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#11
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Jim and I are conducting a Spar workshop on november 22-23, 2003 in Marion
OH. Carbon Rovings are a very poor spar material since you cannot pull all of the strands straight and through testing it was found to have a wide variance in tension and compression. Compression being the worst case which is about 40,000 to 80,000 psi... one may get to 120,000 psi in tension but this is highly unreliable so you must degrade the enitre layup ..... but just what do you degrade it to... it depends on how well the person laid it up. With the Tests that Jim did, they where extremely careful and where professional fabricators in fiberglass and those results where very discouraging.. With carbon rods there is no guessing as to what numbers you are dealing with. Also the process has been thoroughly tested. The other problem is that with the windrose construction you are directed to drill through the carbon. This is an absolute mistake!! This is dangerous. If you are asking an amateur builder to construct the spar with roving you have a situation where the result is highly unpedictible and then drilling through the spar increases the danger that the spar could fail. As for instances of actual breaking in the air..... I'd rather build it correctly first and not have to site facts of dead pilots and broken wings. See our testing of the carbon Monarch spar on our web site for example. All of the Maupin plans need upgrading to deal with modern practices and show that indeed the aircraft have been throughly tested. From what I have seen the Maupin/ Culver designs where never thoroughly tested and with the 15 meter windrose was never even built by the designers. Jim Maupins designs were wonderful concepts and should be studied with that in mind but they are not a refined aricraft as they exist at present. Having siad all of that we in north america do need some brave designers to challenge the dominance of the German ships. It takes a lot of money, guts, research and hard work. and do not expect much support from your fellow soaring pilots! That Jim Maupin got outside the normal glider design was very welcome... that he tried new concepts was great...that he opened up the light sailplane was terriffic..... but we need dedicated people to continue these ventures. It will not be profitable... and plans should not be sold unless you have a craft that has been thoroughly tested. However people can donate to the cause and be an active part of a development. -mat -- Marske Flying Wings http://www.continuo.com/marske |
#13
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"Bob Kuykendall" wrote in message But on-topic, what's really at issue is not the tensile strength, but the compression strength. The way I understand the issue from personal conversations with Jim Marske, hand-laid carbon rovings often drastically under-perform their predicted values in compressive strength. The problem is that it is very difficult to achieve the proper fiber alignment and resin content and properties under hand-layup conditions. The result is compressive strength (the kind you usually need in an upper spar cap) in the neighborhood of only 40 ksi or 60 ksi. Jim says he did coupon tests to validate these numbers, and I certainly believe him. Excellent stuff Bob..... I also believe Jims numbers......... Compressive strength is the upper spar cap limiting constraint. Rovings or tow, are admittedly difficult to use, BUT can be used successfully. Personally, I find that Unitape cures many of Mats concerns about orientation and resin content. In fact it is so commonly used that free tow is basically off the market. 20 some odd years ago when carbon was expensive,new,untested, all you could get was dry tow. Bunches better than not having it, but definately not state of the art today. Todays woven goods are vastly superior to the old stuff. I hesitate to condemn its use in spars. It might be a bit less efficient than rods but definately usable in todays structures. Especially by the workman at home. NOW lets also get down to brass tacks and admit to each other that the mode 1 failure of upper spar caps is BENDING caused by a collum buckling. When this occurs the substructure supporting the spar caps deflects and the cap fails in bending. A good example is when the shear web ruptures as the bending loads go up and the spar cap can't absorb the local bending load. The failure looks different than a compressive failure (because it is) inasmuch as it looks like a snapped pencil. Scott. |
#14
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To add to the discussion of Carbon in use with spars.
One of the important elements that should not be forgotten is the deflection. With Carbon rods you can accurately predict the deflection which I believe is very important in glider wings. I have produced a spreadsheet that will predict the amount of rods needed and predict the deflection. A good deflection is about 10 to 12 " per G and with hand laid up rovings you may overcompensate to account for the poor quality of rovings and produce a very stiff wing. And incidently I own and maintain the site and work in partnership with Jim. -mat -- Marske Flying Wings http://www.continuo.com/marske |
#15
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On Tue, 21 Oct 2003 17:30:24 +0000, Robert Ehrlich
wrote: Marske Flying Wings wrote: ... The use of wood in a glider is not really recommended any longer, nor is the combination of wood and fiberglass. Ineveitably the wood and fiberglass tend to separate. It is better to use an all fiberglass structure and I even avoid the use of foam. ... However a lot of fiberglass gliders are using wood (mostly plywood) for holding various elements of the linkages from stick/spoilers lever to the control surfaces they actuates. I had just a look inside one of our Pegases where the seat pan has been removed for its annual, and it is made this way. IIRC this fuselage is a copy of the ASW20, never heard of separation of wood from fiberglass, neither in Pegases nor in ASW20's and there is a lot of them flying since a lot of time. Wood inside fibreglass can be a problem for repairs. The wood will shatter inside the fiberglass and you end up removing good glass to repair the wood. I was told this by Harry Schneider (glider designer, manufacturer and repairer) at Gawler about 30 years ago and subsequently saw an example of what he was talking about. Mike Borgelt |
#16
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#17
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Doug hoffman wrote:
My point was that materials other than the rods can still be safely used (providing they are used properly, etc., etc.). There is no reason for Mat to be condemning the perfectly legitimate use of the carbon rovings as used in the Windrose and Carbon Dragon. Guess what, the Windrose 13 meter doesn't even use carbon. It uses fiberglass rovings. The point I am making is that there is no way you can honestly compute the strength you have using carbon rovings with hand layup especially for someone who has never done it before. Even a fiberglass pultrusion would have been better as Jim used in the early Monarchs. The other problems is that you cannot compute the deflection at the wing tips which should be about 10-12 ". So if an amateur is building the wings for the first time he could easily mess it up by not being consistent in the pulling of the carbon or fiberglass tow. I have found that there is no underestimating the bad buidling practices of some homebuilders. I have had to correct a number of homebuilts! Doug hoffman wrote: Also, when using the rovings as specified for the Windrose I or II or CD one avoids the complexity and possible source of error of having to design and build an additional structure to tie the main spar pins/bolts to the carbon rods. One needs only to drill bolt holes straight through the roving layup. Simple, easy. Which was one of the design criteria for the glider. Once you drill through the carbon you have little tensile strength left at that point since you are relying on the fiber to give you the strength. The carbon should be properly bonded to a well engineered fiberglass block ,and that is what you drill. Never drill through the carbon!!!! Remember also that you cannot put iron and carbon together... galvanic action corrodes the bolt. Rather than present those wings that have broken in the air, a better method is to see the results of the testing of the Marske Designed spar in the Genesis that went to 18 G's before the fitting broke on the test stand. Jim had tested each of the componet parts proving that this method would not come apart after many years of use. And this was all predictible. There are no studies done on the longevity of the windrose style of spar. So in my estimation using rovings can be dangerous for the first time builder and especially so if you drill through it. I do realize there are still a number of glider manufactureres building spars with carbon rovings but gradually they are changing to accept Jim Marske's proven Carbon rod spar. See the article in the SHA newletter from the Silent Glider manufactuere. -- Marske Flying Wings http://www.continuo.com/marske |
#18
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"Marske Flying Wings" wrote in message ... 1) The point I am making is that there is no way you can honestly compute the strength you have using carbon rovings with hand layup especially for someone who has never done it before. 2) The other problems is that you cannot compute the deflection at the wing tips which should be about 10-12 ". 3) I have found that there is no underestimating the bad buidling practices of some homebuilders. I have had to correct a number of homebuilts! 4) Once you drill through the carbon you have little tensile strength left at that point since you are relying on the fiber to give you the strength. The carbon should be properly bonded to a well engineered fiberglass block ,and that is what you drill. Never drill through the carbon!!!! 5) Remember also that you cannot put iron and carbon together... galvanic action corrodes the bolt. Good Morning Mat. 1) You can certianly do it. It is accomplished in the same manner that all other materials are checked. You just have to decide on a value to use. You say you have tested and shown values as low as 30KSI. Cool, use that and move on........ 2) Your wind bending allowables may be 10 to 12 inches, but that is a subjective opinion. Many all glass birds deflect more with no ill effects. Yours is a subjective opinion presented as a fact......... 3) Agreed, But these clowns would be the same fools who couldn' glue in the rods properly either, so I'll forget about misassembly. Your DAR or Tech Councellor will stop this from being a problem. 4) The loss of tensile strength is very dependant on the reduction in laminate cross section. Unless you have the particular layup schedule in front of you, you can't make an informed decision on the reduction in tensile strength. If the laminate gets thicker or wider, all bets are off. If the laminate is the same in cross section at the hole location as it is outboard of there, you still need to run the numbers to see if the remaining cross section meets the design loads. Again, lacking that data, neither of us can make an informed decision. You are correct that you will experience a local reduction in tensile strength, but it may still very well reach the design limits and be of no consequence. 5) This is a narrowminded statement. You would be correct if you put the fasteners in "dry", but only a fool would do that. Many steel and titanium fasteners are installed through carbon panels on Boeing and MDC jetliners. I have installed them. The trick is to install them "wet" with an approved sealant to provide the barrier you need. Entirely doable and cost effective. Proclaiming that you can't do it is in my mind incorrect, but we can disagree on how you present the facts on this one. Back on point. There is no reason you can't use hand layed up carbon successfully. Yeah it requires care, but so does riveting , welding, dope and fabric and all the other processes involved in amatuer constructed aircraft. Some people find it easy to work composites well, some are very challenged. Blanket statements saying it is unsafe to do so is in my mind bull****........... Scott Correa |
#19
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Scott Correa,
Yes those things can all be done and at one time where probably accepted as normal practice.... but we have better methods and materials today so lets use them. Jim Marske's Spar design is the best design I have seen so lets give him credit for such good work and encourage others to use this. As for dangerous, that is for each of us to decide: but for me I want the best technology, methods and proven methods with predictable results and that is exactly what the Windrose lacks in its present state....there was no mention in the plans of coating anything with epoxy and the carbon tow was not increased at the point where the bolts went through. Update the Windrose plans, do a lot of test flying... that will go a long way to remedy this design. -mat -- Marske Flying Wings http://www.continuo.com/marske |
#20
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Mat Redsell Writes:
[snip] As for dangerous, that is for each of us to decide: but for me I want the best technology, methods and proven methods with predictable results and that is exactly what the Windrose lacks in its present state....there was no mention in the plans of coating anything with epoxy and the carbon tow was not increased at the point where the bolts went through. Umm, page 8 of Jim Maupin's instructions. Third paragraph: "If everything checks out, drill the spar stubs for the 5/16" bolts, and bolt in place. I recommend using lots of epoxy in the bolt holes and on the nuts." and the carbon tow was not increased at the point where the bolts went through. Oh please. Did you really build one of these? I'm not even going to bother rolling open the plans to give the measurements. The hot-wired slot in the foam, for the spar caps, is a *lot* bigger at the spar root than anywhere else. I am tiring of this, Mat. What kind of technical education did you say you received? It must be impressive for you to berate Irv Culver's calculations as you are doing. |
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