.