Then some clever soul came up with the idea of an automatic differential cutting system, and that seemed the way to go. There are many variations along this theme, and possibly the b e s t o f a l l i s t h e“ Te k o a F e a t h e r c u t u n i t” http:// www. tekoa. com / feather-cut-c-2-p-1-pr-1. html a commercial unit, but being me, I had to make my own! When cutting a tapered wing, it is important that the wire travels proportionally and evenly through the foam, exiting the other side parallel to the edge, otherwise distortion of the airfoil surface will occur. The system is a simple gravity driven adjustable ration mechanism that ensures a uniform cut through the foam. It consists of a metre long arm
Fig 2. Differential weighted arm and pulleys for pulling strings. graduated in centimetres, pivoted at one end and suitably weighted at the other. Two pulling strings are then attached, one at the metre mark( 100 %), and the other at the predetermined distance to provided a shorter pull. These strings then travel over two pulleys at each end and attach to the cutting bow wire, and when the weighted arm is released, the strings draw the cutting bow evenly across the cutting templates and the wire should exit the foam block parallel to the edge. You can support the bow by hand, but if you have a wide enough table, then a jockey wheel at the back of the bow allows hands off. The sequence is to get everything set up, check it twice, release the weighted arm from its stop so that it is held up by the foam, and then turn on the power and away she goes. Don’ t forget to catch it and turn off the power as the wire exits the foam! You have to make two cuts through the foam block, cutting the bottom surface first, then add the top template and cut the top surface.
Fig 3 / 4 Foam Cutting table and cutting bow setup. Sounds simple, yes well it is, but you can be sure for a start that if something can go wrong, it will and you don’ t get a second chance with that piece of foam!! Get a hook-up and you get a nice fat grove, what else, turn the power on before releasing the arm... ooops melt, put the former on the wrong way round... B * gg * r, have the pulling string not over the pulley.. damn!! And so the list goes on, but practise does make perfect, and the results are worth striving for. Just make sure you have a good supply of foam from a friendly builder or demolisher!
Fig 5. Cutting templates. I make my cutting templates using the compufoil program, there are others available and also free programs on the web. Two formers are needed for each end, a bottom surface profile and a top surface. I make mine out of 3 or 4.5 mm MDF and swage a strip of thin litho backed with double sided tape over the cutting edge. Some use Formica or the like, it is important to get a smooth hard slippery surface for the wire to travel on. Each pair of formers are matched, drilled and pinned to the end of the foam block with two 60mm nails. The foam block needs to be cut with a hot wire to the accurate size and thickness using some vertical guides for the edges and a flat former for the thickness, both these operations being done manually.
Fig 6. Thicknessing the foam block using a 65mm deep flat template each side.
Fig 7. Trimming the block to shape using two vertical bar guides off the table.
To calculate the differential pull from the lever arm, divide the Tip chord by the root chord and multiply by 100, eg 150 div by