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polyurethane sailplane

#1
Hey new member and literally just learning the very basics, I haven't spent any money so far on my first project because I already had the materials laying around. I have shaped a rough glider shape minus the wings so far but my material was polyurethane insulating boards kinda the same stuff they make some surfboards out of, I was wondering if anyone has tried using this befor and if so does anyone know a good way to seal this foam, I have stumbled across the idea of using epoxy resin, would this be any good or am I on the wrong path completely.
 

KFC

HodgeHound Mercenary
#2
I don't know much about surfboards, but if the foam is porous, it will soak up epoxy resin like a sponge and your glider will gain a huge amount of weight very fast.
 
#3
Polyurethane is pretty much waterproof as its used alot for insulation but the dust on the survice after sanding is quite thick is epoxy 'resin' used for rc aircraft or is that too heavy.
 
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xuzme720

Dedicated foam bender
Mentor
#4
You'd have to keep the resin layer VERY thin as it builds weight fast.

Try using a blower nozzle on a compressor, or canned air if that's all you have, to blow off the foam after sanding and before covering. Another possible covering option for lighter weight is tape. You'd need to test a few types to make sure you have one that will stick to the foam you're using but with a tiny bit of tension it can strengthen the foam remarkably. Common types for covering are the colored packing tapes or even the thicker clear packing tape. Reinforced tape has fiber running through it for added strength but it's also heavier...
Sometimes a mix is the best option, reinforced tape on the leading edges and regular packing tape covering the rest...If you can find some that had a good bond to your foam.
 

xuzme720

Dedicated foam bender
Mentor
#6
After seeing the lines of your parts so far, I doubt tape will work very well for you as a covering. You may have to resort to very light cloth and resin. I know one method to keep it from building up too much is to prewet the cloth on some wax paper or plastic before applying the cloth to the airframe so the soaking up by the foam is kept to a minimum.
 

xuzme720

Dedicated foam bender
Mentor
#9
I should have specified Fiberglass cloth above...Although silk and dope has also been used as a lightweight covering material.
 

xuzme720

Dedicated foam bender
Mentor
#11
Make sure you test anything you might use on a scrap piece of foam as many chemicals can do funky things to foam...
 
#12
Cool, thankyou. Do you know of any flitetest videos or others of how to finish/seal using these methods would be cool to watch how others do it
 

xuzme720

Dedicated foam bender
Mentor
#13
Flitetest doesn't have any that I remember but a quick search of the Tube found this.
.
Many others related to it came up on recommended if you click the "watch on YouTube" button...
 
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rcspaceflight

creator of virtual planes
#14
Plastic Dip spray might work too. It'll leave a thin latex coating on your plane. I think all that you'd really need is a thin material that has high tensile strength. Basically so the foam doesn't chunk off in crashes. Something just to keep it together. NOVA, the PBS TV series, had an episode that showed how taking foam and putting plastic around it improves it's strength significantly. The two materials work together to compensate for their weakness.

But whatever method you use, make sure you test it out on a test piece first. Spraying on Plastic Dip may melt or warp the foam.
 
#15
Polyurethane is a bit of a strange foam to work with for stuff like this and I have tried some strange things with it so far to make it stiff or almost rigid, I started by buying a polyurethane varnish (thinking it was 'for' polyurethane) turned out its for wood and other craft stuff I have tried super pva, and pva with single layers of toilet paper... yes toilet paper, the pva on its own seemed to make the outer layer of polyurethane softer rather than harder, and the idea of toilet paper come from using it with super glue in previous fixes it gives the glue some more texture(theres probably a more technical term for that) and hardens like mad but a plastic film does sound like the best option just need to find the right film to use.....and im rambling but thanks
 

rcspaceflight

creator of virtual planes
#16
I had a random thought, what if you put the plane into a plastic grocery bag and then heat shrunk the bag over it. Or use Saran Wrap. I could be wrong, but I think a thin plastic coating will make a world of difference.

Still test it out on a scrap piece. But don't just see if it coats it smoothly, beat on the piece to test it for strength.

I know that wrapping foam board (with the paper removed) in packing tape makes a huge difference. I'm just not sure how much it'll help when the foam is a lot thicker.

I'm curious as to what you used and how well it worked so be sure to keep us posted.
 

Craftydan

Hostage Taker of Quads
Moderator
Mentor
#17
I know that wrapping foam board (with the paper removed) in packing tape makes a huge difference. I'm just not sure how much it'll help when the foam is a lot thicker.
I've used packing tape on stripped, laminated, and sanded Adams foamboard -- the strength is good, but most of that comes from the lamination. the real advantage of the tape is durability. Naked adams foamboard will scratch and dent from looking at it wrong and the tape will absorb that damage. That being said, all my cracks in any wings/airframes appeared along tape seams that overlaped less than 1/8 in.

I have seen reports of people using celophane wrapping paper w/ spray-on contact adheasive with good success, but if you want to go *really* lite, use the grocery store produce bags w/ spray adheasive -- so far the best strength per weight in the microlite coverings, and plenty strong enough if backed by a solid support structure.
 

rcspaceflight

creator of virtual planes
#20
I like how all of us keeps reminding the original poster to test it out on a scrap piece first.

Oh, and be sure to test it out on a scrap piece first.