Filling plywood fins with foam

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tomsrocketry - 27 Mar 2007 02:26 GMT

Hi,

I'm getting my 1/2 scale honest john ready for a launch in June and am
trying to finish my fins. I am building the fins to have the signature
airfoil incorporated into them and have run into an expected
problem.The fins are made from a 3/16" core with a 3/4" high spar
running down the center. Their are 6 lightning holes cut out of the
center, 3/16" layer, and there are two, 1/16" skins that get glued on
each side. I used the technique shown on this website here if it
helps.

http://www.vahpr.com/n3_p2.htm

I drilled 4, 1.5" holes on each side of the skins, and planned to use
these holes to fill the fins with 4lb density expanding foam that I
got from mr. fiberglass. When I mixed up the foam, I planned to pour
it in quickly through these holes, then, quickly put a piece of 3/4"
plywood on each side to press against the fin as the foam expanded.

Even after holding it for about a minute, the foam still managed to
bulge out the skin at the rear of the fin.

Does anyone have a suggestion for me as to how to avoid this problem
for my other fins? It is really frustrating, and this is about the 8th
method I have used for trying to re-create these fins.

Any help would be appreciated,

Thanks,

Tom Sak

www.tomsrocketry.com

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Thomas Koszuta - 27 Mar 2007 15:42 GMT

Hot wire cut the foam for the cores first, then add the spars and skins.
I've made wings from 60" to 120" using this technique. The 60" wings were 4
oz. using white foam and 1/32" light balsa skins. Very common technique for
RC aircraft.

I've built my own bow and power supply fairly inexpensively and have created
several wings with them. Vacuum bagging makes the glue requirement very
low, but stacking weights on the cores works well too. Making the templates
is the hardest part. I use formica, but you can use plywood. You can
probably even use your ribs as templates. Just watch for the "kerf" - the
hot wire makes a cut lightly larger than the wire because the foam melts a
bit. Your ribs may need to be sanded a bit and strictly speaking, your
scale would be bit off.

Links to examples and techniques:
http://www.charlesriverrc.org/articles_foam_vac.htm
http://winshipmodels.tripod.com/foam_core_with_spar_2.jpg
http://winshipmodels.tripod.com/foam_wings.htm
http://world.std.com/~racores/RCSD_articles.html#lowcost
http://kirstant.orcon.net.nz/
http://www.dbalsa.com/foam_wings/foamwingmain.htm

Search for "hot wire foam cutting" for more.

Signature

Tom Koszuta
Western New York Sailplane and Electric Flyers
Buffalo, NY

> Hi,
>
[quoted text clipped - 29 lines]
>
> www.tomsrocketry.com

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nitram578 - 28 Mar 2007 02:45 GMT

What I use for fin/wing templates are demostrator/ads from a local carpet
store. They toss these out every so often and just by asking they give them
to me for free. They are a plexglass type material and work for fins by
themselves. For an HJ I built the fin can for two of them glued butted
together and then cut on a scroll/band saw worked great. These are about
18'' W by 24"L and anbout 3/16" thick with whatever type of flooring they
are advertising at the time.

> Hot wire cut the foam for the cores first, then add the spars and skins.
> I've made wings from 60" to 120" using this technique. The 60" wings were
[quoted text clipped - 53 lines]
>>
>> www.tomsrocketry.com

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Jim Yanik - 27 Mar 2007 19:16 GMT

> Hi,
>
[quoted text clipped - 29 lines]
>
> www.tomsrocketry.com

leave the holes unplugged and let the foam expand out of them,and trim it
flush.
You probably used too much foam to begin with.

Signature

Jim Yanik
jyanik
at
kua.net

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Model Forum / General / Rockets / March 2007

Filling plywood fins with foam