SonexBuilders.net

I'm confused and need some help understanding my fuel tank.
I have a Sonex Legacy w/conventional gear. It recently came to my attention that If I fill that tank all the way up that once the plane is level after rotating then the fuel is pushing on the cap and on the vent.

Today with 12.5 gallons on board I lifted the tail to level the plane (filler cap removed) and dumped fuel on the hangar floor. So please tell me how I am supposed to think about this, and deal with it. Should I just fill the tank and make sure the cap is well sealed then go fly or do folks just not ever fill their tanks?

Oh, btw - After researching this and finding some interesting info on vent check valves, I'll be ordering one. Amazon seems to have a lack of 3/16" teflon balls.

Thnx

Whatever the conclusion;

Should you fill mogas make absolutely sure there can never be any fuel spill either from filler or vent on the windshield- it will crack almost at once. Avgas is much less troublesome. Also make sure there is never any remaining mogas spill in the fuel cap well- it will suck out during flight and crack the windshield.

I’m the first to to admit that the above is easier said than done- after 600 hrs #0525 will have it’s fourth windshield installed this winter. Only consolation is the windshield is fairly easy to replace. Still, some mogas tolerant material would be a great boon!

Thx

You can fit an extra gallon in the taildragger if you fill it to the cap. Just make sure the cap is tight. The check valve is a good idea too.

I found that if I fill my tank to the top and then it sits in the sun, the fuel will expand and leak out the vent as well. So, I usually allow for that if it is going to sit on the ramp for a long time prior to flight.

When I start it right up and start to taxi, the level seems to come down enough that it is not a problem. I like to fill all the way.

Brock, I fill the tank to within 2" of the top of the filler neck which amounts to 16 gallons usable plus a conservative half gallon of "unusable" fuel. The fuel vent rises 3" above the top of the cowl and points forward. Only once or twice in the 16 years I've been flying my Sonex have I seen any fuel escape during flight and that may have been when I departed with a full tank of fuel and then dived the aircraft soon after takeoff. Also, I regularly pick up the tail of the plane with 13 1/2 gallons of fuel on board (fuel cap on) and have never seen any fuel escape at this time. My fuel cap is a well sealed Peter Anson manufactured one.

How high above the cowling is your fuel vent located and is the opening pointed into the relative wind?

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

Beware the common check valve in the vent line, its spring may be too strong. I think peter sells one he has made that has gotten good reviews. The original old vent per Sonex plans that sticks up above the cowl and it bent to face forward also works but is not fool proof. I just sent the Sonex Foundation a bunch of my drawings and photos from way back that had been posted but may have been lost in the transition. I have been told those should be up this week. I made a somewhat unusual anti-siphon vent that has allowed me to fill to near top with tail on the ground without spilling any. It is or should be listed under fuel vent and filler box modifications. Looks complicated, but is made from brass from a model shop and a bit of solder.

David A.

BRS wrote:Oh, btw - After researching this and finding some interesting info on vent check valves, I'll be ordering one. Amazon seems to have a lack of 3/16" teflon balls.

McMaster Carr carries PTFE balls. They come 50 to a pack. I still have 49 of them around here somewhere.

Thanks for all the great feedback.
I ordered McMaster balls a few hours ago. I was going to order a check valve from Peter but I already have the fittings. Though I'll be ordering a fuel cap from Peter.

Still, I get the feeling that my tank won't hold 16 gallons. Though I didn't try too hard to stuff it, yet.