I'm sure if Jim were still with us and interested in making improvements to the CD he would be using the carbon rods. Most of the wood can actually be eliminated altogether, and keep in mind that the carbon rods have a VERY high modulus, so the implication is the wood would hardly pick up any load anyway -- it mostly serves to stabilize the rods, but they could be stable without the wood if packed into the corner well. The load is proportional to the A*E of the wood and spar. The Graphite is orders of magnitude stronger than the wood. I used 100 Ksi as the allowable when running through the calculations. If someone needs a unsophisticated spreadsheet to calculate the number of rods, I'd be happy to pass the piece of crap I created in a couple of hours on to whomever wants it -- there really isn't too much to it.


I also did calculations for a wing with a 14% or so bump in area to increase the payload keeping the same original wing loading (spreadsheet calculated both wings). Basically it's the same wing only pushed outboard and more added to the root. I think the tip has a 23 inch cord and the root is something like 64 or 65 inches with a total wing span of 47 feet (it's been a few months since I looked at it, so just going by memory).


OK, I just looked for the spreadsheet, and it was on a laptop that was packed off to the states and the back-up hard drive isn't reading right now, so I'll try and fetch it if anyone is interested. Phil may consider sharing his spreadsheet -- I vetted the wing calculations and they are OK. He was using a higher allowable, but also a larger safety factor -- in the end our sizing seemed very similar (original wing was done as a check). Anyway, The compression allowable will limit the strength of the wing bending, so I just used 100 Ksi to be on the safe side (upper side of wing will be in compression for the up-bending cases.


Phil -- did I send you a copy of mine? If so, could you send it back? (LOL, I'm such a dope!). There is potential to tweak the CD to be more composite (less wood that has a tendency to warp)and stronger and more stable for about the same weight. I think some light-weight glass would have to be used along with the carbon (outboard wing), and the graphite is REALLY strong, but sometimes too strong for the application -- especially outboard where the loads are low. I'm not really interested in building one any time soon (too many other projects are further up the list), but if someone need a little help vetting some calculations, I can try to help.
Cheers, Kenny

--- On Thu, 2/2/12, Rick Mullins <This email address is being protected from spambots. You need JavaScript enabled to view it.> wrote:


From: Rick Mullins <This email address is being protected from spambots. You need JavaScript enabled to view it.>
Subject: Re: [Carbondragonbuildersandpilots] "Carbon" Dragon?
To: "This email address is being protected from spambots. You need JavaScript enabled to view it." <This email address is being protected from spambots. You need JavaScript enabled to view it.>
Date: Thursday, February 2, 2012, 6:59 AM




I'm sure that Jim thought the carbon roving was necessary for strength. He was very weight conscious, almost to the point of absurdity sometimes. He suggests at one point cutting a 3/32 notch in some spruce to save weight, which would only have been a couple ounces at most. The roving you add weighs much more than the spruce you remove to install it. Spruce only has 5000 PSI compression and 9400 PSI tensional strength. At the time he designed the CD the value he used for the strength of roving was 90,000 compression, but Jim Marske's tests proved it was substantially less than that. More like 50,000 PSI, but that is still 1000 times stronger than spruce. I installed carbon in my spars but I started replacing the roving with carbon rods early in the construction process. It's very easy that way. Run the cap strips over a table saw to cut a notch and glue them in. Much simpler, easier, stronger, and saves weight over the roving.

The flapperons have to be very rigid. The control attachment is right at the root and the flapperon is something like 22 foot long, so there would be a large twisting force on it. The carbon is probably required to keep it sufficiently stiff. As to the control rods, they are in line with Jim's "keep it as light as absolutely possible" philosophy. Aluminum would work fine, but you have to make one of these carbon tubes to appreciate how strong and light they are. I used the carbon sleeve material that is available now instead of wrapping cloth. I used two layers of it and when it was done it was so unbelievably lightweight that I had serious doubts it could be strong enough, so I tested one to destruction. It is WAY stronger than it needs to be and it is easily 1/4 the weight. I'll weigh one and compare it to equivalent aluminum and post it when I have a chance. And at Jim's composite class he showed us a clever way to make the rods. Instead of putting the carbon over aluminum and using acid to eat it out. he built them over PVC tubing that was waxed and sprayed with PVA. The next morning while the epoxy is still a little rubbery, use a hair dryer to blow hot air through the PVC for about 15 minutes. The epoxy will first start to get a little tacky again, then finish curing. Because the PVC expands so much, when you let it cool, the PVC shrinks and with a little twist the carbon tube will pop right off.


From: Dewey <This email address is being protected from spambots. You need JavaScript enabled to view it.>
To: This email address is being protected from spambots. You need JavaScript enabled to view it.
Sent: Wednesday, February 1, 2012 7:35 PM
Subject: [Carbondragonbuildersandpilots] "Carbon" Dragon?




First of all let me say that I am looking for edification and information, I am NOT looking to offend anyone. With that said, here goes.
I own a set of Dragon plans and construction manual and there are a couple of things that I wonder about. The first is this, except for the tailboom is there really a point to the carbon roving that is incorporated into the airframe ostensibly for structural reinforcement?
The roving can not possibly develop anywhere near its theoretical strength, mainly because it's impossible to keep all of the fibers straight and under consistent tension. would not omitting all of the roving and making the sparcaps of solid or laminated Spruce result in a structure that is just as strong at very close to the same weight? The carbon roving in the flaperon ribs and trailing ribs seems a bit gimmicky also. The flaperons already seem sufficiently robust in construction without the carbon. Lastly is the control parts that made of carbon like the torque tube, etc. Is this really needed? Why not aluminum? The ULF-1 uses no carbon, is it weaker than the Dragon? The Dragon's design seems quite normal for a wooden sailplane of its weight except for the roving.