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Author Topic: Plastic Bird  (Read 2001 times)
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piecost
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« Reply #50 on: May 21, 2020, 08:02:40 PM »

I finally went flying in the evening an enjoyed very still air bedore the weather was due to break. As predicted; I broke both my new wings and the original wing survived. I am not upset since the failures point towards improvements to be made on succesive models.

The homemade carbon tube fuselages were a success. When fitted to my first wing (with the original shape tail) the increased stiffness increased the tail effectiveness  which allowed the CG to be moved back to a similar location to the Lazy Bird and Yashinskiy models.

I had sanded the flaps on my first wing and reduced their stiffness so they flexed as intended. This increased the launch height. Javlin and discus launchs worked Ok. I next swapped for a fuselage with a Yashinskiy tail and was supprised that the model was still in glide trim. However discus launches reasulted in a barrel roll to the left. This was similar behaviour to that when I installed a Y tail without dihedral. Perhaps the 20 deg wing dihedral requires more than 10 degrees on the tailplane to prevent this ocurring. I don't inderstand how this works.

The Plastic Bird 3 model showed much promise despite the glass cloth delaminating from the foam prior to even flying for the first time. The wingtips felt far too fragile, but did not break. Javlin launches sometimes got to a good height but transition was unreliable. However, the glide was lovely and floaty. The flaps seemed to work well.  Unfortunately; it would not turn, even with lots of rudder. I started to bend one flat down more to acheive a turn but broke the wing before making much progress. The RH upper spar delaminated just outboard of the dihedral brace and balsa shear web. I was kind of expecting a failure here,  but on the bottom spar cap, not the top (as per my second model when discus launched).I regretted not extending the shear web further outboard but this may not have helped. I think that employed a twice as thin, but twice as wide spar cap may have contributed to the failure. I theorise that the stiffness of the cap itself is important in resiting it peeling off the foam. The thinner but wider cap has less stiffness and is more likely to try and peel of. But it had more gluing area than a thicker and narrower spar; so who knows?

Plastic Bird 4 broke after only a few.launches with the LH wing bending down with a crack on launch. The bottom spar cap peeled a layer of foam and buckled. This was again at a stress concentration; just outboard of where the cap went from 2 adjacent pieces of carbon to 1 and where the balsa shear web terminated. I am not sure if more carbon would be better than extending the balsa shear web further outboard. This wing lacked the glass cloth leading edge covering. I don't think that this caused a problem with the wing twisting so will leave it off the next wing. Especially since I had such problems getting the glass to stick to the foam.

My conclusions are that;

The homemade carbon sock fuselages are a big improvement over fishing rod tips. They are stiffer and the increased tail effectiveness allows a more aft CG with resulting saving in nose weight.

My Yashinskiy shaped tails made in foam and carbon were robust enough (so far) and of similar wieght to using 5 lb/ft3 balsa.

The yashinskiy tail combined with a wing with 20 degrees of dihedral cannot discuss launch without barrel rolling to the left.

My new wings employing the tapered flat plate sections were an improvement over my earier attempts at aerofoil shaped wings. The better quality of hotwiring allowed a more precise control over flap thickness and stiffness.

Both new wings seemed to have stiff and strong enough dihedral braces. Perhaps I can save more weight by omitting a layer of carbon from the brace on future wings.

The wing without glass cloth broke before I could establish if it was too flexible in twist. But i will omit the glass on the next wings since it saves much weight and hassel in manufacture.

Both new wings failed with the spar caps buckling and peeling the foam. Increasing thw spanwise extent of the shear web may help and be lighter than employing more carbon.

Perhaps i will make some new wings in a batch with more extensive shear webs. I may inset some Rohacell instead of balsa extending much further outboard. I could see if the thinner carbon cap sticks better to this than the blue foam.

If I omit the glass cloth on the leading edge then I can employ the elegant Yashinskiy curved leading edge.

I have so many ideas to try that I am risking trying too many at once and not knowing which ones work!

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Re: Plastic Bird
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OZPAF
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« Reply #51 on: May 22, 2020, 05:52:55 AM »

I haven't digested all of this interesting report Piecost but have a couple of comments re the shear web and the spar buckling.

Definitely the risk of buckling of the caps reduces as the caps are made thicker and this will require more support from the shear webs.

Instead of using balsa for your shear webs it may be better to use say even  light glass cloth at 45 deg. I have done this in a 20" RC foamie which was fully aerobatic and never looked like breaking the wing spar. My shear web of 3 oz cloth was not much thicker than paper and it was place in a knife cut between the sanded out cap positions and the carbon laid in either side of the web which had been trimmed to be just below the level of the wing surface. It doesn't need much glue on the shear web to hold it in place.

This would not be all that much heavier and I feel would be a lot stronger.

Good luck with your plastic bird.

John
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piecost
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« Reply #52 on: May 22, 2020, 10:10:47 AM »

Thanks for the tip John. Am I correct in thinking that the spar web protruded into the spar pocket and the caps butted against it? Were the caps wet layup or pultruded? I will keep this suggestion in mind, but thought of some ideas last night when I couldn't sleep.

Unlike my 2nd wings failure where the foam crushed; both new models failed by the caps peeling from the foam. Actually the foam itself peeled away rather than the glue failing.

I may experiment with a full span 1/32 verrical grain balsa web. This may not give more resistance to the cap peeling off, but the increased ristance to crushing may be good enough. Indecated by the wings failing immediately next to where the web terminated.

I will also try a Rohcell web the full width of the caps. This should resist cruahing and the material should not fail in peel, I hope. For the latter the foam leading and trailing edges will be butted against the spar assembly. I figured a way of sanding the web to get the spar caps flush with the foam wing profile.

Tim
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« Reply #53 on: May 24, 2020, 06:27:47 AM »

Quote
Am I correct in thinking that the spar web protruded into the spar pocket and the caps butted against it? Were the caps wet layup or pultruded?

The shear web did run into the cap cutouts in the foam and thus bonded to the carbon filaments when they were laid up wet in the groove. I split a 12k tow - 1/2 on top and the other on the bottom. I used to lay the tows down on the sticky side of a length of masking tape(secured at each end with more tape). The tows were then wet out, excess resin blotted of with paper, and then still stuck to the tape - transferred to the cap cutouts and pressed flat.

I sanded in the cap cutouts which were in the form of a flat triangle with the max depth at the central shear web. From memory I allowed 1mm2  area for 12k  - so in this case I had 0.5mm2 top and bottom.

It may be a little heavier - but I think it is a better way to go with foam as you are not relying on a foam carbon joint for buckling support of the cap - the triangular shape provides that and the foam supports the shear web in buckling and this requires very little if any glue.



John
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