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There is little if any structural benefit in using carbon fiber cloth and the extra stiffness (along with higher cost, higher difficulty to work with, etc.) can be a detriment...... |
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And coming back to my primary point - the CF wasn't there for structural benefit, but rather finish benefit: no paint cracking due to flexing of the underlying structure. Was it extra work? Hard to tell, but it was nothing compared to the rest of the canopy work. ETA: I don't think the two points are incompatible - it comes down to what the builder wants. |
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One layer has very minimal impact on overall stiffness. I should have been more specific to say that I would recommend not using carbon for the entire thing. |
So I think I have this thing worked out, at least I hope I have because I'm about to go and lay some composites down. If the coefficient of thermal expansion of perspex is 0.000075mm/mm per degree kelvin, and assuming the canopy side is 1000mm long (round figures here) then for a 40 degree celsius maximum desired temperature and assuming I bond at 20 degrees celsius, the canopy will expand by 1.5mm. Sikaflex has a 500% elongation to break per the data sheets, and IIRC a working elongation up to 300%, thus of this canopy can shear expand up to 1.5mm compared to the skirt, then basic trig suggests that a 0.5mm spacer should be adequate to allow the bond to shear this amount (ok, the hypotenuse of a 1.5mmx0.5mm triangle is 1.58mm, but close enough).
Thus, in summary, I intend to lay up some electrical tape first to a 0.5mm thickness to fill the void where the sikaflex will be in the future (this step may not even be necessary as you should get enough flex in the skirt even with carbon in the mix), then cover the tape and the void between the canopy and the fuselage with packing tape tape, lay up the glass/fiber skirt, pop it off when cured, tear off the electrical tape spacer (if I decide to use it), replace this with some 0.5mm small spacers (fishing line perhaps) then finally bond the canopy in place with sikaflex. At least in theory. The only downside I can see to this is the fact that you will have a 0.5mm thicker layup than you would otherwise., but there's no reason why you can't put a nice slope on the sikaflex bead. Time to put this into practice. Tom. |
I'd put a large factor of safety on that elongation. That 300% isn't a "working" number, it's elongation to failure. You don't want it to fail. Do search in VAF for the thickness that other people are using even without the carbon, and I think that you'll find that the gap really ought to be considerably larger.
Dave |
Hi Dave. The elongation to failure for 295 UV is definitely 500%. I checked VAF threads and it appears that many are using no spacers at all for joining the skirt to the canopy.
http://www.vansairforce.com/communit...ad.php?t=98485 Thus I fail to see why 0.5mm won't be better than nothing. I just don't want an unnecessarily high step up to my already thick skirt and targa strip. Although there is little linear elongation in the carbon/fiberglass skirt, it does have the ability to flex outward to some degree. The spacing specs issued by Sika are assuming mounting to a far more rigid marine structure. The world has 10 minutes to convince me otherwise, as I'm about to lay down the last layer of tape. Tom. |
But thanks for your feedback anyway Dave. That's the whole reason I'm hashing this out on the forum and not making the decision in complete isolation.
Tom. |
I increased the thickness by 50% to 0.75mm. Don't say I'm not good to you Dave.
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Glue
Put a few screws in and it will be alright.
Bob |
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