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quinn
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+xWhy is it a problem to just cut the material so it has a 45 degree fiber orientation? Orientating the fibres at 45 degrees isn't hard to do with regular woven fabrics? You just then need to have an overlap of material between the two halves to compensate for the loss in strength of the cut fibres. You'll get a bit more wastage per square meter of material, but you also wont have to fabricate a reliable bladder and can just use conventional materials. It will be far easier and more reliable to fabricate, so in my opinion it will actually work out cheaper. yeah that would probably be fine. i think a continuous sleeve would be stronger rather than 2 overlapping halves, but probably not too much difference in strength. the part im having a hard time picturing is how to get the multiple layers to overlap properly within the mold when joining the 2 halves. Is there a good way to do this? as for my most recent bladder idea, its not nearly as complicated as the way i explained in my original post. it would just be 2 layers of plastic seamed together with a soldering iron by guiding it around a cardboard template to form a general shaped balloon that is over sized. earlier i watched a video of a guy using this method to make a bladder for a fuselage, really quick process with a few cents worth of plastic drop sheet. this over sized bladder would be sucked down to a male positive plug and then sleeve applied to the outside and wetted, then inserted into the mold and inflated. To me it seems simpler and allows for easy use of the sleeve so theres no need for trying to get seams overlapping correctly inside the mold. Also the advantage of a lighter part with no breaks in the weave. i guess im just trying to see the advantage of doing it with seams, unless its easier than im picturing
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Hanaldo
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Overlapping seams are very easy with pre-preg, especially for a part as simple as this. You layup both halves of the mould separately. One side you trim flush with the edge of the mould, and the other you leave overhanging the edge by 10 or 15mm. Then you simply fold the overhanging material inwards before positioning the other half of the mould on top of it, just being careful that no material gets pinched in the join. The material will want to return to it's original position, which will position it where it needs to be. You can then either press it firmly into place if you have a tool that you can reach inside the cavity with, or just let the vacuum do that for you - not normally recommended, but on such a simple shape it would work.
Yes, the part may be slightly heavier from the extra material compared to a seamless construction. Realistically you are talking about grams though, which could very well be offset by the fact you are using pre-preg with a precisely controlled resin content, compared to using wet-laid sleeve.
I've seen the video of the RC glider fuselage, and I can tell you from experience that those style bladders are not nearly as reliable as you would like them to be. It takes practice to cut the plastic with a soldering iron so that it creates a good seam, and even then they often burst as soon as you build up any positive pressure. You are wanting to position this bladder inside your sleeve before you wet it out, so what happens when your material is all wet out and the resin is starting to cure and you're positioning your mould halves together and you pump up your bladder and - pop. It develops a tiny hole in the seam and wont hold pressure. All the time and materials that you've committed are wasted. The materials are not expensive, so it may be worth trying for you - for me I couldn't deal with the wasted time and the frustration with making bladder after bladder and only having 1 in 5 work properly.
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Steve Broad
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+xYou are overcomplicating things by a lot. First: a 45° biax cloth (or 45° cut weave) wrapped around the core with just 10mm overlap will give you exactly the same stiffness than a similar thickness. I would just make it on a positive mould/mandrel/plug. If you got a cnc, you could easily machine if from an aluminium piece much smaller than it would be needed for the mould. Than laminate the sleeve over it, compress it by wrapping it wich tape and remove the plug after the resin is cured. Filament winding would give you the ultimate result however. Maybe you could make the aluminium mandrel, and speak to someone with an winding machine (someone here has got an x-wonder, if I remember right). This should not be too expensive. I may be missing something here but, as the boom is wasted in the middle, how would you get the mandrel out? Perhaps made in two parts would work?
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oekmont
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It is 30mm in diameter at one end, and 40x48 at the other. In between the shape should continuously change diameter, because that is the smartest way to do it. Should go off the mandrel quite easily.
With the bladder technique you will likely not get that very last bit of fibre content you would achieve with positive methods or prepreg. This easily negates the advantage of getting rid of a small overlap.
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Steve Broad
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+xIt is 30mm in diameter at one end, and 40x48 at the other. In between the shape should continuously change diameter, because that is the smartest way to do it. Should go off the mandrel quite easily. With the bladder technique you will likely not get that very last bit of fibre content you would achieve with positive methods or prepreg. This easily negates the advantage of getting rid of a small overlap. So how do you pull the mandrel out if the ends are bigger than the middle? I obviously don't understand how mandrels work :-)
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Fasta
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Seems the simplest method if you were happy to use a bought pre preg tube would be to have the tube supplier make your tube to your laminate specs with the 45 deg torsional layers you want? Or you could supply a pre preg tube maker with your own custom alloy mandrel? One good method for internal pressure in moulds is simply a solid silicone block. With your female moulds, layup a mock laminate from fibreglass or other like sheet wax to simulate the correct thickness plus a bit of extra clearance, close the moulds and pour cast a silicone block. Then you can do a pre preg laminate with overlapped split join and insert the silicone block when you close the moulds. No bags, no vacuum needed, the silicone expands a lot with heat. Of course this will only work with pre preg. $200 of alloy is very cheap and high quality mould compared to the effort of trying to make a composite split moulds that are of lesser quality and durability IMO.
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oekmont
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Steve, 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.
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quinn
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+xYou are overcomplicating things by a lot. First: a 45° biax cloth (or 45° cut weave) wrapped around the core with just 10mm overlap will give you exactly the same stiffness than a similar thickness. I would just make it on a positive mould/mandrel/plug. If you got a cnc, you could easily machine if from an aluminium piece much smaller than it would be needed for the mould. Than laminate the sleeve over it, compress it by wrapping it wich tape and remove the plug after the resin is cured. Filament winding would give you the ultimate result however. Maybe you could make the aluminium mandrel, and speak to someone with an winding machine (someone here has got an x-wonder, if I remember right). This should not be too expensive. Yes, machining an aluminum positive would definitely be affordable. It's like 1/3 the amount I would need for machining 2 negative halves. I do like the idea of doing it that way, but a couple questions. The boom goes down to very little taper for the last 300mm or so, zero taper for 50mm. What kind of force is it gonna take to get it out of the mandrel? I know they do actual tubes this way but use a hydrolic press to get the mandrel out. I guess I can machine some kind of apparatus that the base of the boom sits against and use a long bolt to pull the mandrel into it as it's tightened or something like that. Other question, when pulling sleeve over mandrel and compressing with tape, what kind of surface finish could I expect from this? I'm assuming I would need some sanding and clear coats?
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quinn
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oh, 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
fasta, roll wrapped 45 degree tube would absolutely work to directly replace the tube im using now to solve the torsional strength issue, but i would still need the boom supports. im taking it a step further by making a tapered boom to eliminate the boom supports. solves the torsional strength issue and also reduced weight and much nicer look.
sounds like the expanding silicone is a good replacement for the bladder, but then baking is required and im back to needing a custom built oven
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Fasta
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+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.
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