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Author Topic: Plastic Bird  (Read 1701 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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piecost
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« Reply #33 on: November 07, 2019, 02:20:17 PM »

I had great fun at this year's nationals playing with my two chuck gliders. Although, I was disappointed that I hadn't had time to replace these models with their more developed descendants. The flaps were too stiff on the first modal and too flexible on the second. I am
hoping that the flaps on a future model proves to be like Goldilock's parridge and is just-right!

A crude repair consisting of 5 minute epoxy to the second wing held up, but after a few flights it failed on launch, just outboard of the previous damage. The wing failed downwards, buckling the lower spar and creasing the glass and foam outboard of the dihedral brace (an
obvious stress concentration).

I was not very upset as it showed the weakness in the wing construction and removed the need to stiffen the flaps. The Plastic Bird 1 wing remained intact despite numerous maximum effort launches. It was noticeably stiffer, being thicker at the root par position, af shorter span and of lower tip chord. The same carbon spar and glass was used on both wings. The flaps were far too stiff and the model still bunted after launch. It only achieved 22 seconds in the competition. I think that it did achieve around 30 seconds in an untimed flight. I employed a balsa stiffener on the aft fuselage, as recommended by Olbill. This really helped the launch in reducing the
whipping of the fuselage and reduced the resultant bunt. But, I believe that the boom was still twisting. A video of an early flight with the boom unreinforced showed the tail twisted by about 30° out of plane with the wing!

I had 3 fuselages with tails attached and swapped them between models as required. I was surprised in that the aluminum tape adequately held the pylon onto the fuselage.

The underfin was susceptible to damage on landing and broke sheared the 0.005 carbon spar at the root. The fin came off on the next launch, which was exciting. I am considering moving it to the top of the boom, as per Yashinskiy. Perhaps this is why his fins are ontop,
rather than for some subtle aerodynamic reason.

On the other fuselage I managed to destroy one of my elegant foam/carbon tails. The RH tailplane tore at the root, from the leading edge back to the spar. This was not a surprise since I employed the same Swept leading edge planform as emplayed on the Lazy Bird balsa tail. With balsa, the strength/stiffness is uniformly distributed throughout the sheet tail. With foam, it is localized in the carbon reinforcement. The spar being located far aft on the planform so as to be unswept; allowing a single piece (on the upper and on the lower surfaces) to be used from tip to tip to hold the elegant curved dihedral. This resulted in the unreinforced leading edge of the foam being cantilevered too far ahead of the spar. Rather than add further reinforcement and resulting mass; I will change the tailplane planform to locate the spar at 25% chord, say, giving a low sweep leading edge and a.Forward swept trailing edge. Unfortunately, this will move the aerodynamic centre of the tailplane forward. This will be somewhat allieviated by the swept forward trailing edge
allowing the tail arm to be increased with the model still fitting, diagonally, in the model box.

I may also try making a fuselage from carbon sock. The aim being to make a fuselage lighter than the thicker rods that I obtained and stiffer than the thinner ones. I have a couple of lengths of 5mm carbon sock from different suppliers and have obtained a fishing rod tip as a
mandrel. I see this as key to reducing the mass to under 10g. Not just due to the contribution of the fuselage mass itself; but also being able to reduce the nose weight. The lighter fuselage needing less ballast to counteract the mass of its rear portion.
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piecost
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« Reply #34 on: November 07, 2019, 02:36:47 PM »

But also, increased stiffness making the tail more effective in the high-speed launch and transition.
This allows a more rearward CG.

If this does not work out; I may try a larger tail so I can move the CG aft as per other models. The tail weighs less than the fuselage and a large relative increase in area can be made with.relatively little weight gain. It must be noted that an equal increase in nose wieght Will be
needed to maintain balance.

My next wing will be inspired by the Yashinskiy model; the leading edge consisting of 3.straight, swept elements approximating the planform. My method of wrapping glass around the leading edge prevents an elegant curve being used. This will be quite complex and certainly the build time will be several times higher than the balsa/foam construction. I will try to match the wing area to find out if I can build down to weight along with being stiff/strong. I will also copy the Yashinskiy section and thickness to achieve similar flap stiffness- assuming
those models use a foam of similar properties to Floormate 200. The foam will be hotwired
spanwise using straight wire guides along the leading and trailing edges. I have already.found out that this is produces a much better quality result than using conventional rib.templates. I am tempted to this new wing without the additional reinforcing to withstand the failure
experienced by the second wing. We will see.
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piecost
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« Reply #35 on: November 07, 2019, 02:40:53 PM »

Proposed new wing
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USch
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« Reply #36 on: November 07, 2019, 05:24:34 PM »

The spar being located far aft on the planform so as to be unswept; allowing a single piece (on the upper and on the lower surfaces) to be used from tip to tip to hold the elegant curved dihedral. This resulted in the unreinforced leading edge of the foam being cantilevered too far ahead of the spar. Rather than add further reinforcement and resulting mass; I will change the tailplane planform to locate the spar at 25% chord, say, giving a low sweep leading edge and a.Forward swept trailing edge. Unfortunately, this will move the aerodynamic centre of the tailplane forward. This will be somewhat allieviated by the swept forward trailing edge
allowing the tail arm to be increased with the model still fitting, diagonally, in the model box.
Piecost,
You could try to keep the spar/roving on the tailplane in one piece but with a bend on the center. Fix the raw tailplane to the board, push a pin trough tailplane into the board at 25% center cord. Pull the roving/spar to the tips, following the 25% line, fix the impregnated roving with tape outside of the tips.

My method of wrapping glass around the leading edge prevents an elegant curve being used.

A solution could be to cut the glass covering the front part of the wing not at 0/90° but at 45/45°. This allows to follow any curve in the wing planform. A little more difficult to handle the cut glass cloth though  Sad

Urs
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piecost
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« Reply #37 on: November 07, 2019, 05:49:22 PM »

Hi Urs,

Thanks for your ideas. I was using 0.005" precured uni carbon sheet for the tailplane spars. If i switch to tow then your idea will work well.

Tow would also conforn to a curve for the wing spar as well. But, I wanted to try precured uni carbon sheet. 0.010" rod is another idea for wing spars with multiple rods at the root reducing towards the tip. A strucurally tailored spar would result. I am too lazy to recess the spars inti the foam and this would cause a higher ridge in the profile.

The wing leading edge glass was biased towards 45 degrees and might indeed wrap around a curved wing. But i am applying it with mylar on the outside which would not. I had considered using thin rubber sheet which might stretch round the leading edge. I do not have high confidence in this.

The biggest success of this project is that the glass on the wing lwadi2ng edge remained well stuck down on the foam and that the leading edge was nice an sharp. I am still supprised that it worked so well.
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« Reply #38 on: November 07, 2019, 06:02:05 PM »

Thanks for that long report PC. lots of interesting info there.

I was wondering if Mylar could be heat formed to go around a curve - perhaps on a separate mould, to help with your LE problem?

Have you considered a very light "Dissier" approach to reinforcing the tail? You would need to pull fibres of carbon from the tow and it could be a bit of a pain however.

Good luck with this interesting approach.

John.
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USch
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« Reply #39 on: November 07, 2019, 06:24:55 PM »

But i am applying it with mylar on the outside which would not.

I think the spherical deformation on a wing is so small that you can easily "mould" the wing with mylar. Thickness, I guess, about 0,4mm.

Urs

PS: tomorrow I will read the whole thread to know what you had done already and what not. Now it's to late  Angry
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piecost
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« Reply #40 on: March 24, 2020, 01:45:10 PM »

The picture shows my first wing fitted with a carbon sock fuselage (as described in the Yashinskiy thread). A new tail was fitted, of the type that failed in the 2019 Nats. I want a back to back comparison using the same tail, of the home made boom compared to the original one. I want to find out if i can move the CG aft due to the tail boom being stiffer and the tail being more effective.
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piecost
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« Reply #41 on: March 25, 2020, 02:55:26 PM »

I followed USch's advice and cutout a Yashinskiy tail out of 0.028" foam and applied 3 1k tows on upper and lower. The length of tow was matched by weight to the 0.004" uni carbon spars used before. I didn't use pins for a guide and paid the price in that positioning the tow was very difficult. I had wetted out and rollered with paper towel before positioning on the foam. This proved too dry so i applied further epoxy in location.  This will likely made it heavier than the 0.004" uni spars.
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piecost
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« Reply #42 on: March 25, 2020, 02:58:07 PM »

Picture of the tail in a food saver vacuum bag
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piecost
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« Reply #43 on: March 25, 2020, 03:08:09 PM »

A Yashinskiy tail unswept to emply a straight spar from 0.004" uni carbon. It looses the fashionable straight trailing edge and looks somewhat retro. Whilst I have had the foam fail in flight; I have not yet broken the spar so each tail employs slightly less carbon. The foam is hotwired thin and quickly sanded from 0.030" down to 0.025" at the trailing edge and tips.
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piecost
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« Reply #44 on: March 25, 2020, 03:12:53 PM »

I wetted the uni carbon with epoxy and vacuum bagged the tail onto a form with the 10 degree dihedral. I found that the spar in the elbo of the dihedral delaminated so i applied more glue and tied a thin fishing line loop through the foam binding upper and lower spars together. The actual dihedral is more like 7 degrees, which i hope is enough to give a good discus launch without rolling.
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« Reply #45 on: March 25, 2020, 07:35:58 PM »

The completed tailplane showing the bottom surface since the tow was neatest on this side. Some weights

Foam.                              0.241g
1k carbon tow 230mm 0.026g
Epoxy.                              0.031g
Total.                                0.298g

The epoxy / carbon ratio of 1.2 is very high since i needed to plaster it on to get the tow to stick at all. It is not a big deal.

I recal Maxouts youtube video stating that each yashinskiy tail half was 0.325g. That sounds too heavy and is perhaps the total for both halfs. I should really rewatch the video to check.

Certainly, my foam and carbon fin weighs 0.073g and the ooriginal balsa example was 0.058g.

So, I recon that i am close to the equavalent weight using 4.5 lb/ft3 balsa. I really need to fly it on a model to see if it is strong enough.



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