How to cut a good foam wing

[Richard Boyd]

Yesterday evening I embarked on a mission to try and cut a thin airfoil wing for a wee glider I have in mind to experiment with.
The span on the wing is 1.9M, this is dictated as thats twice the lenght of my bow.
The material I am cutting is styro foam.
I have a tranformer with 2 heat settings,

Now reality !
I cut quite abit of foam and have not yet succeeded in producing a wing !
The wire is dragging when I am cutting, it is a problem for me to have both arms working at the same speed !
I donot want to spend anymore money on this project

Any tips ?

Richard

[Ron]

No tips Richard.
But I'm also very interested in cutting foam, and have been put off so far by just the sort of thing you are describing.

Any ideas anyone  :?

Ron

[Richard Boyd]

Hi Ron, we can both learn together !
I have only a small garden shed to cut foam and it also houses the lawnmower etc.
I donot want to be building a monster machine to cut the odd wing lol
Simple/stupid for me please.

Richard

[Richard Boyd]

Hi Ron, I spent a few minutes searching and found this video, its perfect for me !

I am of to the garden shed to burn some foam.

 [youtube]http://www.youtube.com/v/ckxnNRSM3YE[/youtube]

[Ron]

Hi Richard,

That video is excellent.
Like to see what results you get.

Ron

[billscottni]

If it's dragging Richard it's one of two things.

Not enough heat

Moving the bow too fast.

I have a couple of video's on Youtube of the set up I use currently, or give me a ring

[Aidan]

It's a long time since I hot wired foam wings. We used to do it as a two man operation. We'd mark graduations along the templates say at 20mm intervals (closer at the LE) and call out the numbers as we went along to make sure we were in sync. I tried doing them solo a couple of times but found it very difficult to get a good cut this way. I'd agree with Bill's diagnosis. Dragging is caused by going too fast for the heat available. Getting the speed right is the hardest bit, especially for wings with lots of taper.

[billscottni]

Try these:
[youtube]http://www.youtube.com/v/kb3Bz7IThkw[/youtube]

[youtube]http://www.youtube.com/v/oVvE9CUALcQ[/youtube]

Good info here: http://www.charlesriverrc.org/articles_foam_vac.htm

[Richard Boyd]

Hi Bill thanks for the pointers.
Ron, cutting these wings aint  that easy.
I have had results with small airfoil wings for my indoor planes.
Yet for gliders I have had problems.
The taper in the wing means the wing tip is burning !
I guess its down to the wire traveling less distance than the root of the wing, hence the burning occurs.
One thing , the foam supplier is a happy man , profits are up !
I am going to change my wire thickness tomorrow and see if the results change any.
 
Richard
if one never tries , one never knows

[Ron]

Hi Richard,

Ah, that's one thing I hadn't thought of, the wire moving more slowly at the wing tip and burning the foam.
The wing tip distance could easily be half the root, so there's a 2:1 speed difference straight away.
Hmmmmm.....
Can't think of a way around that.

We need a wire that's 1.4 times the cross sectional area at one end from the other, that way the resistance would be 0.707 of the other end, and the power would be halved.
Think I've got that right :?:

But back to the real world...
Nope, still can't think of a solution :x

Ron

[Aidan]

Hi Richard,

Ah, that's one thing I hadn't thought of, the wire moving more slowly at the wing tip and burning the foam.
The wing tip distance could easily be half the root, so there's a 2:1 speed difference straight away.
Hmmmmm.....
Can't think of a way around that...

You're going to get a little more melt at the tip one way or the other. The only way to eliminate that is to leave the template at the tip a fraction oversize. The big problem I remember having is pacing the small end so that you didn't get ahead or fall behind the pace of the big end. When doing it with two people the guy at the long end sets the pace by the feel of the resistance. If the graduations at the long end are 20mm apart and the taper is 2:1 then the graduations at the tip would be 10mm apart. As long as you hit the corresponding graduations together the wing should be the correct shape. The short end is the difficult one. It's difficult to maintain a steady slow pace with no real resistance. If the templates aren't good enough you can also find the wire snagging and then jumping on when it comes free.
Difficult to do well with two people. Extremely difficult, in fact nearly impossible, with one.

Aidan

[billscottni]

if it's an extreme taper, you may be better off with a single point cutter, see the video I posted earlier.
One end is fixed and the foam blank is positioned along the wire so that the two ends exit the foam at the same time

[Aidan]

if it's an extreme taper, you may be better off with a single point cutter, see the video I posted earlier.
One end is fixed and the foam blank is positioned along the wire so that the two ends exit the foam at the same time

Doesn't work if you want washout or transitioning aerofoils though!

[Richard Boyd]

Hi Richard,

Ah, that's one thing I hadn't thought of, the wire moving more slowly at the wing tip and burning the foam.
The wing tip distance could easily be half the root, so there's a 2:1 speed difference straight away.
Hmmmmm.....
Can't think of a way around that.

We need a wire that's 1.4 times the cross sectional area at one end from the other, that way the resistance would be 0.707 of the other end, and the power would be halved.
Think I've got that right :?:

But back to the real world...
Nope, still can't think of a solution :x

Ron

Hi Ron, for small wings that have depth of profile the cutting was pretty successful !
The wing profile for my glider has a very thin profile at the root 20mm and then the idea was to taper it to 4mm at the tip.
I have been up extremely early and have tried all the different gauges of wire that I have in the workshop, the results stay the same, melted foam.
The results are not what I wanted.
For this style of wing I think I would have been better spending my time cutting the ribs and building a traditional wing.

Bill and Aiden , thanks for the help and insight into cutting winds.
One point on the cutting method I was using with the anchor point ,
I needed a long wire to cut the wing as the further away from the anchor point the wider the wing tip is cut !
Aiden I was using only one templete as per video I posted earlier.
I thought this method would be the simplest for my pea brain, yet it has lots of pit falls.

Bill I may take you up on that offer after all.

Richard
leave it up to the experts