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