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quinn
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+x+xoh, i missed a bunch of posts. yes, the boom is tapered the whole length except for a bit of zero taper at the ends, so yes a mandrel would come out. hanaldo, you make good points about the bladder, indeed i would be pretty much screwed if i went through all the layup and then had a leak when inflating. a lot of risk there. sounds like prepreg seaming is no big deal, it make sense now that you explain it, but still the oven size problem. i badly want to make the sleeve work, aside from saving the few grams, it would sure look sweet having it absolutely seemless. i know saving a few grams sounds rediculous, but in this case it matters. With performance rc helicopters used for aggressive 3d flight, you want absolute minimal moments of inertia for snappy responsive cyclic inputs. removing grams at a far distance from center of gravity (out on the boom) makes a huge difference. im using titanium fasteners in the tail case to save like 3 grams and its worth it. All that being said, if the sleeve isnt gonna work, thats just how it is i guess. and like mentioned, i could end up heavier by trying to do wet layup even though theres no overlap. maybe i should consider the homemade oven with light bulbs idea for the prepreg with seams. i imagine you need to monitor this closely to maintain proper temp, or have thermostat controlling the light bulbs You could also consider putting electric elements in the back face of an alloy mould? Search the poster Bladerunner in this forum, I think he has some posts about it. No oven needed. ah, interesting. yes i can see how that would work. some heating elements, temp sensor, and a cycle to keep needed temp. ill look into that for sure. its not that im not willing to spend a couple hundred bucks for really nice molds, its just the fact that its a prototype that may need design adjustment. i guess its probably most likely that the mold design will be fine how it is, but maybe layup and thickness need to be adjusted which doesnt require new molds, so ill consider going aluminum. definately would make a really nice finished product. sounds like prepreg in a 2 part aluminum mold with either vacuum or silicone pressure its going to give the best results within my abilities. sleeve or filament winding i think has higher potential, but not within my abilities to get it to its full potential
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Steve Broad
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+xSteve, maybe I missed something, but to my understanding, the boom isn't going to be thinner in the middle. The diameter never drops under 30mm. Except for design and aerodynamic decisions there is really no reason why it should be. So you could easily pull out the mandrel to the bigger side. To get something really light, you should keep the material stresses constant over the length of the part, wich will take you to a smooth wall thickness reduction towards back rotor. The tapered crosssection basically works the same way, but to my estimation doesn't completely compensate the difference in crosssection loads. The anti torsion layer however should be constant over the whole part. No, It's me misinterpreting the photo, it looked to me that the centre section was thinner than the ends but it was just an optical illusion :-)
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Steve Broad
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Group: Forum Members
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+x+xSteve, maybe I missed something, but to my understanding, the boom isn't going to be thinner in the middle. The diameter never drops under 30mm. Except for design and aerodynamic decisions there is really no reason why it should be. So you could easily pull out the mandrel to the bigger side. To get something really light, you should keep the material stresses constant over the length of the part, wich will take you to a smooth wall thickness reduction towards back rotor. The tapered crosssection basically works the same way, but to my estimation doesn't completely compensate the difference in crosssection loads. The anti torsion layer however should be constant over the whole part. No, It's me misinterpreting the photo, it looked to me that the centre section was thinner than the ends but it was just an optical illusion :-) How do you get a smooth exterior finish when using a mandrel, without adding additional weight?
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oekmont
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It's easy: you don't.
But all regular roll wrapped tubes are made that way. With a bit practice and the right materials you can get a finish like commercial tubes, wich for most people will be good enough.
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Fasta
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+xIt's easy: you don't. But all regular roll wrapped tubes are made that way. With a bit practice and the right materials you can get a finish like commercial tubes, wich for most people will be good enough. Do you have to finish it at all? The spiral tape finish is often quite acceptable to look at although it would have poor UV resistance if left in the sun. Since you are still developing/prototyping don't worry about it.
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oekmont
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Most research I've read so far comes to the conclusion, that uv resistance has a very low to zero effect at the mechanical propertys of crp. The fibres itself got very strong uv resistant molecular bonds and protect the deeper layers of the resin from degradation. So the major problem is the yellowing of the outer resin. An aesthetic problem.
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quinn
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+xMost research I've read so far comes to the conclusion, that uv resistance has a very low to zero effect at the mechanical propertys of crp. The fibres itself got very strong uv resistant molecular bonds and protect the deeper layers of the resin from degradation. So the major problem is the yellowing of the outer resin. An aesthetic problem. Aesthetics is definitely a concern. Doesn't need to look great as a prototype, but i need to develope a process that will eventually be capable of making a really nice looking part once prototyping is done. Is this sort of thing ever done with both a male and female mold with calculated wall thickness between? With a tapered part like this, I could imagine that making a very efficient part. Layup is done on the male (could be sleeve) then is drawn into the female mold with with some kind of draw bolt mechanism to really put some intense pressure on the walls. Obviously that would only work with a tapered part, but that's what I have. Right now prepreg with seams sounds like the most realistic option and I just need to figure out the heating, but I want to explore any other options that could potentially make a better part
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Fasta
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Sounds like maybe you should just commit to the quality split female alloy moulds and make a start. The moulds can be used with a number of different techniques so you can change your mind or try other things with the same moulds. Your going to have to just get into it and learn along the way, that's how it goes. Maybe just make the moulds a little over length at the ends to allow for some excess part or fitting in bladder ends etc. Positive pressure for internal bladders from bag material or other, inner tubes? Expanding core like expancell Expanding silicone core Internal bag with external bag and vacuum. Others?
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quinn
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Group: Forum Members
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+xSounds like maybe you should just commit to the quality split female alloy moulds and make a start. The moulds can be used with a number of different techniques so you change your mind or try other things with the same moulds. Your going to have to just get into it and learn along the way, that's how it goes. Maybe just make the moulds a little over length at the ends to allow for some excess part or fitting in bladder ends etc. Positive pressure for internal bladders from bag material or other, inner tubes? Expanding core like expancell Expanding silicone core Internal bag with external bag and vacuum. Others? yeah thats true, no matter what im gonna need some nice female molds so i guess start there and experiment. maybe even start with fiberglass while experimenting to keep cost down. so if doing this with vacuum, i assume you just have to push the bag through the length with a long stick or something, then the remainder of it gets doubled back over the outside of the mold? so what i really need is a long tubular bag big enough for the mold to fit inside, and a little more than double the length of the mold?
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Fasta
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Group: Forum Members
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+x+xSounds like maybe you should just commit to the quality split female alloy moulds and make a start. The moulds can be used with a number of different techniques so you change your mind or try other things with the same moulds. Your going to have to just get into it and learn along the way, that's how it goes. Maybe just make the moulds a little over length at the ends to allow for some excess part or fitting in bladder ends etc. Positive pressure for internal bladders from bag material or other, inner tubes? Expanding core like expancell Expanding silicone core Internal bag with external bag and vacuum. Others? yeah thats true, no matter what im gonna need some nice female molds so i guess start there and experiment. maybe even start with fiberglass while experimenting to keep cost down. Exactly, it's more about refining the process. Heated moulds will work great but you may as well build a small oven as then you have a more versatile setup for making other parts too.
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