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Old May 26, 2013, 06:27 AM
Arrarrar!
josh18's Avatar
Australia, NSW, Wagga Wagga
Joined Jan 2010
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Cool I like it!
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Old Oct 07, 2014, 02:47 AM
PB of 733kph
ALEX HEWSON's Avatar
Christchurch, New Zealand
Joined Jul 2005
5,467 Posts
How have the PU cored wings been holding up over time???
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Old Oct 07, 2014, 05:29 AM
AvB
Wind, hill, ... I'm keen ...
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Australia, QLD, Woody Point
Joined Nov 2006
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Funny you should ask because I've been thinking about this subject in the last day too and have been chatting to Tim, Shane, Kev and Brent about it. Probably because the new curvy Scratcho mould is sitting there and after making the first wing recently, I'd now really like to make a light PU wing from it.

Long story short, the PU wing I made early last year is astoundingly successful. It's done huge amounts of flying and has been my "go to" plane for most light to moderate sessions. There probably hasn't been a flying day over the last year that it hasn't been flown. Love it and haven't had to do any repairs to it at all. I've been flying it heaps at Shorncliffe yesterday and today and absolutely love it. It always turns heads - very agile and fast.

It's not a DS wing - with only a 100gm glass/ 100gm carbon skin, no spar. It's a bit bendy, but for fast frontside and mild DS it's fantastic. The edges which this time were bonded with a thin bead of Techniglue are rock solid and the TE is fine. No splitting of the skin as happened with the first version.

I haven't made any others since that one, so can't give more feedback. But I reckon it's a no brainer ... an insanely effective and quick way to make a wing that is pretty light, more durable than hollow mouldies, and will give a long life and a lot of bang for the buck.

I'd like to find a foam that has a longer working time, but this might not be possible. Tim has been researching foams in the background for quite some time and it's very complex. Lots of variables.
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Old Oct 07, 2014, 05:34 AM
PB of 733kph
ALEX HEWSON's Avatar
Christchurch, New Zealand
Joined Jul 2005
5,467 Posts
From reading back over the thread, it is the fatest way to make a robust wing from molds. I would love to make a heap fro my mold for the locals to fly frontside at places like Cass, maybe get some slope racing going again etc.

YOu still using the HXT900s on ailerons etc???

Will have to make a wing if you come over here. Would really really love the advice and to do it with you for the first time
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Old Oct 07, 2014, 05:42 AM
AvB
Wind, hill, ... I'm keen ...
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Australia, QLD, Woody Point
Joined Nov 2006
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I've only made 2 successful wings with the PU. The first one was super dooper light and didn't use the bead of glue around the edges. It had the HXT's for aileron and $7 MG14 metal geared servos - none of which ever had to be replaced! It was a pretty rough plane - I had to keep glueing and taping the edges together where they split apart ... but man I got so many hours of flying out of that thing, repaired many times. After a recent break I decided to pension it off so it's in the cupboard but honestly with a bit of tape and CA it would fly again! It looks terrible. (that's the one that we hacked the tips crudely to a curve).

The second one, the one I fly now, uses better servos ... can't remember but I think it's the 555's for ailerons and Bluebird BMS 385Dmax for flap (best cheapish combination for the Scratcho).

Every stab and fin I've made has been filled with the PU since I learned the technique, except for the AB fin. It just saves so much work for smaller parts, and they're so strong and easy to work with, it's a no brainer.

However I do have to warn that when you start to talk about "foaming PU's" you have to be wary ... there are hundreds of variations all with vastly different properties. The one I've used is the bog standard basic cheap expanding foam used to fill boat cavities etc. The one I use is Erapol GP330. It expands to a max of 33 times in ideal conditions, but I only give it 11 or 12 times volume to expand, so it's highly compressed/ back pressured which gives it a much denser firmer consistency than the raw foam allowed to expand fully. It's critical that it's spread over the whole surfaces very very fast, so you need 4 people to do it properly, but you only need them for a few minutes! Once the mould is closed, you can pop the part out in a couple of hours. Theoretically you can do the whole wing from paint to pop out in a day.
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Old Oct 07, 2014, 06:01 AM
ETS....Energy retention system
timbuck's Avatar
gold coast australia
Joined Aug 2008
5,504 Posts
I still think using the heavier foam , letting it expand in both halves and cutting it off at the parting plane , one layer of glass through the centre and joining it is the most simple and more repeatable and easy ways of doing PU.
No rush , no fuss type build. You can do 1/4 of a wing at a time or each half etc.
also this really takes away any problems with gassing out , which some PU's do as they are cross linking over time.
Also this ensures to get a perfect skin coverage every time. As you basically make to much and let it over fill each half.
I found covering the parting plane with thick wide electrical tape saved it from any scratches when slicing the foam to the parting with a general wood saw. You slide it through the foam more than saw it , if you know what I mean.
For the average bloke in his shed this way really does make it easy and it works every time , due to there is no much to go wrong. And if it does , just scrape it out and do the foam again. Joining is a breeze. Pull about 10mm of foam around the LE and about 5mm off the TE both sides , full with splooge , add a piece of 100g glass or carbon over one side so it lays onto the flange , and join it.
It does take a little longer , but not really much at all. And 99% of guys on here if they had a mould would make a perfect wing every time , one shed , one mould , one guy.
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Old Oct 07, 2014, 06:46 AM
Themadartist
Australia, QLD, Narangba
Joined Nov 2012
235 Posts
Very interesting discussion guys.

Just wondering Tim, will the expanded foam cut with a hotwire or does it need to be a 'mechanical' separation ???

Cheers, Steve.
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Old Oct 07, 2014, 06:50 AM
ETS....Energy retention system
timbuck's Avatar
gold coast australia
Joined Aug 2008
5,504 Posts
Turns to glue if cut with hot wire
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Old Oct 07, 2014, 01:30 PM
Building with TLAR precision
Zenmaniac's Avatar
near Madison, WI
Joined Jan 2009
2,666 Posts
Somewhat related foamed tail

Some great stuff on this thread, thanks for starting it AvB!

So I'm starting a project to make a molded Orca pitcheron -- or I'm well into it. The mold is made, and I've been thinking about someway to slow the plane down. I've decided on a split rudder mechanism, but since the Orca has a "T-tail", I've been thinking on what to do to give the vertical stabilizer some internal support and still allow for a split rudder.

I decided to make a foamed internal support foundation for the V-stab, using some 3lb PU. This will be pre-cast, and then placed in the mold when the fuze is being made, and not foamed-in-place during the build.

I made a second mold of just the tail section, and the first plan was to anchor a 0.8mm fiberglass sheet inside the mold, and let the foam surround it. Turned out that if there was unequal distribution of the PU in the mold, it just pushed the FG sheet wherever it felt like. Didn't work. Plus, despite leaving the top open for overflow, it seemed that the top hardened first, and continued expansion of the foam expanded the mold besides.

Trial #2, without the FG sheet. I think will work fine. I made some expansion holes in the side of the mold, though not too much foam blew through. I may cut in a 1/2" FG strip, glue it into the TE of the foam, and cut out some of the foam -- the current weight of the foam is 1.8 oz -- that's going to require a bunch of nose weight.

Comments, ideas, before I start the first fuze?

-= Dave
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Old Oct 07, 2014, 04:05 PM
supreme being of leisure
ZAGNUT's Avatar
Tel Aviv, Israel
Joined Jul 2004
2,438 Posts
used to be a manufacturer here that made their balsa sheeted foam core wings with this method. somehow they would hold the skins in the closed mold and then add in the foam. once in a while a sheet of the skin would be too wide or out of place so there would be an overlap between it and the adjacent sheet....the foam pressed it all into place just like overlapped veneers in plywood. it would leave a tiny gap on the outer surface but the covering would hide it just fine.
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Old Oct 07, 2014, 10:16 PM
Twisted and Confused
flyonline's Avatar
Joined May 2003
4,328 Posts
Don't bother with a split fin on the first one, just build, fly and enjoy. Mine was a real joy to fly and the easiest of any TWF I've ever flown(ed) and landed. Just make sure you've got a good LZ for the first few times or practice landing without any aids beforehand

If you still want a split rudder, I'd lay up a normal tail, trim it to size of the rudder and tack them to the inside of the mold with double sided sticky/3m77 (same as for a canopy insert). Wax/PVA well to fill all gaps and over the top of the insert then lay up another tail over the top. All going well it will be a near airtight fit.
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Old Oct 07, 2014, 11:58 PM
Building with TLAR precision
Zenmaniac's Avatar
near Madison, WI
Joined Jan 2009
2,666 Posts
Quote:
Originally Posted by flyonline View Post
Don't bother with a split fin on the first one, just build, fly and enjoy. Mine was a real joy to fly and the easiest of any TWF I've ever flown(ed) and landed. Just make sure you've got a good LZ for the first few times or practice landing without any aids beforehand
Yeah... there's the rub. There aren't that many choices for "good" slope sites around here -- some hills for foamies and such, and the one GOOD hill within 100 miles has a very limited LZ which is littered with antennas and dishes . See below.

I can land my Carbon Bird there with good crow control, but I fear I will need some way to dirty the aerodynamics of the Orca.

I like your idea of the "insert"... maybe on v.2 ... or maybe this one...

-= Dave
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Old Oct 08, 2014, 06:12 AM
Arrarrar!
josh18's Avatar
Australia, NSW, Wagga Wagga
Joined Jan 2010
3,392 Posts
Hey, I did a couple of wings in the method Tim described, but used no glass in between, just PU glued together. This worked great and was the best combination of foamy and mouldy I have made. Only issue is that I hacked a bit of my mould flange with the saw when cutting before I learned how to be careful! Its a great, quick method
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Old Oct 11, 2014, 04:06 AM
AvB
Wind, hill, ... I'm keen ...
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Australia, QLD, Woody Point
Joined Nov 2006
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Report related to PU Scratcho here: http://www.rcgroups.com/forums/showt...660287&page=29
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