REFLECTOR: Leak

["Al Gietzen" <alventures@email.msn.com>]

Builders,

I'll share this experience in the hopes it may save some time for someone
else.  I did (I thought) thorough prep of the stakes before closure, and
used my "foolproof" closure method (see also
http://home.utm.net/alventur/airplane/strclos.htm).  Yet a leak test with
air at typical fuel pressure head showed a small leak.   Leak rate of air
was somewhere around .5 to 1 cu. in./hr; small enough to be very hard to
find (I spent some hours trying), but I believed too large to ignore;
especially if it might be coming into the cabin area.

I concluded the way to find it was to use a bit of detectable gas in the
tank and a leak detector.  The EPA not withstanding; if you know someone in
the AC/refrigeration business, a bit of freon works well.  After searching
for a couple of hours (mostly eliminating false alarms), I found a couple of
pinhole exits coming through the inner skin of the lower strake near the
outboard end, near the leading edge of the foam where the inner and outer
skins come together.  This is about 7" outboard of the outboard bulkhead.

Going on a suggestion from Mark Machado to dam it at the outboard bulkhead,
I chose to cut in through the outside skin under the outboard bulkhead, to
head it off at the pass, as it were.  I ground in just to the inner skin
from the leading edge back about 3 inches into the foam area. With the leak
detector it was very easy to find that the leak was a single channel (maybe
less than a square millimeter) between the skins of the lower strake right
at the edge of the foam where the two skins come together.

It continued right along the leading edge of the foam where I chased it
inboard for about 10".  At that point I pulled about 0.5 psi negative
pressure in the tank and applied thickened epoxy, which stopped the leak;
then back filled the cutout with microglass.  Then covered it all over with
a couple plies of fine BID.

Somewhere along/near the leading edge of the foam I had a pinhole entrance
through the inner skin. My conclusion is that the little continuous channel
between the skins in that location is likely not unusual.  It is a natural
path for air to exit when the skins are bonded.  Anyway, since the inner
skin is (was in my case) quite porous in that area, I'd suggest it worth
your while to apply 3-4"strip of fine BID inside the strakes from the
leading edge back over the tapered area of the foam core to preclude the
problem.

Happily continuing on . . . .

Al Gietzen