I had a wonderful time at the contest in Tullahoma Saturday. I ended up
flying my venerable Oly 99; I had good flight times and so-so landings. Met
some new folks, enjoyed flying with my old friends.
I flew the Oly because I discombobulated my brandy-new BoT on it's first
flight of the contest. :( Beautiful launch, but once she got off the line,
she went into a nose-down right spiral that I couldn't recover from.
Minimal damage, really: split the fuse aft of the wing, broke the wing-bolt
mounting blocks loose and broke the right wing near the tip and the left
wing at the polyhedral joint. Fixable.
Fooey. Couldn't imagine why this happened: everything was checked and
double-checked prior to the contest and things were perfect. I figured this
was a combo of "stuff happens" and "dumb thumbs".
On the drive back (3 hours, plenty of time to cogitate), I got to thinking
about the damage to the wings. Both wings did have impact damage to the LE
from a forward direction where they hit a bush and the ground. However,
that left wing also shows breakage of the poly joint, the direction force
seems to be from the bottom.
Ah ha. I suspect that she went into that unrecoverable spiral because of
failure of the poly joint on the left wing. As you may recall, I went to
great lengths of strengthen the wing spar on this BoT. CF laminate added to
the spar caps, end grain balsa shear webs, Kevlar tow wrap, and so on. This
was my first strong wing, built with info I gleaned on RCSE.
However, since this was a learning experience spead out over several months,
I did incorporate one "achilles heel": I enclosed it the 1/8" ply poly
joiner in vertical-grain balsa shear webs, making sure that the spar at
that joint was strong and well-glued. The inner-panel side of the poly
joint is Kevlar-wrapped, but I didn't wrap the outer-panel side of the poly
joint since that would have involved cutting into the lower D-tube sheeting
on that panel. I actually thought of potential problems with those ply
joiners as I was gluing the wing tips on and finishing the wing, but since I
had a series of "maybe I ought to..." delays, I decided that the stock,
unwrapped outer joint would be "good enough".
It wasn't. The poly joint on that wing failed by pulling the sparcaps from
the full-width vertical grain balsa shear webs. The glue joints were good:
the balsa web pulled apart. Had they been Kevlar-wrapped, I suspect that
they would not have failed. Do correct me if my thinking errs.
What to do? Repair the poly joint and take the time to wrap both outer poly
joints in Kevlar tow. I'm even considering a layer or two 2-oz glas cloth
on the bias under the Kevlar, but this may be an extreme over-reaction to
toasting a plane.
Since this was my first "strong wing", I was being careful and was launching
it just a little stronger with each launch. This was a stronger launch on
an unfamiliar winch and I may well have gotten a bit frisky on the ole
pedal.
Oh well. Goes to show, you can take care of 99.9% of the details, but it's
that .001% that'll bite you.
When I had earlier reservations about using the ply poly joiners, I also
thought about using .125" CF laminate plate in place of the 1/8" ply. I may
use the CF plate on the next phase "KillerBoT" wing, but I think that the
ply will work for this wing. Opinions also welcome, since this is the time
to fix that.
All in all, enjoyed the event, and glad I figured out "why?". :)
--Bill
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