jbhart(at)onlyinternet.ne
Guest
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 Posted: Sun Mar 20, 2011 9:14 am Post subject: FireFly Broken or Detached Outer Wing Braces. |
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Kolbers,
| Quote: | From my flight log:
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November 9, 2010 - Removed propeller for cutting to 48 inches. Washed the
FireFly, and while washing heard a noise. Found that both rear outer wing
corner braces have broken.
March 19, 2011 - Cut the wing fabric over the rear diagonal 5/16 inch OD
brace to the main spar. The left brace broke at the cross brace attachment
bracket, The forward end was loose in attachment to the spar tube. The
right brace was not broken but the attachment holes in the brace tube were
torn out and the forward end was free. Both braces show that they were in
high compression in that one cannot move them back into place with out over
lap or, in the case of the space between the attachment end and the spar, a
gap of about 1/8 of an inch.
Most likely cause is that when the fabric was shrunk before painting it put
this brace into severe compression and moved it out of direct/straight
column. This makes the brace very unstable and unable to resist side to
side vibration. As result the bowed brace vibrates and swings around the
direct line between the attachment points. This in turn causes local tube
flexing, and finally, fatigue failure at an attachment point results.
To repair, one must move the spar attachment point from inside to out side
the spar tube. Long rivets should be used to penetrate both walls of the
bracing tube and the spar tube. Also, the same end of the small tube brace
should be filled with JB Weld and the same substance used between the
contact point of the tube. This will prevent the joint from moving, resist
brace torsion and the pulling of the rivets through the tube wall.
To repair the lower end, one can slide a slightly larger tube over the
original tube, fill the gap with JB Weld, and use several pop rivets to
hold it together.
None of this will keep the tube from going into compression on a new build.
But much of the movement and/or vibration about a column can be minimized by
using a tape and/or woven fish line and/or a diagonal metal tube brace from
the end bow to the middle of the suspended brace length and on to the back
side of the main spar.
Jack B. Hart FF004
Winchester, IN
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