25th July 2010 - Tip ribs cut out

I finished cutting two complete sets of wing tip ribs out of 12mm birch ply today. The picture shows one complete set plus the tip and tail ribs from the second wing. All the cutting was done with the CNC router using a 6.35 (1/4") cutter. The router was programmed for a 8m cut width so the machine does two passes at each height this ensures that the cut to final size is small and means that the finish and accuracy is superb. The program is set to leave 2.5mm thick holding tabs to hold the parts in place as the cut completes. It looks just like a model aeroplane kit (only bigger!) and removing the pieces is easy using a miniature saw blade in the Dremel. I've uploaded the coordinates of the ribs to the Yahoo Tailwind Group site if they are of use to anyone.

18th July 2010 - Wingtip test fit

First job today was to calculate the dimensions for a stub rib rear section which forms the rear inside edge of the wingtip (top right of the picture). This is positioned 1" outboard from the last main rib to allow the aileron mass balance to fit between it and the wingtip to streamline it in normal flight. As before the dimensions for the stub rib were fed in the the CAD/CAM package and a test piece cut out on the router. Then it and the three tip ribs were located on the spars in their correct positions. A blank for the trailing edge was made by sawing along the diagonal of a 2" x 1" section of spruce to make two triangular sections. When angled at 45 degrees along the line of the wingtip trailing edge this section matches the wing section nicely and will need relatively little finish sanding. The ends of the trailing edge were cut to 45 degrees and the outer end pinned to the tip rib. Then the trailing edges of the two intermediate ribs and the stub rib were carefully marked and cut at the required angle. A piece of 3/8" spruce was cut to form the leading edge and the front of the ribs bevelled to match. The picture shows the wing tip assembled on the spars with just one panel pin in the tip rib the only thing holding it together.

Next job is to procure some 15mm birch ply, cut the final ribs for both wings, buy some Aerodux glue and then assemble and glue the two wingtips.

16th July 2010 - Wing tip ribs

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.

11th July 2010 - Left wing structure finished

Work is proceeding slowly and I'm also behind on blogging. The main structure of the left wing is now complete and glued up. However, this will be the last construction I can do with epoxy. I developed an allergy to epoxy when building the Vans resulting in contact dermatitis which became infected - nasty. This time I've been very careful to make sure my skin is covered and to wear gloves but I've still developed another bad reaction. Reading into the literature, it seems for anyone using epoxy the question is not if they will become allergic but when. Forced air breathing and full body suits seem to be the order of the day for professionals - hardly practical at home. So the rest of the construction will use a resorcinol based glue (Aerodux 500).