Hi Roger,
Sorry for the slow reply - I'm just home from a long weekend of hang gliding in Kerry where we had stunning weather, flying and scenery - I'm still on a high!
The CAD renderings are really good - well done for taking the time to transpose them from the original drawings. I would love to have been able to CNC cut out all my parts (especially the wing ribs and molds for the carbon fibre) - it would have saved me a *lot* of work! Spinning the 3-D render around, it looks like you may have missed out one of the thickness dimensions on the lower longeron - it appears to be a thin ribbon. This longeron is a composite to several 1/2" square pieces spliced together... which then gets planed down on its outer edge to conform to the slope of the pod sides. The lower longeron is 1/2" thick x 1" wide for the mid-section of the pod, tapering to 1/2" square at either end. Other than that, it looks really great!
I fabricated all the ribs for the wings, tail, rudder, tail-boom, etc out of 5mm closed-cell PVC foam board encapsulated in one layer of carbon cloth either side. These have proved their strength in destructive load testing, surviving to just shy of 10g loading. The original wooden CD had a 5g design maximum with an 8g ultimate (failure) load limit.
If I was going to start from scratch (and had a CNC set-up) then I would CNC all my molds from something like High Density Poly Ethelene (HDPE) - the tough plastic they use for kitchen chopping boards. This would be much tougher than the MDF I used and much less work (you wouldn't have to treat the surface with anything except a thin coat of PVA release agent.) My MDF molds required 4-5 coats of high gloss floor varnish (sanded between each coat) to seal this pourous material and create a nice shiny finish. HDPE wouldn't require that extra effort and the epoxy resin probably wouldn't stick to it (even without the PVA) as long as the surface was smooth.
In truth you would probably build a wooden CD (per the original specs) a little faster than in carbon... maybe. It's the preparation required for vacuum infusion that takes the time - making and prepping the molds, making the foam cores, cutting the cloth, bagging up the parts and checking for leaks, etc... but, as you can see from my videos, you can infuse a dozen or more parts in one go and there's no messing with wet epoxy (which is very nasty stuff if you get it on your skin.)
I would strongly recommend avoiding any wet lay-ups if you possibly can. The vacuum assisted resin infusion process is infinitely cleaner, safer and less time dependant (you're not racing against the clock before your epoxy starts to go off, as you are when doing a wet lay-up. With vacuum infusion you only mix the epoxy once you are 100% happy with all your parts and the integrety of your vacuum bag.
The only note of caution I would add is that Ireland is a lot cooler than South Africa (where you are I think?) and my slow cure (120 minute pot-life) resin was often still mobile 6 to 8 hours after mixing - a warmer climate will shorten the pot-life of your epoxy. If it's too warm (25C+) then you might want to cool down your workshop with an air conditioner while you are infusing so that the resin has time to infuse all the parts in a multi-part infusion. Once it's fully infused then you can let the heat back in for a quicker setting time!
I usually leave my parts under vacuum for 24hrs with an electric blanket to bring up the temperature to between 25C-30C or until the resin in the vacuum tube going to the resin trap has has fully cured and is brittle. After that the parts are de-bagged and tossed in my electric oven to post-cure up to 80C (you'll need to talk to the epoxy manufacturer (not the supplier) about an appropriate post-curing regime of time at temperature.) Post curing makes sure the epoxy will not start to go soft (and weaken) at higher operating temperatures (like when your CD in left in the sun!) In any case, you will need to spray paint all your black carbon white, to reflect as much heat as possible.
Definitely make your wing spars out of carbon - don't use the original method of wet-laid carbon roving in spruce spar-caps - you will *never* obtain anything close to the theoretical strength of the carbon. I used this method for the lower longeron in my pilot pod and, although it worked well enough, I was very unsatisfied with the process. It's almost impossible to control the amount of resin in the roving and I ended up with different numbers of tows in each longeron (25+ in one and 30 in the other.) I'm planning on redesigning the pod next year, after I get the current one flying! I like the Swift's pod structure - aluminium tubing with clip-on farings. I believe there's a weight saving and a strength gain to be made there.
I'm currently working on the bomb-bay doors of my pod at the moment. I'm very close to covering the whole pod in Kevlar and moving on to the wing spars. However, my roll of Kevlar is stuck in a mate's flat while he's off swanning around the French Alpes in his Swift - lucky blighter!
Hi Philip
I just wanted to say hi and show you my virtual progress on the CD. It’s taking a while working in the evenings but it’s not a total waste of time because I can CNC cut all the profiles when I am done and I have checked the fit on the computer.
I have also been researching and found a company that can supply very good 6mm plywood for the formers. The wood complies with BS 1088.
I think I also found a lead for an alternative to spruce but I just need to go round there and see the wood for myself. I found a place that used to import White Deal but havn’t brough it in for more than a year now. Bummer.
Also , one of my existing suppliers stocks 12K Carbon Roving in 4kg spindles and any other resins or carbon cloth I will need for the project.
I would really like to try and build most of the plane from carbon where possible so I was just wondering what your thoughts on the process are. Is it much more difficult of time consuming when compared to building the wood/Carbon version?
Oh, and I was looking at some of your hang gliding YouTube videos (I have already downloaded all of the CD build ones for reference later) and it looks like you have a great time with your mates on the slopes J
Hope you are having a good day. How is your build coming?
Kind regards
Roger
Roger Hardie [B.Tech Industrial Design] +2782 780 3535
www.rogerhardie.com PO Box 968, Fourways, 2055
