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Old Feb 16, 2013, 12:43 PM
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Old monokote question

Howdy, all...

I'm in the process of bringing a Great Planes PT-40 back to life that I built but never flew about 20 years ago. Folks on the forums here have been very helpful getting the OS Max FP-40 up and running, the Futaba radio, receiver, and servos are all working, and are roughly trimmed correctly per the PT-40 manual.

Next question... I have two pretty substantial holes in the wing due to some cats we used to have (that have gone to that great kitty litter box in the sky, thankfully.. of old age, don't worry... ). After reading the forums, I've got a pretty good idea of how to go about patching them. Over the 20 years of just sitting there, the Monokote has developed some bubbles and wrinkles. How critical to good flight is a completely mirror-like surface? Should I poke a bunch of pinholes in it and re-iron it down? I remember when I was building it it seemed like every time I'd tack a bubble down, another one would appear next to it... I got kind of frustrated. When I finished it back in '94, it came out looking pretty good finally, but it's not as good now. Thoughts? Would it better to recover the whole plane after 20 years? I hope that's not necessary, but I've got more patience now than I used to and I'd probably do a better job.

Thanks..

Barry
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Old Feb 16, 2013, 01:07 PM
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Canada, BC, Langley
Joined Nov 2007
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If ironing out a bubble causes more bubbles, the iron is too hot. I prick a hole at one edge of the bubble, then use a coolish iron, just enough to activate the glue and iron toward the pinhole.
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Old Feb 16, 2013, 06:42 PM
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Fla.
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If you patch any holes : cut your patch just a shade larger then the hole, then round all the corners before sealing. Sealing also brings up a point : after you have applied the patch seal all the edges real good . The round edges assist in keeping fuel out especially if sealed real good.

You might get by using your heat gun to make the plane look better. Just take your time and keep the gun moving, don't keep it in one spot long. Good Luck ENJOY !!! RED
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Old Feb 16, 2013, 07:03 PM
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Denver, CO
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If the covering was overheated it will likely never be really smooth. I would do just the patching needed to fly it. After a few flights the covering will take a new seat. So working to get the wrinkles out before a few flights might be a waste of time. If you really like it, a recovering might be in order. Some wrinkles will probably not be noticable in flight, unless they allow the wing to twist.
Best to measure the temperature of the iron. I use an infared thermometer. Most coverings packages have a recomended temperature on them. If unknown, and you have some scrap pieces, how it reacts to the iron, can give a good idea of the temperature to use.
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Old Feb 17, 2013, 07:53 AM
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Thanks for all the tips, everyone. I'll patch it, then go after a couple of the larger bubbles on the fuselage with lower heat... it sounds like the problem might have been me being a little too enthusiastic with the heat setting the first time. The wing is pretty good except for the two holes, although this trainer is supposed to have 1/2" - 3/4" worth of wing twist or wash and I recall having trouble achieving that. The twist is supposed to help with stability and self recovery. Is it essential? I don't have a heat gun, and tried to do it with my wife's hair dryer. I also tried just rapidly moving the hot iron back and forth over the surface of the wing will twisting the wing. That was back when I first built the plane in '94. I guess I should remeasure, see if any twist still exists, and try again once it's patched, eh?

Thank you all again,

Barry
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Old Feb 17, 2013, 11:42 AM
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Wilson NC 27896
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You are asking if the washout is necessary on a PT-40 trainer to fly? Not really. If the plans called for it I'd try and replicate what was supposed to have been there if possible, but i wouldnt sweat it if you cant get there. Its difficult ( not impossible) once the wing is built to get 1/2-3/4" of washout to stay into the wing by twisting & shrinking the covering. I'd finish repairing your holes & go fly. On future builds try shimming the trailing edge before you glue all your wing ribs in place & you'll have a better chance of keeping your washout in the wing.........Gene
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Old Feb 18, 2013, 01:24 PM
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Quote:
Originally Posted by gene6029 View Post
You are asking if the washout is necessary on a PT-40 trainer to fly? Not really. If the plans called for it I'd try and replicate what was supposed to have been there if possible, but i wouldnt sweat it if you cant get there. Its difficult ( not impossible) once the wing is built to get 1/2-3/4" of washout to stay into the wing by twisting & shrinking the covering. I'd finish repairing your holes & go fly. On future builds try shimming the trailing edge before you glue all your wing ribs in place & you'll have a better chance of keeping your washout in the wing.........Gene
Thanks for the tip re/ washout, Gene. I'll keep the shimming trick in mind for the next one! I measured it, and the washout is pretty much completely gone.

The good news is that I just finished patching the holes and ironing down most of the bigger bubbles and blisters. In body-shop lingo, it's about a 10-foot paint job.. <grin>... which I guess for this scale is not too good, but I'm happy with it.

Engine's running, holes are patched, electronics are good to go.. now I just need the snow gone so I can start taxiing a little bit..

Thanks for all the help!

Barry
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