Fuel Tanks and Strake Sealing
06-06-2009
Sealing the strakes is always a tricky job, and finding leaks can be time consuming and frustrating. I managed to work out one method that worked pretty well for me. Hopefully it will for you, too!
Start with an altimeter, a length of flexible tubing, and a Tee fitting. You'll also need some way to reduce the size of your air compressor's output tubing. A short length cut from a rubber air hose seems to work well if you use the right flexible tubing - the tubing slides inside the air hose and makes a decent seal, and the air hose goes over a hose barb on the air compressor. You must have an air compressor with a good valve - you'll only be tapping 1psi or so from it!
Use this setup to pressurize the tank by hooking up the third leg of the Tee fitting to the vent line. Remember that air takes time to flow through such a small line. I've found it worked best if I pressurized the tank slowly (2 minutes or so) until the altimeter read about -1500ft.
Stop here and clamp off the air hose so it can't leak through the air compressor fittings - they're a common leak source! See if your tank holds pressure. If it does, you're done - great! If not, keep reading.
Forget the soap. You have a lot of surface and joint length to hunt for leaks, and the soap just makes a mess. Save it for later confirmation once you think you have one. Instead, go get a "TIF 8800A" gas leak detector. Don't get a cheap one - this is the real deal. It's very sensitive, has adjustable sensitivity, and detects a wide range of gases. These sell for $100-$200 online, but I've seen them on eBay for as little as $40. In my opinion, this tool is indispensible! While you're at it, get a disposable BBQ lighter, the kind about a foot long, that your local market probably sells for a few bucks.
Back in the hangar, carefully crack open the BBQ lighter and you'll find a hefty cylinder of butane and a short length of tubing. You could get a butane lighter refill cartridge, instead, but they're more expensive and you really don't need much here. Stick the butane cylinder's tube up the vent line, and pull up on the cylinder's valve to open it for a few seconds. (Again, it doesn't take much!)
Now re-pressurize the tank. Using your gas sniffer, probe slowly around likely leak areas. You'll find that you can use very high sensitivity levels if you move slowly. (It also helps not to be using propane heaters or have open fuel or solvent cans around your test area.) Likely leak areas include the sump, water drain fittings, and along the kerfs in the leading edge.
I didn't fill in the outboard dead spaces with foam - I left them open. I'm glad I did, because a major leak showed up here. If you did, you may find as other builders have that you have a leak elsewhere that migrates along the kerf lines and shows up here. Instead of drilling into this area, if you've filled it, I'd suggest drilling 2-3 small holes through the outer (only!) skin of the strake, near the leading edge, and right on top of one or more of the kerf lines. Butane is a very light gas, and even though Divinycell is supposedly "closed-cell", I've found the leaks migrate through the top skin pretty easily. Making a few test holes is a quick, painless check, and it will get covered over when you glass the leading edge. You might as well find out early!
As you explore, keep checking your tank pressure. Make sure you have enough to keep forcing air (and butane) out any leaks, but don't overpressurize the tanks either! I found -1500 to -1800ft worked well.
Once you know you have a leak in this area, finding it is a matter of drilling more test holes to zero in on it until the leak detector goes nuts. Then use the Dremel and a sanding drum to open up a small portion of the skin, and carefully sand down to the inner skin. If you're right on the leak, the detector will go nuts, and you can use some soap to verify that you have a spot. Open it up enough to do a proper layup, and put two wet plies of BID over it - an inch square is fine. It's not structural. Fill the area back up with foam, sand flush to the outside skin, and it'll disappear when you glass the leading edge.
Perfecting this method took several work days and probably 25-30 hours, all total. Had I had this method written down ahead of time, and known it would work, I could probably have found AND fixed all of the leaks in just a few hours. It's that easy.