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diagonal cuts appearing in covering during flight

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diagonal cuts appearing in covering during flight

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Old 05-30-2003, 07:55 PM
  #1  
waagbuck
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Default diagonal cuts appearing in covering during flight

I have a 40 size great planes super sportster that I built from a kit two years ago. I have an 0S 40 fx engine on it and I typically fly it with a 10x6 or an 11x5 prop. It flies great, lands great, overall a wonderful airplane.

The last two times at the field I've noticed small diagonal cuts about 3/4 inch long appearing in the covering in the wing. The covering is monokote. One cut was midway out on the right wing, and the other was at the second to last rib bay. The cuts are appearing in open areas, ie not directly over the ribs, and I notice them only upon landing. I didn't notice any performance issues on any flight. In fact, the most recent cut appeared during one of four flights on a given day, and I didn't notice it until loading it up at the end of the day. I've covered them with clear plastic tape as a repair and have not had a problem.

I'm curious as to why these cuts might appear.
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Old 05-30-2003, 08:34 PM
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ChuckAuger
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Default My guess..

The wing is flexing..see if you can't flex the wing and make the cuts get wider.
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Old 05-30-2003, 10:09 PM
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Stick Jammer
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Default diagonal cuts appearing in covering during flight

Are the cuts on the bottom or top of the wing?
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Old 05-30-2003, 11:31 PM
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JimTrainor
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Default diagonal cuts appearing in covering during flight

My best wild guess: Pressure on the open bay will cause a stress to develop across the chord, and along the span. The net result is a diagonal stress in the covering material - and you are seeing the result.

It could just be from normal wear (but I really have no idea how monokote ages). Or maybe tiny punctures that open up diagonally?
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Old 05-31-2003, 03:09 AM
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Default diagonal cuts appearing in covering during flight

Did you overheat the covering while ironing or shrinking it? Or have you re-shrunk it a number of times to get it tight again? You may have "stretched" it to it's limits (pun intended. ) and it's showing up as stress cracks during vibration or wing flex.
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Old 05-31-2003, 01:14 PM
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Default more info

Thanks for the input everyone.

The cuts are on the top of the right wing.

This was the third plane I covered with monokote, and has been restretched at least once. I'm sure I could have overstretched it to get rid of wrinkles, but I haven't had any of this type of problem on the other planes I've covered.

I guessed it would have to be caused by wing flexing as well. If it's just a question of repairing the cuts in the covering, that's easy enough. I was wondering whether I ought to be worried about this type of flexing. No one so far seems concerned about it. Should I be?
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Old 05-31-2003, 10:15 PM
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Default diagonal cuts appearing in covering during flight

whether I ought to be worried about this type of flexing
Well, I guess that depends. IF it's wing flex I'd be concerned. But that hasn't been determined. Can you easily flex the wing by hand? Only the right side? Have you cartwheeled it, or had some other crash that may have weakened the structure?

Were you flying extreme maneuvers before seeing this? (Hard, fast rolls, fast dives with quick up elevator, or full power loops.)

It may have been a bad batch of Monocote. Some of there more recent stuff seems to have been reformulated, and in my opinion is garbage. Requires much higher temps to adhere and shrink.

I've also heard of Monokote becoming brittle over time, but usually much longer periods than you're talking about.
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