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Old Mar 04, 2013, 10:40 AM
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Knoll53's Avatar
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­Composites – Where learning never ends !

­The bottom skin was laid up in one session using a separate mylar for each side. Splice reinf. was added to the foam core at the root joint with 3M77, wet out and mylar lay ups laid on top. The epoxy started to become quite sticky during the lay up.

BTW, very happy with the new epoxy from US Composites (3:1mix). I had a small cup left over which cured very quick. To test the new epoxy, I placed this 1 cubic inch sample of epoxy in a big vice to see how much force was needed to smash it. Amazingly, I could not smash it in the vice. I did not use a cheater bar. Then I took a cold chisel to it while in the vice. Just little chips came off. Finally I put the sample on the conc. floor and demolished it with the cold chisel. Wow. This is unreinforced epoxy. Just think what it will do with carbon tow !
­
­The lay up went pretty much as planned (and there was a lot of planning) except for the following:
­
  • The yellow paint did not cover or hide the reinf. and the black Sharpie marks as well as hoped. It's kinda like having X-Ray vision.
  • ­The homemade vacuum bag apparently had a small leak and only 15 lbs. of pull was achieved. The most obvious result was that the mylars did not pull down around the curvy LE as normal. This will create some added work at the LE. Not the end of the world.
­
­Considering the vacuum leak, very happy with the result. I'm always amazed at how well epoxy removes paint from waxed mylars. But, then again, I'm easily entertained.

Kent

Plank 2.5 - Wing layup - bottom (6 min 26 sec)




This is the sound I live for.....mylars coming off.

mylar removal (0 min 55 sec)
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Old Mar 04, 2013, 05:44 PM
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T.D.'s Avatar
Vancouver, Canada
Joined Jan 2008
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Looks good Kent.

Something you might try on the leading edges is to bring the mylars to a point where they are almost touching at the LE and when you lay up stagger the cloth back a few millimetres from the LE of the mylar.

Use the usual bias wrap on the foam core LE and when you bag the wing you will achieve an accurate leading edge with very little finish work required.

To hold the mylars in perfect position I push short thumb tacks through the mylar/layup into the foam core near the root/tip LE.


Tony
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Old Mar 04, 2013, 06:26 PM
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Knoll53's Avatar
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Positioning the mylars IS a sloppy mess. I trimmed the glass to the edge of the mylar. Can't imagine how you could have the glass come up short of the mylar edge. What's the trick?

The thumb tack idea would help a lot. I guess that would be a well waxed thumb tack. But even the thumb tacks wouldn't have helped this lay up at only 15 lbs. vacuum.

Thanks for the tip.

Kent
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Old Mar 05, 2013, 09:36 PM
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Spar Groove

With the bottom skin on, the spar groove can be cut without separating the cores. The knife cannot be too sharp for this task.

Kent
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Old Mar 08, 2013, 06:23 PM
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Wire hole

So I COULD HAVE burned a wire hole a couple of weeks back using Phil Barnes method but nooooo..... I forgot.

Plan B worked out quite good. Used a special wire tool on a soldering iron and straight edge. The little groove will be filled with a balsa strip and sanded smooth.

Kent

wire groove (0 min 0 sec)
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Old Mar 09, 2013, 02:30 AM
I don't like your altitude
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http://woodlandscenics.woodlandsceni...ow/item/st1447
One of the guys on s/b edf's has been using this as a filler etc.Might be ok for fillets and the like.I'm going to try some,see how it performs.
Neat job with the servo wire slot,I need to buy one of those irons.
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Old Mar 09, 2013, 12:03 PM
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UK, Coventry
Joined Aug 2005
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Well your build is going well. Work and family have got in the way of mine. The drawing is finished, just need to add the details and notes. We also have a cold spell on the way which puts starting back a little longer as I don't last long in a cold workshop. Might get the templates started as I could do those in the house.
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Old Mar 09, 2013, 01:01 PM
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Yes, here in California, we are currently suffering thought daytime temps in the mid 50s (F). So in the UK, thats got to be low 40s.

I think that if I lived away from the coast where winter actually happens, I'd be the first to kill two birds with one stone and turn my work shop into a near sauna with some sort of fireplace. Homey and warm....that would be nice.

The only time I heat my workshop is when I build a "oven" for an epoxy cure.

Kent
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Old Mar 09, 2013, 01:13 PM
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Around 0 Celsius, I never quite got fareheit. We are supposed to have snow on the way. Not much but it is still blooming cold. The irony is in summer the shop gets too hot at times. At least it gives me time to list and order what I need, and another reason I don't do much composite stuff like yours.

Martin
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Old Mar 09, 2013, 09:02 PM
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I was thinking of using lightweight filler for the little groove at the wire hole Stuart, but this area is highly loaded and I'm already not too happy about having a big hole there. I choose to fill the gap at the surface with balsa set with a thin layer of PU glue. Very strong. The PU glue sands nicely, fills little gaps and is stronger than the foam. Also, if I had used the light weight filler, instead of the balsa, I'd probably end up filling the entire hole with filler and I'm yet to pull the wires, so I need that hole.

Should be ready for the top skin in a few days.

Kent
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Old Mar 10, 2013, 03:31 AM
I don't like your altitude
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Yes,good point about the strength/ clear channel.I'm hoping this stuff will be a bit more "structural" than lt filler,we'll see.
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Old Mar 10, 2013, 01:50 PM
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Fish string

I casually taped the fish string for the servo wires in place.....by the time I was done with fiddling with the wing, one of the fish strings had gotten pulled inside the wire hole....out of reach. Actually it got caught on something and the whole string was laying on the floor.

Happily I'm a construction worker and I've seen this one before. I just stuffed the end of the string into one end of the wire hole (at the root rib) and put a shop vac at the other end and sucked the string thru the wire hole. Works on electrical conduit too.

Now I've got a more solid anchor at each end of the fish string.

Kent
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Old Mar 12, 2013, 10:39 AM
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Heidelberg
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Looking nice Kent!

Some while ago they prohibeted selling PU to the general public here in Europe because of some cancer producing component. Anyway, people like to use here in Germany a mixture of 24h epoxy and micro-balloons. Though I have not tested it myself, I've read often that this mixture fills very well empty spaces, is lightweight and is sanded easily. Some use it even as a filler .

Cheers
Andrés
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Old Mar 12, 2013, 11:56 AM
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Sorry to hear about the restriction on PU glue. Thanks for the tip on health concerns. I always wear gloves when handling it and a respiratory when sanding it. Here is the US, corporations rule the government so we would never get such a warning.....it would hurt sales...

Epoxy and microballoons is my least favorite glue/filler because it
  • lacks adhesion
  • is brittle and weak
One hard landing and all the microballoons shatter or simply de-bond.

PU glue has
  • fantastic adhesion
  • is expanding and gap filling
  • light weight
  • relatively tough and strong
  • and sandable like balsa.
If you land so hard to break the PU glue, then you've already got bigger problems.

Kent
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Old Mar 12, 2013, 12:02 PM
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I get it: PU is elastic while Epoxy is hard and brittle. Makes sense!

Andrés
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