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Old Oct 18, 2012, 09:51 AM
Caution:Makes sharp left turns
Troy's Avatar
United States, CA, Lake Forest
Joined Feb 1999
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Glad to hear you're back on it. I had to pause on my wing for a couple weeks for side projects but should be finishing it up in the next couple weeks. I was really impressed with Pdawg's wing project and we have another small pylon wing for future molds and foam experiments.
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Old Oct 19, 2012, 12:26 AM
AvB
Wind, hill, ... I'm keen ...
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Australia, QLD, Woody Point
Joined Nov 2006
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Ta-dah ... my first-attempt, expanded in closed mould, PU filled wing and tails are now built up into the lightest Scratcho I've ever made. AUW is 869gm, for a 64" (160cm) span. Some description and pics here.

http://www.rcgroups.com/forums/showt...3#post23041823
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Old Oct 19, 2012, 12:48 AM
Entropy is happening!
Jim.Thompson's Avatar
Australia, NSW, Bellingen
Joined Aug 2008
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Quote:
Originally Posted by AvB View Post
Ta-dah ... my first-attempt, expanded in closed mould, PU filled wing and tails are now built up into the lightest Scratcho I've ever made. AUW is 869gm, for a 64" (160cm) span. Some description and pics here.

http://www.rcgroups.com/forums/showt...3#post23041823
That is impressively light Andrew! I'll be trying to achieve something less than 1000gram using a similar method for my developing Wompoo fully moulded plane.


Jim.
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Last edited by Jim.Thompson; Oct 19, 2012 at 06:34 PM. Reason: question removed.
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Old Oct 19, 2012, 03:19 AM
AvB
Wind, hill, ... I'm keen ...
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Australia, QLD, Woody Point
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Read back to the start of this thread Jim.
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Old Oct 19, 2012, 04:16 AM
Entropy is happening!
Jim.Thompson's Avatar
Australia, NSW, Bellingen
Joined Aug 2008
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Originally Posted by AvB View Post
Read back to the start of this thread Jim.
Ok Andrew. I'll read it all again.
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Latest blog entry: Yet another light plank!
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Old Oct 20, 2012, 09:20 PM
AvB
Wind, hill, ... I'm keen ...
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Australia, QLD, Woody Point
Joined Nov 2006
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I have laid up a Scratcho stab and fin and just put the GP330 PU in but the weather is hot here now so it was a different experience to last time! The working time was shorter so by the time I had spread 12 ml of mixed liquid on one mould side it was beginning to thicken up and foaming a bit and of course smearing it around deflated it. So I mixed a few ml more a d added it in. Judging by the amount that squirted out of the flanges once I'd clamped the mould up I don't think there was any shortage. For the fin I did a half mix for each side which worked well.
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Old Oct 21, 2012, 04:10 AM
AvB
Wind, hill, ... I'm keen ...
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Australia, QLD, Woody Point
Joined Nov 2006
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I maidened the very light Scratcho with the PU wing today. Short report: http://www.rcgroups.com/forums/showt...1660287&page=4

One further comment on PU construction: I unfortunately had a disaster with paint sticking in my tail moulds today. That's another story, but in pulling the moulds apart it struck me just how easily the parts split and ripped open. The skins stayed stuck in their moulds and the PU ripped. I think likely that parts joined only with PU will tend to bust apart more on hard impacts, compared to the ones I've been making with blue foam cores and Ampreg edges. I think I'll be sticking with the latter technique for "good" builds, but the PU method is just so dang quick and easy I can see myself using it for things I'm not going to care as much about.
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Old Oct 21, 2012, 03:39 PM
Caution:Makes sharp left turns
Troy's Avatar
United States, CA, Lake Forest
Joined Feb 1999
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AvB, I don't recall from your posts but how long after laying up your skins did you pour foam?
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Old Oct 21, 2012, 05:28 PM
AvB
Wind, hill, ... I'm keen ...
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Australia, QLD, Woody Point
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About 24 hours. Tear off the bagging stuff and trim the flanges with a very sharp paint scraper/ broadknife when they're hard, but still a bit green. I'm using LC3600 and it's hot weather, so that's at perhaps 6 or 8 hours, optimally. Then leave them till 24 hours at least to add the PU and clamp.
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Old Oct 26, 2012, 02:46 AM
AvB
Wind, hill, ... I'm keen ...
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Australia, QLD, Woody Point
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I was flying my light PU wing Scratcho at Shorncliffe today, and had a mysterious lockout and it crashed down onto the rocks at the bottom of the cliff. I'd been having a great time with it till then and don't know why it didn't respond, but I have to confess that I was holding the TX in a way that might have pointed the aerial axially at the plane. Not sure.

Anyway, it was good for the durability research. The crash made a big mess of the outer third of one wing ... the skins are opened up right around the LE, tip and TE and peeled like a banana right off the brittle polyurethane foam. Bits of foam broke out freely.
I have to say that the PU foam fill technique (with light foam anyway) is absolutely nowhere near as durable as the blue foam core + ampreg around the edges. My other SCratchos would not have suffered such extensive damage. But then, on the other hand, it would have taken about 4 times longer to make the wing too.
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Old Oct 26, 2012, 03:51 PM
PB of 733kph
ALEX HEWSON's Avatar
Christchurch, New Zealand
Joined Jul 2005
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AH bummer! Sorry to hear Andrew
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Old Oct 26, 2012, 05:46 PM
AvB
Wind, hill, ... I'm keen ...
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Australia, QLD, Woody Point
Joined Nov 2006
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Yes thinking about this again I'm loathe to drop the technique entirely, because it's so fast, and the wing's other properties are great - particularly stiffness, weight and workability. What we need to do is work out a way to lock the skins together at the periphery.

I could try putting a bead of splooge, gelcoat or Ampreg all the way round. My concerns are:
1) if it becomes airtight, the expanding foam won't be able to push out the edges and this may lead to voids? But if I let it expand up to the level of the flange before closing the mould, this shouldn't be an issue.
2) The pressure from the PU might push the goo out of the edges.

Only one way to find out!
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Old Oct 26, 2012, 06:11 PM
Composites guy
North OC, Ca.
Joined Jun 2005
1,182 Posts
We typically put a light glass( .75 oz) or a polyester spunbond material( think Bounce dryer sheet) into our adhesives to improve flow control and to add bond line thickness control.

May work in your application as well,
Scott
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Old Oct 26, 2012, 06:19 PM
Composites guy
North OC, Ca.
Joined Jun 2005
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Try a light glass in the bond line.
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Old Oct 31, 2012, 08:13 PM
Keep it Simple Stupid
razortoe's Avatar
United States, NC, Raleigh
Joined Jul 2011
244 Posts
I'm reminded of advice I read elsewhere, "Build to fly, don't build to crash". I think the build-speed + "strong enough" characteristics here lend themselves to simply having a spare or two on hand You've come really far developing/learning/creating with this technique, with mastery short at hand. No tool or material is ever going to be perfect for every possible use or application. Don't worry so much about the shortcomings, instead focus on perfecting/enhancing the good qualities, and it'll take you much further.


(while providing inspiration and fascinating reading material for us mortals)
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