COMPUTER FOAM CUTTING:

Presently it would be hard  to win events with wings that are constructed using old techniques of balsa laminated foam cores. 

The latest wings are usually constructed using Carbon Fiber cloth - and Epoxy Resins, which is vacuum bagged in metal moulds that were constructed with CNC milling machines. The laminations used in this process is 3/4 oz Fiber Glass - 1/32" balsa and then 4 oz Fibre Glass.

There are a now a few people who are very successful in constructing blue foam wing moulds instead of metal. These wings are molded in two halves – a top and bottom wing.  Full-length spars are then strategically positioned to strengthen the wings. The molded components are then pieced together and joined to complete the wing.

Anyhow the building techniques have changed from what they were.  And all I know is that the models are getting faster – and the only major change has been these new wings.

So if you want to go fast I suggest that you get hold of one of these new vacuum bagged wings.

 There is  another approach, and that  is to foam cut a core and cover that with carbon fibre and glass cloth.

This method still requires some sub spars to strengthen the center section of the wing –  to eliminate wing crushing in that area.

This methods main disadvantage is it requires  the cutting of new cores for every wing.

What I am detailing below is how the cores are cut – using a computer controlled device to cut wing cores. Some people are using this method by utilizing the core shells as wing moulds,  instead of going to the expense of making metal wing moulds.  I personally think that this is good idea because no one that I know truly believe that the have a perfect wing, and to produce inexpensive moulds is good thinking - because tomorrow you may wish to change your wing design.

I was speaking to Vern Gibson who has mate Andy Smith, who is well into Gliders, and  is a part owner with Mark Hunt of a CNC foam cutter. This cutter has been set up to cut model aircraft wings for all types of aircraft. 

So I went to Andy’s home – and here he was out in his garage working on his computer controlled hot wire foam cutter. 

All he had to do is key the co-ordinates of the airfoil into the computer together with the plan form of the wing, and set the machine going – Walla!! Out comes a foam core.  Well I suppose it is not really as simple as that – as a few trials are done before they get it right.  However once everything is sorted out – all they have to do it place the foam on the cutting table and you will get a perfect core every time – which accurately produces the same wing core again and again.  One good thing – you only have to input the variables once for one half wing, and then you just ask the program to reverse everything for the other side.

It is fascinating to watch this machine slowly cut the foam.  Incidentally they use blue foam instead of the usual white – this blue foam cuts beautifully – I think that I will use it in future!  I was told that it is too heavy – but I can assure you there is not that much difference in weight – and it seems to be much easier to use.

Here are a few details as supplied to me by Andy Smith:

The machine is called a Styrocut and the software that comes with the machine is called Styrowin.
This Styrowin package also handles cutting text letters in foam
The machine come complete with table, software, power supply, bow and interface.
The price about 2 two years ago was $4000 including delivery from Thailand.

The website is http://www.cbhobby.com/

No templates are required; hundreds of airfoils come with the software. It is compatible with Compufoil and new foils can be downloaded from the Web.
The software requires a (min) Pentium 100 PC with Windows 95/98.
The machine is capable of 0.1mm accuracy. However limitations to this fact is that with tapered wings the burn rate at the tip is greater than the root. To compensate some trial and error is required to thicken the tip section.
Different foils can be used for root and tip and for large wingspans several panels can be cut and butted together.
 The machine can cut up to 1000mm panel with a 600mm chord. Experience has shown that highly tapered wings greater than 2:1 (Tips less than half the root) are not possible to cut. This to due to the angle on the wire required would go out of bounds of the table. The software handles washout if required. The machine will cut white, blue, EPP and Spider foams.
 
         The best foam in the high quality/high compression strength class suitable for Pylon is Spider foam but as this is very hard get and very expensive.
 The best alternative is Blue foam. It has much higher compression strength than normal white foam. The small trade off is that depending on density, Blue is normally a few grams heavier than white.
 As a guideline the Blue foams by DOW called Styrofoam are a good choice.
Many grades are available but a popular choice for wing cores is a grade called  "SMTG".  It comes in 2500mm X 600mm X 50mm. The "TG" stands for tongue and grove.  Prices vary but $45.00 is typical per sheet.
In Victoria you can obtain Blue Foam from One Stop Plastics next door up from Saturn Hobbies in East Bentleigh.
  

So once you have keyed in  the wing section co-ordinates into the computer  - together with the wing tapers and plan form.  You can bring up a picture of the wing on the computer screen.

It then is a simple matter to place the foam on the cutting bench, and test the machine our for cutting speeds.  At this stage you will possibly use up a bit of foam to get it right.  However once you have all the variables worked out – you can accurately produce wings exactly one after another.

One thing surprised me was the slowness of the cut.  Andy has the machine set so that it the actual cutting wire does not even touch the foam – the heat of it melts the foam ahead, as it passes through the foam.  He also uses a stainless steel instead of the usual nicrome wire.

Once you have started the machine – all you do it just sit there and the wire bow automatically cuts the top and bottom of the wing – in a single pass - both the top and bottom of the wing.  Then you place another wing blank for the opposite wing half into the machine and reverse everything on the computer to cut the other half of the wing.

There are also some tales going around that the fumes from this blue foam are deadly – Andy produced some documentation that the foam that he stipulated above is okay from us to use.  However I for one would not be sticking my nose close to it whilst it was cutting.

So now all you have to do is purchase the foam cutter and off you go – perfect wings every time.

 
 You can contact Andy at andysmit@netspace.net.au who would answer any of you questions.