Foam cored wings...

[Jim (Chairman)]

All,

Does anyone have any experience of hot wire cutting foam cores for balsa skinned wings?

Got some insulation foam, and some ni-chrome hot wire, so I thought I might give it a try...

Jim

[DannyF]

Hi Jim, not something I have ever got around to trying, I have meant to.
The one thing I do know is that some foams (polystyrene is okay) give of toxic fumes when cutting so be careful!

Cheers
Danny

[Jim (Chairman)]

Had a go last night starting to make some foam cored wings.

Cut two pairs of light ply root and tip ribs.

Cut two blocks of Celotex insulation foam. Checked the manufacturers product info and there doesn't seem to be any toxic chemical or fume issues.

"Bored" a wing spar tube (6mm OD carbon) through the two blocks:



Used a battery drill, little height block and some marks on the bench to keep the tube running in straight. Worked OK.

Glued the root and tip ribs to either side:



Both blocks uncut weight 325 grams!



So I guess they will be down to about 100 grams for the pair once they are cut out.

I will have a go at hotwiring the cores out of the blocks tonight.

BTW these "wings" are 425mm long each so that would produce a span of about 850mm or 34". The aerofoil profile I used came from a software package called "Profili 2" and the profile is known as "12% Joukowski" which looks very similar to an Extra 300 wing profile I found on some plans.

Maybe I'll end up building an Extra 300 34" fuz...?!

Updates to follow...

[DannyF]

Looks good Jim, what are you using as a power source for the "bow"? I have a LiPo charger that has a hotwire setting, not that I have explored it.

Cheers
Danny

[Jim (Chairman)]

Danny,

I use a 0-35 volts 5 amp bench PSU.

Using the ni-chrome wire I have which is 32 swg / 0.274mm diameter I need a little over 2 amps at 5 volts if I remember correctly. That's for a 30 cm length.

Nichrome Wire from Ebay: http://www.ebay.co.uk/itm/121174932501?_trksid=p2059210.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT

To end these cores I will use about 1 metre of wire, pinned down to the bench at one end only. The loose end will have a dowel handle.

Left hand hold the foam with the tip end towards the fixed wire end. Right hand guides the wire around the two ribs / templates and keeps tension.

Seen it done on YouTube to great effect. I might make a mess of it but the foam is cheap and it only took an hour last night to set up the "experiment"...

Also, once the cores are (rough) cut they can be VERY easily shaped / smoothed using very light sanding blocks or foam sanding pads.

If I manage to get two wings that LOOK like they might be useable, I will try to skin them in very thin balsa, cut two a servo hole in each (9g servos) and cut the ailerons out...

Great! Then I've got to make a fuz!

PS: Here's the foam board from Wickes: http://www.wickes.co.uk/Celotex-50mm-High-Performance-Cavity-Wall-Board-450x1200mm/p/190546

The paper backed foil covering peals off really easily.

[Jim (Chairman)]

Danny,

Checked my little table frame hotwire last night and the wire is about 25cm long and I run it at 12 volts 2.25 amps.

That cuts soft padding foam to go inside your TX / flight cases.

Just for your reference...

[Jim (Chairman)]

Tried to cut the wing cores last night.

DISASTER! 

Things did not go well.

Using about 90cm of Ni-chrome wire running at 25 volts 4 amps. At that voltage / current the wire in free air is not quite glowing.

Tested on small bits of Celotex insulation board and all seems well. Testing using digital readout soldering iron, Celotex seems to melt about around 225 - 250 degrees C.

Went for a cut on my 425mm wing sections and that's where it all goes wrong.

Tried to go in along the leading edge, hot wire cuts in about 5mm and then just stops!

Physics, Jim, physics!!!

The material is an engineered insulator! As soon as it melts onto the hot wire, it totally insulates the wire from the rest of the foam!

There is no way to use as hot ware to cut this stuff, so I gave up and am now shaping them by hand. To get a rough shape sanded into a wing took about 1 hours but makes A LOT of dust!

When I get back from Poland (next week) I will repeat the experiment, but this time using EPP that we know will cut cleanly with a hot wire...