Glue it to it

wrote:
[...]
Once the early designers appreciated the advantage of the one over the
other they moved immediately to true monocoque structures of molded
plywood, welded steel tubing and so forth, but the structural integrity
of the 'box' structures combined with their simplicity of fabrication
makes the method ideal for homebuilders even today.

-R.S.Hoover

Which reminds me. A little googling will bring up an Avions Mudry Cap
10B (also known as Apex), the left spar of which failed in Texas a few
years ago (c. 2001), killing the pilot. The problem was compression
fractures in the upper box spar cap and some other damage which could
not be (or was not) inspected and repaired, even after compliance with
several service bulletins requiring inspection and repair near the wing
attach fittings at the wing roots. According to the NTSB report, none
of the glue bonds failed; the failures were compression cracks in the
Sitka spruce and other woods in the main spar. Having read the reports
and seen the extensive photos of the failures, I think if I had a Cap
10, I would rebuild the wings or ground it.

"jls" wrote in message
oups.com...

wrote:
[...]
Once the early designers appreciated the advantage of the one over the
other they moved immediately to true monocoque structures of molded
plywood, welded steel tubing and so forth, but the structural integrity
of the 'box' structures combined with their simplicity of fabrication
makes the method ideal for homebuilders even today.

-R.S.Hoover

Which reminds me. A little googling will bring up an Avions Mudry Cap
10B (also known as Apex), the left spar of which failed in Texas a few
years ago (c. 2001), killing the pilot. The problem was compression
fractures in the upper box spar cap and some other damage which could
not be (or was not) inspected and repaired, even after compliance with
several service bulletins requiring inspection and repair near the wing
attach fittings at the wing roots. According to the NTSB report, none
of the glue bonds failed; the failures were compression cracks in the
Sitka spruce and other woods in the main spar. Having read the reports
and seen the extensive photos of the failures, I think if I had a Cap
10, I would rebuild the wings or ground it.

Compression failures are due to either an over-stress condition, poor
design, or poor materials. Compression failures have happened in every type
of aircraft structure (metal, wood, and/or glass).

Why would you ground your hypothetical CAP due to this one instance, which
was probably caused by an overstress at some point (assuming no pertinant
facts were omitted from this synopsis) ?

Beyond that, if you rebuilt the wings, how would you know that the next
person who flew it didn't overstress it on his/her first flight?

KB

Kyle Boatright wrote:
"jls" wrote in message
oups.com...

wrote:
[...]
Once the early designers appreciated the advantage of the one over the
other they moved immediately to true monocoque structures of molded
plywood, welded steel tubing and so forth, but the structural integrity
of the 'box' structures combined with their simplicity of fabrication
makes the method ideal for homebuilders even today.

-R.S.Hoover

Which reminds me. A little googling will bring up an Avions Mudry Cap
10B (also known as Apex), the left spar of which failed in Texas a few
years ago (c. 2001), killing the pilot. The problem was compression
fractures in the upper box spar cap and some other damage which could
not be (or was not) inspected and repaired, even after compliance with
several service bulletins requiring inspection and repair near the wing
attach fittings at the wing roots. According to the NTSB report, none
of the glue bonds failed; the failures were compression cracks in the
Sitka spruce and other woods in the main spar. Having read the reports
and seen the extensive photos of the failures, I think if I had a Cap
10, I would rebuild the wings or ground it.

Compression failures are due to either an over-stress condition, poor
design, or poor materials. Compression failures have happened in every type
of aircraft structure (metal, wood, and/or glass).

Why would you ground your hypothetical CAP due to this one instance, which
was probably caused by an overstress at some point (assuming no pertinant
facts were omitted from this synopsis) ?

Beyond that, if you rebuilt the wings, how would you know that the next
person who flew it didn't overstress it on his/her first flight?

KB

Read the reports and you'll begin to see the problem. There's not that
big a Cap 10 fleet here in the USA but after the fatality, more bad
wings were found. Read the reports before you make your judgment. I
didn't make mine until I had seen them.

I agree that metal structures can suffer compression related failures.
At my employer, a leading Regional Jet manufacturer, a situation came up
where landing gear legs were cracking due to localized stress during
hard landings, which were yielding a small zone of metal at the peak
stress point, within surrounding metal (300M steel) that did not quite
reach the compression yield point. As soon as the load was removed,
this set up a huge internal "force fight" between the yielded material
and unyielded material deep within the forging, leading to a cracked leg.

This requires the fitting to be capable of reaching a compression yield
limit before buckling, with just enough force applied to yield part of
the structure but not all of it. I think in the case of most metal wing
spars, the compression critical part of the structure will either
totally yield in compression or will buckle, leaving a bent wing.
Therefore, generally with metal wings after a wingtip strike, or
overstress in flight, the rule is if it ain't bent it's ok. With a wood
wing you have to somehow detect the compression failure within the wood
by inspection. This is the principal weakness of wood structures from a
practical operational standpoint.

John

Kyle Boatright wrote:
0, I would rebuild the wings or ground it.

Compression failures are due to either an over-stress condition, poor
design, or poor materials. Compression failures have happened in every type
of aircraft structure (metal, wood, and/or glass).

Why would you ground your hypothetical CAP due to this one instance, which
was probably caused by an overstress at some point (assuming no pertinant
facts were omitted from this synopsis) ?

Beyond that, if you rebuilt the wings, how would you know that the next
person who flew it didn't overstress it on his/her first flight?

KB