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Post by jirvin4505 on Feb 8, 2010 2:49:16 GMT 10
This is how I support the fixed end of the bow for cutting deltas etc.. [/size] [/quote] Thanks for the diagrams must try that. I have typically used a cheese head screw driven into a blindnut secured to the bench. I adjust the screw height and rest the bow in the screw slot sean. Yes some more gotchas . However I prefer vanishing over drop bar with tapered cores. The tip panels of your Sg11 Dlg were cut with vanishing technique plus I do the tails feathers the same way (notoriously difficult to cut) @ graememarion. I want to visit your shed looks like you are doing some good stuuf there! Cheers jeff
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Post by graememarion on Feb 8, 2010 14:00:08 GMT 10
Sorry Jeff, not a lot happening at the moment. My wife has Alzheimer's. She has lost her short term memory and panics if she can't physically see me. I first started flying radio controlled models in 1956. This was with a single channel models and a rubber powered escapement. Had to teach myself to build. Had to teach myself to fly. There was no-one to give instruction. Model aircraft have always been a hobby of mine. In those days it was a case of teach yourself and improvise. There was no ARTF. I delivered news papers before school to finance my hobby. I get most pleasure from flying models that I design myself. I have never been competition minded. Couldn't afford it. The top competition models are usually too expensive for me. Call me a pessimist if you wish but I believe that the only models that will never crash are the ones that you leave at home. ____________________________________ I'll grant you that high density foam would have greater resistance to compression than low density foam but I have always thought that the foam was there to give the skin its aerofoil shape. If you want bending and compressive resistance add a spar or two. The most expensive models have moulded wings. Moulded wings are not filled with high density foam they are hollow. Compression resistance is achieved by installing spars in the high stress areas. I was once told that anyone could build a bridge but an engineer would do the job with the least materials. I think that trying to increase the strength of the wing with high density foam might not be the best use of materials. Then again what would I know? Go for it. The fun is in doing your own thing.
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Post by thevon on Feb 11, 2010 21:06:16 GMT 10
Too busy to do anything much this week. Tonight after a day full of visits for a field study we're doing, despite being tired we were determined to squeeze one more in. It was a good case but it took ages and it was almost dark as I backed out of the driveway of their house, feeling pretty drained, and crunched the back corner of my nice new Subaru into a brick fence pillar.
Anyway, I have cut 2 longer bow legs from the other half of the 3/8" steel.
An electrician guy I know who works at the hospital told me ages ago he'd get me a proper regulated vacuum pump. It didn't materialize so I finally built my own setup. This week he came good with the pump! It's nice because you can turn a dial to change the vacuum while it's running. But it runs continuously whereas my system with a regulator (like Shane's) switches off while the vacuum is maintained.
My cabinetmaker kitchen guy Brad (great guy) got me some old, thicker laminex sheet for cutting templates. They're making it thinner and thinner (and sometimes softer) these days. This stuff is 1.3mm and the other stuff I've been using is 0.8mm.
Graeme, I'm no expert but I think it's accepted that the harder foam core does confer more strength. Failures are mostly compression cracks and anything that reduces the ability of the skin to buckle inwards will raise the fail point. In fact the function of the spar in a hollow wing is not to create bend resistance on its own, but to hold the skin up and resist it buckling. I don't read the tech threads much but I understand they say that the hard foam creates an "I Beam" effect over the whole core.
The hollow molded wings are highly accurate and reproducible whereas vac bagging involves many potential inaccuracies. If it was possible to combine the accuracy of a molded skin and a foam core, I reckon I'd pay double. When you hit a hollow skin it can only dissipate the force around the shell, causing widespread delam and other damage. But a vac bagged wing with a hard foam core is incredibly strong. The vac bagged Zippers, my Ditza etc have some big bangs and just keep flying. Damage is often limited to a "ding" in the leading edge, rather than a big "dog bite".
One challenge however is to brace the wing over the servo cutouts. On my Ditza wing and Sean's Berzerk, the only place I've seen a crack is over the servo cutout/ control rod cutout sections.
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Post by thevon on Feb 13, 2010 11:03:06 GMT 10
OK I'm boasting now! Had some success with cutting 60cm cuts with a different wire. I went to BCF and noticed they had a different type of wire - it's Chromium Nickel (is that the same as Nichrome???) and the remarkable thing about it was the much higher breaking strain compared to stainless. More than double the capacity for the same thickness. I got some (it's only $7) and tried it today. I made another bow with the longer legs, and put heaps of tension on the wire - no problem. The wire is much more high tensile/ springy than the others I've used, and is a coppery colour. I also tried the method of hanging the bow to the ceiling rather than using a tracking wheel. And I fitted a cheap old multimeter to the transformer/ switch board, so I can dial the voltage exactly. The DC volt gauge came with the switch when Sean gave it to me - but it doesn't do anything! I made up a couple of rough templates so I could do a test cut on one of the (many) dud beds. I set it up and it cut beautifully. It moves along more smoothly with the hanging bow. I started on about 7 volts and cut it back to 6 part way through, but it didn't seem to make any difference. The wire is so tight that it comes out of the foam completely straight at the trailing edge. On opening it up, the result was excellent. You can see the covering of "hairs" which are a good sign! I'm pretty sure that when I use good templates the finish will be perfect. I wonder if it would be any use using one of those infra-red spot thermometers to work out the optimal wire temperature? Then if you changed the bow you could adjust to the same wire temp without any experimentation.
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Post by jirvin4505 on Feb 13, 2010 11:33:16 GMT 10
OK I'm boasting now! Had some success with cutting 60cm cuts with a different wire. I went to BCF and noticed they had a different type of wire - it's Chromium Nickel (is that the same as Nichrome???) and the remarkable thing about it was the much higher breaking strain compared to stainless. More than double the capacity for the same thickness. I got some (it's only $7) and tried it today. . yahoo Avb. Yes that is one of the good wire. Real stiff and springy. You cuts look good cheers jeff
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Post by thevon on Feb 16, 2010 13:20:33 GMT 10
A bit of progress. I've put 2.4mm fibreglass kite rod along the main leading edges, and also embedded it in narrow trenches on the top and bottom.
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Post by sean on Feb 16, 2010 13:33:46 GMT 10
Nice one Andrew! Looks almost ready to bag?
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Post by graememarion on Feb 16, 2010 20:25:34 GMT 10
Andrew,
When I am about to cut wings I rest a scrap piece of foam on the hot wire.
I then adjust the temperature of the wire. When it is correct, gravity will cause the scrap foam to be cut through at the speed that I want my wings to be cut.
After adjusting the temperature, I pluck the hot wire with a finger nail. The hot wire should sound like a guitar string being plucked. My bows are easy to tune hot.
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Post by thevon on Feb 16, 2010 22:02:20 GMT 10
Aha, thanks Graeme, good tip.
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Post by shane on Feb 16, 2010 23:38:09 GMT 10
Looking good Andrew ;D what Graeme said about the wire temp is good idea also when working out amount of pull on wire, i only have enough to stop the wire pausing from friction on the template, use a gray lead pencil to lube the template also.
Keep going ,Shane.
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Post by Steve on Feb 20, 2010 6:26:10 GMT 10
Andrew
I've been using something similar wire wise and it's cut quite a few cores for me flawlessly so far without breaking. I tried many other wires, but had trouble with breakages, poor cutting etc. until I found the wire in a box of fishing gear my grandad had left for me. I try and go with as much tension as possible and recently built a new bow with a turnbuckle in the middle the same as shown above so I can really crank on the tension, yet let it off when not in use (a problem with the ones with wire arms).
Have you got a digital charger like a swallow/novus etc? I use my swallow on NiCd or NiMh charge and you can dial up the amps you want. It also is a safety catch in case the wire breaks or something shorts out.
I've also found that cooler and slower (generally) gives better cores, though I'm no expert. That also helps with overburn at the tips as well.
Steve
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Post by thevon on Feb 20, 2010 19:05:17 GMT 10
My wing is in the bag! My clever builder friend Chris dropped in yesterday and I showed him the foam cutting etc and he said, hey,let me know when you're gonna do it - I'm interested!
So he spent the whole arvo over here helping me laying it all up and getting it into the bag! Very exciting! I even painted the mylars!
I don't think it's gonna be perfect. Apart from the fact that I used the mangled Innegra cloth that had been chewed up by the pups, we had great trouble getting the masking tape to stick to the mylars, despite multiple wiping clean with acetone and metho. I think the mylar leading edges protrude too much and will be a bit messed up.
Can't wait till tomorrow to check it! How long is safe to wait? Is 12 hours enough?
Thanks Steve. I agree, I have heaps of tension on the chromium nickel wire and it has completely cured the problems. I dial the heat back till it is just cutting, and not much weight on the gravity bar.
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Post by sean on Feb 20, 2010 19:16:25 GMT 10
That's good to hear. Having the mylar a bit forward of the LE shouldn't be a problem, provided it isn't slightly forward of the TE.
12 hours is OK but if you can stand waiting longer definitely do it - the epoxy is still a little soft after 12 hours, meaning it's possible for a warp or bend to develop after you take it out of the bag (though unlikely). 24 hours might be safer and even longer than that seems to give a better finish, for me anyway! Most epoxies keep hardening for around 7 days.
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Post by thevon on Feb 20, 2010 21:38:21 GMT 10
A few photos ... here is Snoopy caught mangling the Innegra and my much valued Clearshield. I sat down at the desk Thursday morning and looked out the window to see this. Wing in beds under vacuum Taking it apart an hour later for a sneak peek. I reckon that leading edge looks good!
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Post by cdsloper on Feb 21, 2010 6:57:14 GMT 10
Looks great Andrew. You certainly did a fantastic job recovering from the first photo. (McGiver eat your heart out ;D) Puppies are great but I'm so glad I got my dog from Animal Welfare League as a six year old so I didn't have to deal with all the training/nappy issues. It also gives you a good feeling inside that you gave some poor unfortunate animal a second chance. I admire you guys doing all this great work cutting cores and laying up your own wings etc. Would love to learn it myself but would like to do it by helping someone who knows what they are doing first. Recent health issues have left me with a lot of chemical alergies so it would be good to expose myself (so to speak) before purchasing and setting up my own equipment.
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