Europa #435 G-RODO Build Journal - 2009 08

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1 Cut away damaged area of outer skin with cutting disc in Dremel. Try removing foam with a knife, but rather too easy to cut into inner skin. Find that cylindrical burrs of various sorts work OK and don't cut too fast on the end to pose a serious threat to the inner skin. Find that the smaller hole does not go as far in as the inner skin, which is good news. Confirm this by getting (with some effort) my arm into the aileron bellcrank inspection hole just past the elbow and feeling the inner face. However, there is a strip delaminated on the inner surface of the inner skin which shows as a stripe across the area where the foam has been removed. Can feel it on the inside and it seems to extend about 75mm each side of the 50mm hole in the inner skin where the damaged pieces have been removed. Extend the cutaway area in both directions but the stripe seem to continue further than expected at the TE/outboard end. Chamfer outer skin with flat face of large grinding wheel in Dremel. It's also good for clearing last of foam from inner skin, although very easy to go too far - have to extend cutaway area in a few places because the inner skin got cut through. 1290.3
3 Phone Roger at Europa about the wing skin repair. He says I probably need to talk to John Bewley of Aviation & Marine Engineering who prepared the original repair scheme, but he is away on holiday this week. He asks me to send a photo anyway in case he can offer help once he's got a better idea of it, and if not he'll pass it on to John Bewley. For the DB-9 adaptor for the wing-leveller servo connector, cut off a piece of 35mm x 35mm x 3mm aluminium angle about 50mm long. Clamp it in the machine vise still set up on the milling table from the test cuts. Fit a 3mm slot drill and check squareness of workpiece. Not quite prefect but adequate for this job. Trim end of angle square, cut the flange to 25mm wide and trim the other end square to 40mm long. Plunge cut connector mounting holes. Mill along short edge of hole (find that 0.5mm plunge increment per cut is fine), then take ends of that cut across to where the long side will be, so that centre will drop out. Plunge cut ends of long edge to help see when approaching them, then mill along length as for short side. 1292.3
4 Unclamp the piece of aluminium angle. File the angles on the end of the slot and check fit of connector. A tiny bit more easing with the file and it goes in very sweetly. Set it up for milling the other face, using a spacer to get it level as the previously-worked flange is now too short to bottom on the vise rods. Reset the dials to zero against the reference edges. Start to repeat previous sequence, beginning with plunging the mounting holes, but after the 2nd one realise they are wrongly positioned. I had got the X and Y co-ordinates mixed up for the first hole, although the Y increment for the 2nd was correct. Repeat with more care and the holes in the correct position just intersect with the wrong ones. Will just have to put up with the unwanted holes - don't want to start again. Continue milling sequence as before, double-checking all settings before making cuts. Remove from vise when complete and file angled ends of connector slot. Deburr all sharp edges and corners. Fit connectors and solder wires between them. Check fit of completed item on servo - all looks OK. Now maybe need to provide insulating cover over the wires. 1294.3
6 Roger e-mails to confirm that he is passing my photo of the wing repair preparation to John Bewley, who should be back on Monday.
7 Soba (or Shobha) rotary table arrives from Chronos. Draw up milling arrangement for TT21 transponder head (TC20) mounting hole, to check that the panel will clear the milling machine column at the extremes of rotation. No problem, the hole can be about 30mm in from the edge without fouling the column.
8 Convert the dimensions on the milling diagram for the TC20 mounting hole to handwheel settings, and work out the sequence of operations. Still not sure how to hold the panel down; it needs to be constrained once the last part of the hole is cut out. Consider that the fixing holes could be plunged 3mm into the (proposed) MDF support, the cutter changed to a larger one and just the aluminium hole enlarged to allow a self-tapper to be used.
11 Phone John Bewley; he hasn't seen the message from me forwarded by Roger, but will be in the office to check his mail tomorrow. Check TC20 fixing screws and they are just under 3mm diameter (4-40 UNC, CSK head) so the idea of enlarging the mounting holes to provide a hold-down while milling is not so good, unless I use a smaller slot drill (which I haven't got in stock now) to make the first hole and then enlarge to 3mm. Order 1.5mm, 2mm & 2.5mm slot drills (and some other stuff) from Chronos.
12 John Bewley replies to my e-mail agreeing to my proposal to lay up BID on the inside of the wing repair. Reply with a sketch of the complete proposed repair to make sure I have fully understood what he's stipulating. He replies saying "Your sketch is exactly how I saw it".
14 Request quotes from US & European firms for short lengths of coax cable suitable for transponder aerial. Measure thickness of brown foam and recess in wing skin. The supplied 5mm foam looks slightly thicker than what is in the wing skin but it's hard to be sure. It needs to be thinned by 10-15 thou (one layer of BID) at least. Consider how to hold it whilst sanding top surface. Start to set up for routing a recess in a piece of chipboard. After dinner and some more thought realise it's easier to attach thin retaining strips to chipboard than to rout out a recess. Bandsaw some slices about 2.5mm thick off a 35mm wide piece of mahogany. Trim to 2 off 100mm and 2 off 280mm lengths. Wrap brown foam in cling-film to prevent glue contamination. Stick mahogany strips to chipboard in position around cling-wrapped foam (Evo-stik has gone off so use UHU clear glue). Remove foam, lay another piece of chipboard on top and weigh down to set. 1296.1
15 Remove weights. Remove cling-film from foam. Check foam for good fit between slips - all OK. Check foam thickness with digital caliper. Varies between 5.17mm and 5.20mm. Total top skin thickness is about 5.25mm (as well as can be measured) adjacent to the cut-back area. One of the tip offcuts is also 5.25mm, but the other is 4.85mm. One piece of dry BID is 0.25mm. The inner skin (pre-preg) varies between 0.35mm and 0.5mm. So, to be on the safe side, the foam should be 5.25 - (2 x 0.5) = 4.25mm. Place foam between slips and rub down carefully with large coarse mounted Perma-Grit sheet. Check at frequent intervals for progress and evenness. When almost there (4.28mm to 4.33mm) turn foam over and lightly rub other (previously untouched) side, to prevent it curling too much and that brings it down to between 4.17mm and 4.22mm. 1296.9
17 Tape piece of carbon paper, blue side out, over the cut-away area of wing skin. Tape a piece of A4 paper over it and rub around the edges of the hole with a mixing stick (brass-rubbing style) to transfer the outline to the paper. Remove paper and carbon paper and cut around marked-out line. Try it for fit in the recess, and trim a few places to improve the fit. Once satisfied, lay it on the brown foam and trace around with felt-tip pen. Put a new blade in the Stanley knife and cut around the outline. Try it for fit in the recess - quite good but needs chamfering on some edges to match the shape of the recess. Sand chamfer with 3M P80 production paper. Re-check and sand more until fit satisfactory. Cut a piece of polyethylene sheet to size of BID required on inside of skin, roll it up and check that I can reach in through the aileron bellcrank access hole enough to roll it across the entire repair area. Awkward, but seems do-able. Abrading the inside of the skin before bonding will be quite tedious, I expect. 1297.9
18 Get various sanding sponges in B&Q for abrading inside of repair area. Start to set up Jim Weir's wire numbering system in FileMaker.
21 Get an offcut of nylon (or maybe it's acetal) at Bristol Model Engineering exhibition to make an insulating cover for the wing-leveller servo right-angle connector. Also speak to a laser-cutting service about panels - they can do lettering as well as cut holes.
24 Order battery isolator switches from Classic Partsworld.
25 Measure up for inside BID for wing skin repair. needs to be 255mm x 150mm. Cut and mark up an oversize piece of polyethylene sheet, leaving enough spare at one end to roll over wetted area. Cut a piece of 250mm wide peel-ply to length of polyethylene sheet. Mark back of polyethylene sheet with limits of BID, ends of hole, and position of delaminated strip on inner surface of skin. Cut BID to 255mm x 150m with weave parallel to edges so as to line up with fibres in pre-preg. Trace shape of hole in foam onto polyethylene sheet and onto BID. Cut out BID shape (although on reflection probably should have left that until after wetting-out). Mark up a piece of polyethylene sheet with shape of holes in foam and outline of scarfed area. Cut a piece of BID to extend well beyond the repair area. Degrease outer faces of repair area with acetone. Check how easy it is to abrade inside of skin with sanding sponge through access hole. Awkward, but seems to be do-able. Remove port aileron to provide better visibility over wing. Need either a mirror or an assistant to help with positioning. Abrade inside of skin some more with the sanding sponge, but when I look at it illuminated by the LED light lying inside the wing, the glaze is not really removed. Fit a Dremel sanding drum to the Minicraft tool (it's lighter and smaller diameter than the Dremel) and try that on a piece of the offcut. Seems very satisfactory, so proceed to abrade inside of skin with it. Need to take great care not to drop the tool, or to let it swing round and damage other areas. Set up USB webcam inside the wing to see if that will give a better view of where I'm working than squinting through the pitot-static mount hole. After some fiddling with position of light and camera decide it's not really any help. A small mirror and the LED light seems to be the best I can do. After several passes with my arm at different depths into the inspection hole, I'm satisfied with the result. Clean encrustation off hardener spout of pump and free its one-way valve by poking with a mixing stick. Pump a few strokes to clear bubbles etc. Mark the brown foam with spanwise lines and cut into 5 pieces along them. That should prevent it springing away from the curve of the skin, and also make it easier to insert without entrapping air bubbles. Ask Robert to come in and hold the large mirror so I can see the upper surface while working from the other side. Mix 2 strokes of epoxy and wet out the BID for the inside on a small piece of polyethylene sheet. Add peel-ply and marked-up polyethylene sheet, then remove 1st polyethylene sheet. Roll up the assembly with some difficulty. Insert into wing cavity and try to position for roll-out. Very difficult to get it steady in the right place and much more difficult to get it to unroll - it won't unstick from the backing sheet. Take it out, unroll, remove plastic sheet leaving just BID & peel-ply but no real improvement. Next try applying epoxy to inside of skin to help it stick as that's part of the trouble. That's a slight improvement but my positioning is now way off so take out and bin the mess that the BID has become. Time to think again. Robert suggests it might work better if the BID was rolled on a dowel. That seems a possibility for better control, and if the skin was wetted and the BID left dry it might be easier to unroll too. Will have to think more about how to do it all, including some sort of handle for the dowel. 1302.0
26 Battery isolator switches arrive. The Lucas replica is nice but pretty heavy. The other one is very similar to the one I got from Karparts but has a fuse link for a keep-alive circuit.

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