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#11
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Shin Gou wrote: I am just wondering...if composite materials are stronger and lighter than steel, why composites tubes aren't glued together to form the fuselage frame just like the traditional 4103 steel tubes being welded together? Any reasons? Shin Gou Rans S-9 Warrenton, VA The better question would be why would you want to use carbon fiber to build a tubular type framework? The real secret is to utilize the best strengths of your material and design to that strength. There is no technical limitation that would prevent using carbon fiber composite tubes to create a tubular framework. The only limitation I can think of is the joints for replacing the welded joints of the steel. That could easily be solved by custom bondable carbon fiber joints. Clamps could substitute for welded tabs and so on. One thing for certain, you would never have to be concerned with rust. One thing though, due to the very limited amount of composite vs steel involved, you would not be saving a lot on weight. |
#12
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Bonding titanium to carbon tubes does not sound like a homebuilable
technique to me. You are right the joints will always be the problem. However it is interesting to study the way in which items such as bicycle frames and yacht steering wheels are made in one piece from tubular sections in carbon ... I won't say any more on this. You can find plenty of info on the web. There are fouth major problems/issues that I can see. First .... if you layup a tube from carbon you cannot put all of the fibres along the tube axis. Some will need to wrap around. As carbon fibre is highly orthotropic (i.e. different material properties in different directions) then the impressive properties of the carbon fibre along the fiber axis will not be attained in the completed tube. Don't get me wrong the carbon will still be lighter and stiffer for a given weight but probably not by as much as you would expect. Second ... the properties of carbon composites are strongly dependant on moisture and temperature conditions. For example the compressive strength of Fibercotes E-765/T700 24K uni prepreg is 147 ksi at room temperature / dry conditions. At elevated temperature and equilibrium humidity conditions in a tropical environment the compressive strength drops to 88 ksi ! Not really any better than steel. Compressive moduli and any other resin dominated properties (such as shear) will exhibit similar behavior. Yout need to design for these low strengths at the extreme environmental conditions in any composite structure and thus at room temperature you will end up with larger than required margins and hence more weight. Third .. composites suffer from microcracking and other issues which will limit the laminate strains to approx 4500 micro strain at ultimate load (a rough number ... complicated issue not room to explain here). What this means in plain english is that you can only use approximately 1/3 of the potential strength of the material if you want a structure with long life. Fourth ... composites have no ductility like metals and I would hate to be sitting in a fuselage made of carbon tubes if it hit anything. The tubes would fracture and splinter and the pilot would be in all sorts of trouble. To control this behaviour you would need to add another tougher composite material such as Kevlar to the laminate (common practise in fwd fuselages for gliders to improve crashworthiness). "BobR" wrote in message oups.com... Shin Gou wrote: I am just wondering...if composite materials are stronger and lighter than steel, why composites tubes aren't glued together to form the fuselage frame just like the traditional 4103 steel tubes being welded together? Any reasons? Shin Gou Rans S-9 Warrenton, VA The better question would be why would you want to use carbon fiber to build a tubular type framework? The real secret is to utilize the best strengths of your material and design to that strength. There is no technical limitation that would prevent using carbon fiber composite tubes to create a tubular framework. The only limitation I can think of is the joints for replacing the welded joints of the steel. That could easily be solved by custom bondable carbon fiber joints. Clamps could substitute for welded tabs and so on. One thing for certain, you would never have to be concerned with rust. One thing though, due to the very limited amount of composite vs steel involved, you would not be saving a lot on weight. |
#13
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LCT Paintball wrote: Round is very strong. The only way you could improve on a simple round tube for strength would be to add material in the direction that you needed more strength, and take away material where the strength wasn't needed. Yep they're called box beams, or torsion beams for those who like an extra sylable. Hollow beams are a very weight- efficient approach to construction. In monocoque design the entire fuselage and/or wing is designed as a box beam. -- FF |
#14
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.......... :-)) wrote: Bonding titanium to carbon tubes does not sound like a homebuilable technique to me. The joints would not need to be titanium but could be made from carbon fiber as well. Bonding of carbon fiber to carbon fiber would present few problems. The majority of tubular frames I have seen are designed so that the stress on the frame remains in compression. You are right the joints will always be the problem. However it is interesting to study the way in which items such as bicycle frames and yacht steering wheels are made in one piece from tubular sections in carbon .... I won't say any more on this. You can find plenty of info on the web. That would be the obvious method if you are constructing the frame from raw materials instead of existing carbon fiber tubes. There are fouth major problems/issues that I can see. First .... if you layup a tube from carbon you cannot put all of the fibres along the tube axis. Some will need to wrap around. As carbon fibre is highly orthotropic (i.e. different material properties in different directions) then the impressive properties of the carbon fibre along the fiber axis will not be attained in the completed tube. Don't get me wrong the carbon will still be lighter and stiffer for a given weight but probably not by as much as you would expect. The solution to this was on display at Oshkosh about three years ago. Ever see the Chineese finger lock? They were using the save weave technique to form the carbon fiber tube, wetting out after forming to desired size and curing. The result was the optimum strength in all directions. Second ... the properties of carbon composites are strongly dependant on moisture and temperature conditions. For example the compressive strength of Fibercotes E-765/T700 24K uni prepreg is 147 ksi at room temperature / dry conditions. At elevated temperature and equilibrium humidity conditions in a tropical environment the compressive strength drops to 88 ksi ! Not really any better than steel. Compressive moduli and any other resin dominated properties (such as shear) will exhibit similar behavior. Yout need to design for these low strengths at the extreme environmental conditions in any composite structure and thus at room temperature you will end up with larger than required margins and hence more weight. Those properties can be programmed to just about any desired result. The accepted standard for most composite design is 2X vs 1.5X for steel equilivent design. The issue will always be a design problem, and the best would be to design to the best properties of the materials being used. Third .. composites suffer from microcracking and other issues which will limit the laminate strains to approx 4500 micro strain at ultimate load (a rough number ... complicated issue not room to explain here). What this means in plain english is that you can only use approximately 1/3 of the potential strength of the material if you want a structure with long life. A property not unique to composites. Fourth ... composites have no ductility like metals and I would hate to be sitting in a fuselage made of carbon tubes if it hit anything. The tubes would fracture and splinter and the pilot would be in all sorts of trouble. To control this behaviour you would need to add another tougher composite material such as Kevlar to the laminate (common practise in fwd fuselages for gliders to improve crashworthiness). Composites have proven to be very crash worthy in practice but I agree, that the carbon fiber tube frame would not provide the same protection of a tube steel frame. It goes back to designing to the qualities and strength of the material. |
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