Fuel Tank - Now what?

Fearless · #1 04-23-2007, 06:39 AM

When I balloon tested my left tank the ballon stayed up for over a day before I took it off. When I tested my right tank Friday I discovered a leak in one corner on the outboard rib which I repaired. I tested the tank again yesterday with a balloon and it kept its pressure for over three hours and then completely deflated (I was working on my flaps and didn't notice it going down). I put on another balloon and sprayed the rivet lines and end ribs and couldn't find any leaks. I checked my marks and it stayed inflated for over three hours keeping the size. When I checked the balloon this morning it had deflated again but was still holding air.

I don't know if I am getting a slow leak from where the balloon is attached on the vent or maybe I have a slow leak in a rivet or rib. Now what?

Thanks,

Mark Burns · #2 04-23-2007, 07:01 AM

Hi Mike,
Are you tracking the temperature? A few degrees will make a HUGE difference in the way the balloon looks. Barometric pressure plays a role also.

I didn't have much luck with the balloon and made a simple manometer out of vinyl tubing. It still changed a BUNCH with temperature.

Keep checking,
Good luck,
Mark

billdianne · #3 04-23-2007, 08:42 AM

try a separate balloon blown up about the size of the one attached to the tank. If they go up and down together it's probably not the tank.

prkaye · #4 04-23-2007, 08:50 AM

I was shocked by how much the relative pressure inside the tank changes with daily changes in atmospheric temperature and pressure. I used the manometer method, and did a prolonged test. After finally getting everything sealed tight, I let it go for about 5 days, checking it about twice a day. The level of water on one side of the manometer fluctuated by about a foot! In the morning it would be 6 inches below my baseline, and then in the evening it would be 6 inches above the baseline.
So maybe your balloon would re-inflate if you left it?

I'm not sure if Billdiane's suggestion of using a second balloon for comparison would actually tell you much. The volume of air in the balloon would be much less than the volume of air in the pressurized tank, and so I would expect it not to be nearly as affected by changes in atmospheric pressure.

I much prefer the Manometer method myself... a bit of extra materials/work, but it's a little more precise, and seems to me a more robust way to do the experiment.

Steve · #5 04-23-2007, 09:19 AM

Be sure to soap spray the fuel cap, balloon (vent) fitting, air (drain) valve, and fuel inlet fitting. My guess its the fuel cap and/or the balloon.

Steve

prkaye · #6 04-23-2007, 09:35 AM

The fuel cap was a really tough one for me. I tried all kinds of tape, but nothing worked. Eventually I got a compressable/expandable 2" rubber plug from home depot... it's meant for plugging PVC. With a bit of grease around the seal it worked great.

WSBuilder · #7 04-23-2007, 10:20 AM

So, why not balloon both tanks at the same time and see if they stay the same size?

John C · #8 04-23-2007, 10:32 AM

The balloon is only good for a short period of time and they leak a lot of air. notice how a helium filled balloon will lose its lift over night. The manometer will be good for a week or so. Temp has a big effect, secondly atmospheric pressure. I broke out the physics and aero 101 books. The manometer's rise and fall were by the numbers (temp and pressure). Try sealing the fuel cap with vasoline or fuel lube. Either will work.

Rick6a · #9 04-23-2007, 11:28 AM

Quote:

Originally Posted by John C

The balloon is only good for a short period of time and they leak a lot of air. notice how a helium filled balloon will lose its lift over night.

You would think a balloon will lose air after a short period of time but not necessarily. As I mentioned in a recent thread, I was curious as to how long a balloon attached to a recently completed fuel tank could remain inflated. It was inflated large enough to rub against the (leading edge) fuel tank attach fitting. In my particular case, I finally removed the (almost) fully inflated balloon after six weeks of holding pressure. It was still touching that attach fitting. Another forum member reported the balloons on his fuel tanks remained inflated for 4 months.

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Fearless · #10 04-24-2007, 06:12 AM

Last night when I got home from work I tried checking the tank again. I finally got a balloon to fasten better on the vent AN fitting. I soaped up the end ribs where I thought I would most likely have an issue. I finally was able to see a slight bubble coming from the screw terminal on the sending unit. After I tightened it up I also retightened the screws on the access plate and the sending unit. This morning I checked the tank and the balloon was still up to where it was originally yesterday afternoon.

I feel better about the tank now.

Thanks for all of the replies.