Pod Construction
Click each thumbnail for a larger pic.
18th July 2005
The first pic is the seat shape after the vacuum bag has been stripped and lines added for trimming to shape. You can see at the bottom edge, I didn't really leave enough space to attach the vacuum bag and it caused us some headaches. I'll have to extend it about two inches before we do the next one. The second is the layup just released from the alloy base. Jukka specifies three layers of glass for this part and that's what we've used. I asked him about foam for stiffness and he assured me that it would be OK. I'm keen not to add unnecessary weight, so we're going with the plans (for the first one at least). He did say that his might be stiffer as he had laminated it in place rather than as a separate part but I couldn't figure out how to do that and still be able to remove the moulds. We'll assemble the parts with flox and tapes and see how stiff it all is. I'm not too sure it'll survive being stood upon for very long but it's not the end of the world if we have to redo it as a foam sandwich for the next ones. The lower - step - section is the worry and if necessary, we may reinforce it selectively with either foam or plywood. The vertical walls are next.
17th June 2005
After the usual struggle, the alloy is glued firmly to the plywood. I hate contact adhesive. I've spent ages scraping the residue (off me) and my fingertips still have black patches. We cut out templates for the seat shape, the forward fuel tank bulkheads and the forward seat bulkhead where the rudder pedals attach. We already have the 3mm MDF vertical seat template I mentioned earlier, so we're ready to start laminating the seat.
15th June 2005
Tonight, I glued and stapled a piece of 1.5mm plywood to the seatform. Tomorrow, I'll glue a sheet of 0.5mm aluminium to the plywood. Originally, I was going to fix just a sheet of aluminium to the form but I reckoned that the soft aluminium probably wouldn't survive moulding four seats. Besides which, everyone who sees it will probably sit on it and this it would definitely not stand. It would also have been difficult to fix the alloy really securely to the form without nails or screws and as we're going to vacuumbag the seats, I didn't want to pierce it because of the possible problems with leaks. I'm going to use a contact adhesive to fix the alloy to the ply and the large surface area almost guarantees a good bond. The smooth alloy surface, without fixings, will probably make the removal of the glassfibre from the mould somewhat easier. It'll certainly make it easier to wax, with nothing for the cloth to hang up on. I'm still not 100% after my recent dose. I get tired very quickly and have to pack up. The workshops loss is the websites gain ;-)
14th June 2005
Not much progress tonight. Just made up the form for the seat. Not very high-tech. Four pieces of 3/4" chipboard held with glued dowels. I laid a piece of 1.5mm plywood on the ribs and had a little sit down. The form seems quite comfortable and I couldn't resist making some engine noises (the first opportunity so far). The shape was extracted from the Solidworks model seen in pic4 and I had it lasercut in 3mm MDF. I then copied the MDF shape using first a jigsaw to get the basic shape and then a bearing guided straight router cutter to finish.
The total width is 500mm which will allow both sides of the seatform to be vacuum bagged in one go. We can then split it up the middle to get the two separate pieces. I'm planning on doing a 6mm foam sandwich for this, as four plies of glass alone won't be very stiff. The foam will add about 25gms to the package but the extra rigidity will be substantial.
I modelled the shape in Solidworks as a sheet metal part - the second pic shows the true shape. Solidworks allows the part to be flattened, thus providing the developed shape seen in pic 3. Mirroring the shape provides the full seat shape without the 200mm central gap. We'll copy this shape onto the sandwich and cut along the dotted line. I don't expect it to be an exact fit but it should be within a few millimetres. The flox and glass tapes will do the rest.
The last pic is the Solidworks model of the inner glassworks; seat, tank and bulkheads. There's quite a lot of work yet before the pod is complete.
2nd June 2005
We spent an evening fitting the airframe into the pod. It's extremely nervewracking and - as a result very tiring and tedious. Cut, try, cut, try, cut, try! You get the picture! One of the difficulties is that the airframe has to enter the pod in such a way, that the side mounts pass above the edge and when they are sufficiently far forward, drop down into place. Sounds easy but takes ages to do. This is made more difficult by the rear bracing tubes which begin to interfere when the airframe is 75% into place.
Anyway, we're very pleased with the result and will be able to mark the mould from the cutouts so we have most of the work done for the next three pods. The raggedy edges sen above are the peel-ply which remains for now. The last pic shows the nosepiece siting in place on the shell. For the next shells, we'll have to adopt the Rutan practice of using several layers of duct tape to leave a recess for joining tapes.
The next move is to build up glass pads under the airframe side plates and then drill for the side and keel front fixing. We can then move on to internal glasswork for seat, etc. In parallel with this, we'll make a start on the next pod.
25th May 2005
We laid up the inner plies a little differently to the outer, given the difficulty we had experienced earlier. We purchased a roll of 500 mm wide 250gm unidirectional cloth and used two layers at right angles to do the 45deg layup, followed by a layer of the bid at 90deg. There's a slight weight penalty (about two pounds on the pod) because we couldn't get a lighter Uni but the ease of working more than compensates. We also wetted out the cloth in place using rollers and found a great saving in time and effort, not to mention the ease of keeping the fibres in correct alignment. When it came time to lay in the bid, we cut a large slit at the rear centre (about 400mm long) and found that it made it really easy to conform. We made a similar cut in the peel ply. We had the usual struggle with the vacuum leaks but after about 30 mins we got 15" at a fairly satisfactory leak rate and retired for the evening. The two pics above show the result. The increase in stiffness is staggering. By the way, it came out of the mould really easily. My ongoing nightmare was it sticking in the mould and both being destroyed.
20th May 2005
We started back into the pod moulding a while ago and I'm just getting around to updating my website now. We thought about the foam sandwich a lot and to eliminate the possibility of wrinkles, we finally decided to do the outer skin without vacuum. We laid up the first ply (90deg) on a polythene sheet and carried it in to the mould, where we spent about an hour getting it to conform to the double curvature and eliminate air-bubbles. We then did the same with the second ply (45deg) but it took much longer to get it into place and eliminate the bubbles. Eventually, we had to make some small slits and
overlap the edges to allow the cloth lie flat. I'm told that this is perfectly acceptable and we'll be much quicker to do so in future. The first pic shows the result with peel ply applied all over.
The next pic shows 15ins of vacuum in the bag - leaks are a bastard to eliminate ;-) The foam specified is 5mm and the sample we tried conformed beautifully under vacuum so we went ahead and ordered four 8x4 sheets from the supplier. What arrived was slightly different, being žin and higher density. The result was the large split seen above. I EMailed Jukka who re-assured me that I could patch it OK. He also said that his JT5 had just glass for about 50mm each side of centre and I could do that if I liked with some extra plies. He also suggested splitting the foam in the centre to assure symmetry. We patched anyway, split the second piece and the result is in the fourth pic. The fifth shows the front section patched and the plywood stiffeners fitted. The epoxy can clearly be seen where it's been squeezed through the perforations in the foam by the vacuum action. Earlier on, we laminated the ply in place, from three layers of 1.6mm three-ply, giving us - including epoxy - just 5mm of nine-ply, nicely curved to fit. We marked them (four sets) port and stbd, just in case. The sixth pic is a close-up of the ply insert.
I made a tool from a scalpel blade and a piece of plastic, to cut the foam exactly 55mm down from the edge of the moulding. With the blade depth set at 6mm, it worked like a charm. You can see it in the last pic above. The quality of the tool leaves a bit to be desired, it was a last minute lash-up when I realised that it wasn't going to be easy to keep the cut depth even and parallel with the edge. In this case, function overpowers form.
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