Fuel Venting from Vent tube

[jiott]

Today I was flying side by side with a friend's Super Cub and he called me on the radio and said "every now and then I see a white vapor streaming back from your right wing." I asked him to move in closer to my SS7 Kitfox and see if he could tell exactly where it was coming from. He did that and said "every so often I see a spurt of fuel shoot forward out of the fuel cap vent tube on the right wing; then it vaporizes and streams back". I immediately checked my fuel sight gauges and saw the right tank showed full and the left tank about 2/3 full. Needless to say I was concerned and immediately turned for home and landed without incident. By the way, the air was somewhat bumpy today, but not what I consider very bad at all. Both tank fuel caps were on tight and proper with fairly new good (the blue BMW or Mercedes) gaskets. Both vent tubes were clear and open.

I had taken off for that flight with both tanks full, but not overly full. I have noticed since day 1 (175 hrs ago) that the right tank always seems to stay much fuller than the left during flight. Eventually the left tank shows empty, but the plane keeps on flying just fine as the right tank draws down. I had quit being concerned about it until today. The actual amount of fuel venting out of the forward facing tube must be very small because I have never noticed any more than a normal fuel consumption for any flight.

In thinking about this, I am sure it is impossible for the forward facing vent tubes to pressurize each tank exactly equally with the corkscrewing airstream coming off the propeller along with other small differences on each side. So a somewhat unequal draindown of each tank is to be expected. But actually losing fuel out the vent really bothers me. I suspect that after both tanks are down to the equivalent of about 1/2 each this burping of fuel out the vent probably stops. I also know that during an aggressive slip it is normal to vent some fuel from the high side wing tank.

My question to you smart people out there is: Have any of you noticed this during straight & level flight maybe on a somewhat bumpy day? Is there anything that can be done about it other than installing fuel shutoff valves in each tank line? I hate to go the valve route because then I will have to actively manage my fuel system.

[jrevens]

I'm curious, Jim... do you have a fuel return line that tees off of the fuel line between the fuel pump & the carburetors, & goes back to the header tank? Since the right wing tank is the one that the vent line from the header tank is connected to, I wonder if that has anything to do with it.

[Av8r3400]

How well in trim does your plane fly? Is the ball exactly in the middle?

[DesertFox4]

Not sure if this will stop your overboarding of fuel but we like to close the four small "vents" in the fuel caps with J.B. Weld. See link to my photo album. http://www.teamkitfox.com/Forums/album.php?albumid=15&pictureid=3211

[PapuaPilot]

The first thing to consider is trim, is the ball centered. I would try to get it centered in cruise. If you want to burn a little more from the right tank then put the ball out to the left a little. This is a trick I used all the time with Caravans and Kodiaks so that I didn't have to turn off a fuel selector for slight fuel imbalances. If I went to a half ball or more it would actually transfer fuel through the header tank to the opposite wing tank.

Like was mentioned if you have a vapor return to your right tank it could be causing the spouting of fuel through the vent as that vapor builds up and returns into the tank. Once the fuel level goes down a bit you shouldn't lose any more fuel.

Sorry but I don't understand the reason of DesertFox closing the holes that are outside of the cap's gasket. :confused: It seems pointless.

[jtpitkin06]

Wow, this is going to be a guess and by-golly response. There's no way of knowing exactly what the problem is so I'll take a wild stab at it.

Let's start by agreeing that fuel cannot come out the vent unless the tank is overfilled and fuel is reaching the filler cap. So the question is, "what is causing the fuel transfer.?"

The possibilities are:

Flying with a wing low
Fuel return from a fuel pump (plumbing)
Imbalance of inlet vent pressure between tanks

Under normal conditions (not low on fuel) the header tank remains full. The vent line is also full of fuel up to the level of the tank it is vented to. If the system has a "both" or simply "on/off" fuel selector valve it is possible for the fuel to transfer from tank to tank. When a tank gets heavier on one side the condition worsens with time until the wing is lifted by the pilot inputs to prevent turning. As fuel is consumed the fuel imbalance gradually corrects itself. When the tanks are full there is little or no imbalance. When the fuel level drops to create an imbalance there is not enough total fuel to fill the opposite tank to the overflow level. So flying with a wing low, though possible, is not likely the cause.

Fuel return from a fuel pump is a possibility depending on how you have your system plumbed. One solution is to have any return line go to the header tank instead of the right tank.

The final area to look is an imbalance in head pressure caused by the ram air vent on the top of each cap. The vents must be adjusted so they are both into the slip stream at the same angle. Even a small angle difference can cause a differential pressure which, under the right circumstances, could push the fuel from one tank to the other. There is quite a bit of pressure in the tanks from the vent lines.

As a start, I would look at the vents and adjust them so they are the same angle. If the vent tube angles are OK then you might try restrict citing one side with a small cap over the inlet with a slightly smaller hole drilled in it to lessen the ram air pressure.

Let us know what you find. This is one of those bugs that can be hard to locate.

Stabbing with blinders on...

John Pitkin
Greenville , TX

[HighWing]

Sorry but I don't understand the reason of DesertFox closing the holes that are outside of the cap's gasket. :confused: It seems pointless.

The fuel caps used to be shipped from the factory with those vents welded shut. The reason being that the caps as shipped from the manufacturer are designed with the small dimple internal vents as the normal venting when used unmodified. When the Kitfox factory modifies the cap by adding the pitot vent those are unneeded and in fact the pitot tube is rendered useless if the ram pressure they generate is released through those internal vents.

[jrevens]

The fuel caps used to be shipped from the factory with those vents welded shut. The reason being that the caps as shipped from the manufacturer are designed with the small dimple internal vents as the normal venting when used unmodified. When the Kitfox factory modifies the cap by adding the pitot vent those are unneeded and in fact the pitot tube is rendered useless if the ram pressure they generate is released through those internal vents.
I believe that the caps, as shipped from the manufacturer, have a couple of holes toward the center that those perimeter holes in effect are connected to. It's those 2 holes that are welded shut when the vent tube is also installed. As long as those 2 holes are closed off properly by the welding, and no pin holes are left, sealing the perimeter holes should not be necessary, as PapuaPilot stated.

[HighWing]

Good catch, John.

[jrevens]

Jim,

It may be a dead end, but it would be helpful for me if you could let us know exactly how your vapor/fuel return line is piped, if you have one. Do you have a special fitting with an orifice in the line? As I asked before, does the line start between the fuel pump & the carburetors, and does it go directly to the header tank or tee into the vent line leading to the wing tank, or something different?

[DesertFox4]

Sorry but I don't understand the reason of DesertFox closing the holes that are outside of the cap's gasket. :confused: It seems pointless.

This has solved fuel overboarding for a couple Kitfoxs and also as Lowell stated if the cap integrity is compromised you lose some ram pressure from the fuel cap which may be causing some to experience unequal fuel drain from their fuel tanks. Might seem pointless to some but has helped in some instances. Sure helped my model 3 years ago. Hurts nothing to try even if it has zero effect.
My new caps will get this treatment before first flight. That and the blue Mercedes cap gaskets.

[jiott]

I really appreciate all the good replies and will try to answer the questions asked:

John Evans-Yes I have the fuel return line, the optional kit from Kitfox installed exactly as they say. It does tee off from between the fuel pump and the carbs thru an orificed (factory supplied) fitting. It goes directly to the side of the header tank thru a check valve. It should have nothing to do with the vent line from the header tank to the right wing tank.

Papua Pilot & others-Yes I will admit I quite often fly with 1/2 ball to the right because I need to add a little more right rudder trim tab. If I don't keep a slight right rudder pedal pressure it flies 1/2 ball to the right. Maybe you guys are on to something here!

DessertFox4-Yes I have used Hysol to close those 4 little holes in each cap based on your previous post. However I do agree that it probably is unnecessary.

JPitkin-It is possible I may be flying with the right wing low at times due to the unbalance when the right wing has more fuel in it than the left, which is quite a bit of the time. I will consciously try to avoid this and see if it helps.
I also noticed that my fuel cap vent tubes are close but not perfectly aligned with each other; will fix that.

I will report back after acting on these suggestions. Meanwhile any other advice or comments are welcome.

[PapuaPilot]

I bet if you line up your vent tubes and get your plane trimmed so the ball is centered in cruise the problem will disappear. ;)

[jrevens]

When I leak checked my tanks, I introduced a small amount of refrigerant into the tanks, and pumped them up to 1/2 psi, then used an electronic leak detector to check them (I was in the refrigeration business for years). I have the blue Mercedes gaskets. I never could get a perfect leak-free seal at the caps. That being said, and in taking another look at the caps, there is another possible path to the 4 vent holes that DesertFox4 pointed out. That is from the 2 rivets holding on the latching piece. There is probably leakage around those rivets, maybe more with some than others. I'm going to seal those holes also... thanks Steve!

[DesertFox4]

John- The rivets can bypass also. I appreciate your in depth testing of your caps. Most don't have the means to thoroughly test as you did.
I believe also the original fuel caps two pieces, before the ram pressure tubes are added, are not welded but crimped together. That is why I also sealed the entire way around the caps crimped joint with J.B. Weld. It doesn't show well in the link to the photo in my previous post but it is there. I forgot to mention that little point. Sorry. You could get a leak on the crimped seam anywhere around that edge.
If I recall, these caps were made for tractors and converted for our use. They likely were never meant to be pressurized in their original configuration.

[jiott]

Have double checked the alignment of my fuel cap vent tubes and tweaked them a little; actually they were very close to being OK. Am now working on a better rudder trim tab to eliminate the yaw tendency to fly 1/2 ball to the right. Will let you know how this turns out.

[jiott]

Just installed my new rudder trim tab and also thoroughly checked my fuel cap gaskets and sealing to make sure there are no leaks to bleed off pitot vent pressure in either tank. Everything checked out good. Then went for a test flight with the new rudder tab. After tweaking the tab bend angle slightly it now flies with the ball centered as perfect as I can get it. It made an immediate improvement in evening out the fuel drawdown rate from each tank. The right tank now does not tend to stay full but comes down with the left, not perfectly even, but probably as even as possible. I will have to say I was surprised that 1/2 ball of uncoordinated flight could make that much difference in fuel flow out of two tanks tied together thru a header tank. I need to go out now with a friend who will follow me and see if my intermittent fuel vent vapor trail has stopped.

[jiott]

My fuel vapor trail has stopped. Now only vents fuel if I do a somewhat aggressive slip with nearly full tanks.