I've decided to make the wingtips from wood and have discussed and agreed this with my inspector. The plans do not give any information about the wing section as it tapers from the standard rib to the tip so I started by measuring the distance between the chord line of the main rib and the top and bottom at 10mm stations all along the profile. This was then input to a spreadsheet on the computer. Then I decided on the depth of the tip rib (determined by the height of the strobe/navigation light unit I've got to fit). The length of of the tip rib is given on the plans, together with the spar position in the rib. This allowed me to calculate length and depth of the two intermediate ribs. The profile of the main rib was then scaled for each of them to be the correct length and depth and the position and size of the cutouts for the main spar calculated. The profiles for the three tip ribs were then input into the CAD/CAM program and test ribs were cut out on some scrap 15mm ply (the same width as a main rib including the gusset) using the CNC router. The final ribs will be cut from 15mm birch ply but will have lightening holes cut out to significantly reduce weight. 
