On Oct 17, 7:06*pm, wrote:
On Oct 17, 10:25 am, John wrote:

I have wondered about that too. *There were issues with the Citabria/
Decathlon aircraft with wooden wings some years ago. *The current
factory changed to metal wings as a result. *(I wonder about fatigue
in a few years unless the wings are life limited.) *Another thing I
have noticed is that most fabric covered aircraft I have seen in
recent years, (Pitts, Decathlon) have never had the inspection rings
cut open. *I was taught in A&P school that you left them covered only
till the first annual. *Otherwise you could not properly inspect the
wings. *I saw a Decathlon that was at least 15 years old that had
never had the panels opened.

* * * *The old Champ/Citabria wood spars had issues with several areas
of the spars, including cracking from the root attach fitting holes,
the areas at the ends of the strut attach doubler plywood (where the
stiffness imparted by the doubler caused stress concentrations in the
spar at the ends of the doubler) and at every rib. The aluminum ribs,
which do not shrink with age, were nailed (in three places) to the
wooden spar, which does shrink across the grain with age, so that the
nails forced the grains apart and cracked the spars spanwise. The AD
demanded a bunch of extra inspection holes and covers, all of which
were a pain to install, especially in the underlying aluminum leading
edge. Some owners opted to do a recover to get a better look at the
spars and some of them found a lot of cracks. These were cracks that
were just about impossible to find unless the fabric was torn right
off.
* * * Those aluminum ribs have corrosion issues at the trailing edge,
where condensation collects and rots them out.

* * * The aluminum-spar wings, retrofittable to older aircraft, might
be cheaper than replacing cracked wooden spars and rotten ribs and
leaky fuel tanks and so on. I think they're heavier, though, something
not very welcome on these airplanes that tend to have small useful
loads already. The front strut attach fittings have an airworthiness
limitation on them, requiring NDT every 1000 hours to look for cracks.
I've never found any cracks in ours, but one of our fittings had a
small flaw in the metal that appeared to have been put there when the
aluminum bar was rolled. The factory milled the fitting but apparently
they don't NDI them after manufacture.

* * * * I'm recovering our 7GCBC now. Found cracked flap/aileron cove
skins on factory retrofit wings with 700 hours on them.

* * * * *Dan

Forgot to mention: Steel-tube airframes often rust out where
the fabric touches the tubing, especially along the lower longerons.
Condensation or rainwater (or mel****er from snow that has blown in
through cable openings and so on) will run down the inside of the
fabric and collect along the tube and start the corrosion. The old
cotton was bad for that, absorbing the water and holding it there.
I've seen rudder and elevator tubing rotted clean through. My old
Auster had a corroded longeron right behind the battery box, where
some acid had gotten free and lay along the tube. More good reasons to
recover.
And some wings use rib lacing to attach the fabric. The lace
is (or often was) usually waxed linen, and since linen comes from flax
it's something mice find edible if there's nothing better on the menu.
Missing riblacing means fabric that will pull off the upper side of
the ribs and will flutter, causing loss of control. And mouse pee will
rot aluminum structure so fast it's not funny. Not having adequate
inspection holes is just asking for trouble.

Dan