By quinn - 7/27/2018 8:41:09 PM
hello everyone, im new to this forum as well as working with composites. I am designing and building performance rc helicopters using fusion 360 and a cnc that i designed and built. i have already finished my first prototype 700 class (700mm blades) heli and it was a huge success but there are a couple improvements i would like to make. One slight issue the heli has is tail vibrations at very high rpm. currently for the tail boom i am using roll wrapped 1mm wall, 0/90 degree carbon fiber tube. It is 30mm OD and 800mm long. it has excellent bending strength but because of the 0/90 weave, it is lacking in torsional stiffness which i am convinced is allowing the vibrations at higher rpm. I have actually confirmed this before by trying the same kind of tube on other existing heli designs which had an aluminum boom as stock. In my opinion, the solution is adding 45 degree weave with a braided sleeve to give it torsional strength. Using aluminum is another option but for best performance possible, weight is very important and use of carbon fiber is certainly going to make a lighter part for same strength. Easiest solution would be reinforcing an existing tube with braided sleeve on the inner or outer surface but i would like to take it one step further. usually these booms are braced by 2 rod supports at an angle reaching about 2/3 of the way down the boom, but a couple of manufacturers have made heavier duty tapered carbon fiber booms that dont need boom supports. Overall it looks better and can save even more weight. this is what i would like to do on my heli. I have attached 2 pics, one of the whole heli with the tapered boom on it, and the boom by itself. the boom is 800mm long, 30mm diameter out at the end and 40mm wide by 48mm tall at the base. this isnt the final design, i just quickly modeled it to give a representation of what it will be similar too. I am thinking most of it will be about 1mm thick wall with some extra layers at the base to bring it up to 1.5mm or so. fasteners will pass through the wall of the boom and thread into aluminum pieces placed against the inner wall of the boom so the boom wall is clamped against the side frames of the heli.
so here are my questions. Considering i have a cnc at my disposal, what method is going to give the best results for an efficient part? As i mentioned, weight is very important so every gram counts, which means i would like to have as low of ratio possible of resin vs carbon fiber. since i need to have continuous 45 degree weave for torsional strength, that means i need to use braided sleeve which im sure complicates things since i cant just join 2 mold halves together after wetted out fabric has been layed in each side.
here is what i picture working best, but i dont know exactly how to do it. 2 female mold halves and an air bladder to push the weave tight to the walls, but since im using sleeve, i need to be able to apply the sleeve layers to the bladder (partially inflated?) before laying it into one mold half, then bolt on other mold half, then inflate bladder. im assuming this means i need to make a custom bladder that can be partially inflated that will be slightly smaller than the final size so it can be layed into the mold after weave has been applied and wetted out. i assume this type of bladder would also be made in a mold of its own, but needs to be smaller than the final mold.
I have plenty of other questions about what to mill the molds out of and how to make custom bladders, etc. but i figured i should start by asking if i even have the right idea of how to tackle this, or if theres a better or easier way. any advice is greatly appreciated.
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By Steve Broad - 7/27/2018 10:55:14 PM
+x+x+xhello everyone, im new to this forum as well as working with composites. I am designing and building performance rc helicopters using fusion 360 and a cnc that i designed and built. i have already finished my first prototype 700 class (700mm blades) heli and it was a huge success but there are a couple improvements i would like to make. One slight issue the heli has is tail vibrations at very high rpm. currently for the tail boom i am using roll wrapped 1mm wall, 0/90 degree carbon fiber tube. It is 30mm OD and 800mm long. it has excellent bending strength but because of the 0/90 weave, it is lacking in torsional stiffness which i am convinced is allowing the vibrations at higher rpm. I have actually confirmed this before by trying the same kind of tube on other existing heli designs which had an aluminum boom as stock. In my opinion, the solution is adding 45 degree weave with a braided sleeve to give it torsional strength. Using aluminum is another option but for best performance possible, weight is very important and use of carbon fiber is certainly going to make a lighter part for same strength. Easiest solution would be reinforcing an existing tube with braided sleeve on the inner or outer surface but i would like to take it one step further. usually these booms are braced by 2 rod supports at an angle reaching about 2/3 of the way down the boom, but a couple of manufacturers have made heavier duty tapered carbon fiber booms that dont need boom supports. Overall it looks better and can save even more weight. this is what i would like to do on my heli. I have attached 2 pics, one of the whole heli with the tapered boom on it, and the boom by itself. the boom is 800mm long, 30mm diameter out at the end and 40mm wide by 48mm tall at the base. this isnt the final design, i just quickly modeled it to give a representation of what it will be similar too. I am thinking most of it will be about 1mm thick wall with some extra layers at the base to bring it up to 1.5mm or so. fasteners will pass through the wall of the boom and thread into aluminum pieces placed against the inner wall of the boom so the boom wall is clamped against the side frames of the heli. so here are my questions. Considering i have a cnc at my disposal, what method is going to give the best results for an efficient part? As i mentioned, weight is very important so every gram counts, which means i would like to have as low of ratio possible of resin vs carbon fiber. since i need to have continuous 45 degree weave for torsional strength, that means i need to use braided sleeve which im sure complicates things since i cant just join 2 mold halves together after wetted out fabric has been layed in each side. here is what i picture working best, but i dont know exactly how to do it. 2 female mold halves and an air bladder to push the weave tight to the walls, but since im using sleeve, i need to be able to apply the sleeve layers to the bladder (partially inflated?) before laying it into one mold half, then bolt on other mold half, then inflate bladder. im assuming this means i need to make a custom bladder that can be partially inflated that will be slightly smaller than the final size so it can be layed into the mold after weave has been applied and wetted out. i assume this type of bladder would also be made in a mold of its own, but needs to be smaller than the final mold. I have plenty of other questions about what to mill the molds out of and how to make custom bladders, etc. but i figured i should start by asking if i even have the right idea of how to tackle this, or if theres a better or easier way. any advice is greatly appreciated. Seems too complicated to me. We made something similar recently, a prepreg carbon/Aramid hockey stick :-)  Split mould plus a bladder (heavy duty long balloon). Mould was made out of fibreglass but for your project I would use aluminium as this will give you a more accurate finish, IMO I think that, in order to keep weight to a minimum and use the split mould method, prepreg is the way to go. Any suitably sized second hand domestic oven will do for the curing process. The trick with using aluminium moulds is making sure that they get up to temperature otherwise the resin doesn't flow properly and the surface finish can be compromised. My experience ( and everyone has different experiences :-) ) has resulted in longer dwell times at initial cure temp prior to going to final cure temp (ie ramp to 80 deg C and stay at 80 for 30 minutes before going up to final cure temp) I have found that this lets the mould temp catch up with the carbon temp allowing the resin in contact with the mould the chance to flow properly. Regards Steve i agree your method sounds MUCH easier, but theres a few issues. Main one being that an 850mm long x 60mm x 60mm or so sized mold does not fit in an oven, at least mine, i checked. second issue is that i really want 45/45 weave running the full length continuously which means a braided sleeve like i mentioned. this is why is doesnt work for me to just lay into the mold halves and put it together. That one is not as much the issue, i would be willing to just try the prepreg cut at a 45 with seams. probably would still have great torsional strength, just not quite as good as the braided sleeve. Next issue, i definitely have the capability to make some really nice polished aluminum molds, but for 2 chunks of aluminum that are 850mm long and 30x60, thats like 200 bucks worth of aluminum. this is a prototype that may need to be revised so thats gonna get crazy expensive fast. i would prefer to use something cheaper at least for the first version if possible. so considering the limitations, oven not big enough, preferably molds cheaper than aluminum, and the ability to use continuous braided sleeve, what would be the next best way to do it? i should also mention im doing this as a hobby. if i wanted to make a few hundred of these, i would definately do aluminum molds and figure out another option for an oven, but i will only be making a few of these. just for my own use and some spares. i would like to keep the whole project under a couple hundred bucks to make this boom. one idea i had for molds was glueing together a few layers of mdf, machine the mold plus na few mm offset of extra material removed, fill it back in with bondo, machine the final shape into the bondo, then give it a proper mold coating In that case, make an oven the right size :-) 50mm insulation board between ply. Line with silver foil and heat using light bulbs. Works for me :-) Never used sleeving. However, if I can make a hockey stick that survives the shock of hitting a hard ball VERY hard I think my method would be sufficient for a helicopter boom. The same method is used for glider fuselages which have to cope with high stress levels. With regard to mould material, you could try EC's EB700 High Temperature Epoxy Tooling Board, but this isn't cheap either. Not sure whether MDF would survive the temps without distorting. It will also give off a lot of moisture that could affect the resin. You could make a pattern using a cheaper foam, then make the two part mould out of fibreglass using high temp gel coat and resins. |
By quinn - 8/1/2018 3:10:05 PM
Another question on this, could a compression mould work? Male mould milled smaller by calculated wall thickness, wrapped in lay up (makes use of sleeve easy) then drawn into female mold with plenty of pressure to consolidate layers and squeeze out extra resin. Currently the ends are the only areas without taper, but I can adjust the design to have a slight taper at least on the inner wall to make removal easier. I would think this would make the lay up process super simple and make an excellent part that has great consolidation and a smooth finish inside and out.
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