Hi Bill,
Sorry to hear of your BoT mishap. I suspect something different than you
do, as I built my wing almost per plan an the poly joint. All I added was
.014 C/F to top and bottom spars and shear webs all the way to the tip.
Anyone in our club can tell you, I do hard launches with it, and zooms at
the top. The only trouble I keep having is bent jointer rods. Just went to
piano wire, seems to work OK. Check the picture of a launch of my BoT on
Andrew Mileski's website, the guy from Canada. Can't remember it right now,
new computer, not bookmarked yet.
Jerry Miller
SOSS-Medford, OR
----- Original Message -----
From: "Bill Harris" <[EMAIL PROTECTED]>
To: <[EMAIL PROTECTED]>
Sent: Monday, September 24, 2001 5:52 AM
Subject: [RCSE] the Tullahoma mishap
> 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
>
> _________________________________________________________________
> Get your FREE download of MSN Explorer at http://explorer.msn.com/intl.asp
>
> RCSE-List facilities provided by Model Airplane News. Send "subscribe"
and "unsubscribe" requests to [EMAIL PROTECTED]
RCSE-List facilities provided by Model Airplane News. Send "subscribe" and
"unsubscribe" requests to [EMAIL PROTECTED]
