The main tool used to fabricate the critical flying surfaces in the Varieze and many other composite aircraft, is the hotwire saw. This is the device that is used to shape the large blocks of Styrofoam Fabrication Billet into the complex aerodynamic shapes that make the aircraft so efficient.
Put simply, the hotwire saw is a bow, strung with a high tensile wire that is heated using an electric current. The wire is then guided through the foam block using templates in the shape of the required aerofoil section. Here's a little primer on the process...
Here's the saw, hooked up to the power supply.
Much of the work for a project like the Varieze involves building your own tools. The hotwire is one such tool. It is simply a substantial piece of pine, two lengths of 3/4" water pipe and a length of .041" Inconel wire.
A current is applied to the rig using a transformer. The wire is then tightened by turning one of the tubes, just like tuning a guitar. In fact the optimum method is to twang or strum the wire as you tighten, when the wire stops changing in pitch, it is as tight as it will go and is ready to be used for the cut.
The plans for the saw are included with the construction manual for the aircraft.
In this picture, you can see a canard core in the background.
Successful hotwire cuts depend on the ability to control the heat of the wire. Here is a circuit which modifies a standard domestic light dimmer switch which is used on the primary side of the transformer to control the current. I am currently using this on a 240 volts AC domestic current and get around 30 volts at 24 amps which is all the power I need. The fuses are a very good idea and were added after my prototype was tested for the first time...don't leave out the fuses!
Here is the main wing trailing edge section ready for the aileron hotwire cut. There is a template at each end to guide the hotwire saw through the foam. The templates are made from formica or laminex, (The stuff your kitchen bench tops are covered with) it won't burn and pit when you run the hotwire over it, it is easy to cut, carve and smooth to a nice finish on the edge and it comes in a range of designer colours and patterns!
If you look closely at the picture you can see two wire stops protruding from the template. These stops are a guide for the hotwire at the start and end of the cut to ensure that there is no undercut due to "wire lag", they are simply glued to the template.
