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Author Topic: Plastic Bird  (Read 1097 times)
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piecost
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« Reply #25 on: February 03, 2019, 03:44:22 PM »

I employed a hotwire fitted with a platform beneith the wire. The platform is adjustable for height. The foam was put on a sheet of MDF and slid under the wire, in a spanwoise direction, to cut. A few attempts to set the gap between the wire and platform resulted in a useable flap blank. I glued to a rather heavy balsa leading edge. Once the hotwire is adjusted correctly then it is easy to mass produce flap blanks. It is much, much easier than cutting using templates.

I will make a full wing using my meagure stock of light 1/8".
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piecost
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« Reply #26 on: February 03, 2019, 03:45:05 PM »

The hotwire and platform
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Mefot
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« Reply #27 on: February 03, 2019, 03:55:28 PM »

That looks like a very smooth result. I was interested to see Yashinski drum sands his complete wing halves. A method that might be worth investigating.
Looking forward to hearing how this model performs  Smiley
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OZPAF
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« Reply #28 on: February 03, 2019, 07:48:26 PM »

In the chance that you do go back to hot wire cutting a foam wing I second Victory's choice of phenolic paper or thin laminex.

I have seen the video on UTube using foil templates for hot wire cutting and I wouldn't recommend that way to anyone. Phil Barnes of the USA was one of the best foam hot wire cutters when it was in vogue for both power and glider RC and his videos are worth watching.

Your balsa/foam flap wings look nice.

Happy flying

John
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piecost
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« Reply #29 on: February 03, 2019, 08:21:13 PM »

Thanks both. I will consider the alternative material if i use templates again. I liked the the litho plate since it was quick to make templates and little time was lost if they needed replacing. I often changed the template using trial and error to get the correct wing thickness. But the time wasted in scrappage may have been spent on making better templates.

Going back to the yashinskiy wing. Interestingly, my plastic bird wing was lighter than the Yahinskiy wing that i made from #7.5 lb/ft3 balsa, but heavier than if i used the #4.5 wood specified on the plan. So, my plastic bird was not too bad, but further lightning must be possible.

I have not plucked up the courage to build a drum sander and will stick to hand finishing. I am thinking of making a sanding jig for the balsa part of the yashinskiy wing consisting of 2 aluminium rails infront and behind the assembled balsa leading edge. They need to be carefully set on a board with a shimmed platform between to set the leading edge at the correct taper. The idea is to sand in the spanwise and chordwise taper before gluing the foam and final sanding.
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Rossclements
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« Reply #30 on: February 04, 2019, 08:26:45 AM »

If you are pulling the foam through the wire, try angling the platform at 70 degrees or so and let gravity pull the foam through. I find sheets can be very smooth, as I always seem to get ripples pulling the foam.
Ross
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« Reply #31 on: February 05, 2019, 11:06:26 AM »

If you are pulling the foam through the wire, try angling the platform at 70 degrees or so and let gravity pull the foam through. I find sheets can be very smooth, as I always seem to get ripples pulling the foam.
Ross

Whatever Ross says to do with hotwiring, follow up on it. He does some of the nicest hotwire cutting I've ever seen.
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piecost
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« Reply #32 on: March 13, 2019, 06:53:59 PM »

I had a brief play with my Plastic Bird and Plastic Bird 2 with more standard (not far forward) CG. The earlier model was happy, it was fitted with its original boom stiffened with balsa. But the second model would not transition from discuss launch. Javelin launch only achieved about 15' but transitioned well. Discus launch neared the 25' ceiling but performed a high speed wing over somethimes bunting to land inverted. Once I achieved a perfect transition just below the ceiling but it was not timed. Must have been around 30 seconds though. I have lost track somewhat but think that I was using an intermediate diameter boom and suspect that  it was not stiff enough with the more aft CG. A light solution would be to increase the tail size rather than use a heavier boom.

I found damage to my Lazy Bird 2 wing. See photo. The RH wing lower surface has a crease in the glass close to the pylon. I think that the wing is twisting and bending nose down and buckling the glass. The carbon spar and dihedral joint seem intact. It is interesting that the inboard wing (when discus launched) is undamaged. Perhaps the tension during rhe launch prevents such buckling. I like the idea of developing a design and strengthening where damage occurs and lightening elsewhere. But, when I resume building I will likely make a Yashinskiy wing with a balsa leading edge and probably abandon this construction. It will be bit of a shame, but I see no advantage from foam/glass construction.

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