gas cap venting

[Bcone1381]

True story - when you pull a high G or climbing manoeuvre in the BH (especially with full tanks) it flushes fuel out of the cap vents at very high pressure. ............. But we did see a lot of fuel going overboard and streaming aft.
)

Tell us more Battson! Do you think its being sucked out due to the lift induced low pressure that the wing is producing one might see it during slow flight? Is it something going on mechanically due to the stress and pressures placed by the wing structure onto the internal tank? This seems like something to look into to me.

[DRLPatrol]

........ This seems like something to look into to me.

I agree. Tell us more.

[whee]

That does remind me that I read in some report about the stress testing of the BH wing that the fuel tank is part of the structure. I can't imagine that the tank is getting squeezed in a high G maneuver causing fuel to be pushed out the vent but I suppose it is possible.

[Battson]

I don't really have much more to tell, we were filming during a formation flight and I did a "break" high and right into a wingover. We were cruising at 130 KIAS.
My father-in-law was flying the other plane, and told me he saw fuel streaming from the top of the left wing as we pulled up and away.
I returned to close formation immediately, and we did an aircraft to aircraft examination. There was no leakage.

I have been assuming the G force pushed the fuel outboard, or perhaps I was a little out of balance (just a little), and it pushed the fuel up against the gas cap and out the vent. From memory we had full tanks, we'd only just departed 10 minutes earlier after fuelling up.

Its possible the lower pressure could have sucked fuel out, that is an interesting theory.

[Bdflies]

Well Battson, here's an assignment for you; Be sure to top off, next time you're doing formation with the Father-in-law. Have him take a high nine O'clock position. Assume a hard over slip and see if it vigorously blows fuel out of a vent. Then, (safely) enter a coordinated 3G turn. Again, having him watch for fuel dumping.
My suggstion of reduced pressure was about half way tongue-in-cheek. But, the more I think about it, the more I think it's possible. At 1g, the pressure differential supports the weight of the aircraft. At 3g, the differential is 3 times the 1g situation, right? Because the cap vents are pretty close to the upper surface, I'm thinking that the vents might be in the affected region. In normal flight, no issue. Get a bit frisky and who knows!
No matter what you learn, from the assignment, the "reduced pressure" theory will be my explanation!

(Someday)
Bill

[Phil]

Or a bit safer option would be to mount one of your cameras on top of the plane aimed at the tank caps.

[Bcone1381]

We will all fly with full tanks, and all make rudder corrections which place lateral physical forces on the 130 pounds of fuel mass. I suspect we are venting more fuel overboard than we think. Could we keep our eyes open for a good check valve or negative pressure relief valve into incorporate into the fuel cap design?

[Battson]

Very true. Yes we are....

[bway]

Wing tanks always have fuel sloshing around when maneuvering because they are not at the center of rotation in either axis. I think I have seen vented fuel caps that have a check valve. I suppose they incorporate a way to vent excess pressure from climbing to altitude, etc. Bob

[Phil]

The check valves that Cessna uses have a tiny bleed hole to allow pressure out. For example after topping off with cold fuel from an underground tank then parking the plane on a hot ramp.

[Battson]

a For example after topping off with cold fuel from an underground tank then parking the plane on a hot ramp.

Big +1

We often come home on a long descent from 10k or higher, land, fuel up to the top - freezing cold tanks and cold fuel from underground - and park in a sweltering hot hanger, up to 100F inside.

Within an hour I will see a constant stream of fuel flowing out of the tank cap vents, covering my wings with green dye, wasting avgas, and generally making a mess of the hanger.

Of course any other vent design wouldn't solve the problem, it would just redistribute the fuel to a different place on the floor of our hanger...

[Bdflies]

The Husky incorporates check valves in the vent tubes, which exit the aft edge of each wingtip. I THINK the fuel caps have relief valve mechanisms, to burp pressure buildups. I'll confirm tomorrow. Works really well. The fuel tanks are tee'd together and feed to an ON/OFF valve. When parked on a mild slope, they don't seem to cross feed. I guess this can be attributed to the check valves. Don't recall fuel dumping, after topping off, either.
When I get a bit curious as to the best way to do something, on the Patrol, I step into the hangar and see how it's done on the Husky. The planes are very similar and the Husky is a very mature and durable aircraft. I'm not the brightest bulb in the box and it's not likely that I'll come up with too many better ideas.

Bill