Chapter 21: Leak Testing


Leaks?  What leaks?

I didn't have any leaks in either strake.

 

My Testing Method

The first and foremost leak mitigation strategy is to ensure that your testing gear is not leaking.  

I first tried the altimeter setup described in the plans.  I attached and hose-clamped a 6-foot length of 3/8" ID vinyl tubing between the fuel line and the altimeter.  I attached and hose-clamped a 4-foot length of 1/4" ID tubing between the vent line and a brass water valve.  I used my lungs to pressurize the tank.

Lo' and behold, there was a slow leak of about "100 feet" per hour.  Sure enough I came back the next day and all the air had leaked out.  So I tried again with the same results.   Hmmmmm.  So I switched everything over to the other tank.   Damned, more leaks!  Oh, BUT WAIT!  It's exhibiting the SAME leak rate.  Hmmmmm.

So I grabbed a fellow pilot who happens to also be my friendly Snap-On Tools guy.  He had a machine used by mechanics to detect the tiniest of leaks in an automobile's vacuum system.  The machine pressurizes the system with dense smoke impregnated with a dye that glows when exposed to a black light.  So we tried it on the Cozy.  We replaced the altimeter with a good, accurate pressure gage, then attached the machine and pressurized the tank to 1.5 psi.  (BTW:  "1,500 feet" is equal to 0.8 psi.).  No smoke!  That's a good sign!  But I was still skeptical, so we let everything sit for awhile.  No pressure drop.  Good!  We repeated the process on the other, and it too held pressure.

So in my case, the leaks were being caused by a crummy altimeter and not by my strakes!  So if you have a leak, check your test set-up first before tearing into the strakes.

 

Tips

Be extremely tedious and strive for the best workmanship possible:

  1. Use at least 2 coats of epoxy and peel ply on all ribs that form the perimeter of the fuel bays.

  2. Use at least 3 coats of epoxy on the inside of the bottom and top skins before installing them.  Peel ply each successive coat.  By the way,  I think it's overkill to just pour and squeegee the epoxy into the bays.  It does no good to have a half-inch thick coating only to miss a pinhole.  You gotta seal all pinholes, so you gotta use peel ply.

  3. Use wet flox when attaching ribs to strake bottoms.

  4. Just before BID taping, apply a coat of epoxy into all junctions that form the tank's perimeters and apply really, really wet flox fillets.  The flox fillets should be smooth and glossy, not lumpy like peanut butter.  

  5. Use really wet BID tapes.  I even brushed on a light bead of epoxy onto the already wet flox and brushed on a light bead down the center of the BID tapes just before applying the really wet BID tapes.

  6. Peel ply every BID tape, especially those around the tank's perimeters.

  7. Pay very special attention to all corners.  Strive for complete epoxy and flox coverage and overlap of BID tapes and peel ply in the corners.

  8. Be extremely sure to coat and peel ply the fuselage sides and forward faces of the spar.  I had peel-plied the fuselage in Chapter 7 and peel-plied the spar in Chapter 14 for this very reason.  I'm glad I did.

  9. Give the pour foam a WEEK to cure completely.  Be SURE the foam is sanded EXACTLY EVEN with the tops of the surrounding ribs.  Too high and you'll have gaps between the top skin and ribs.  Too low, you'll have a gap past the foam and straight to the spar.

  10. Use the cap strip method for the top skins.  This ensures that the ribs and top match exactly.  The cap strips also provide more surface area for adhesion and allows the wet flox to flow more evenly without being squashed out only to dribble down the side of the ribs.


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