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Old Sep 24, 2011, 05:38 PM
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Help!
First scratch build

Hi Everyone

I have been building and flying arf's and kits for a while with much success and now i'm ready for the real thing

but the only problem is that I don't really know where to start, I have searched this forum and can't really find any beginners information.

The first thing that baffled me was how you guys get the airframes to be so 'even' then I found out about these cad programs.

The CAD programs seem great at allowing you to plot out a plane but the few that I have looked into, I can't seem to find out how to print out the design, it seems like you would be able to print out a template for each section that you can then follow in foam but I can't find that option, I'm thinking that the fuse for example will have a paper former template every couple of inches or something along those lines?

Do the programs do this can you print out sectional guides to follow or am I on the wrong page.

Also what weight of fiberglass should I use, i see everyone uses different, what should i be using on a fast 90mm!

I have seen what looks like people building the plane in foam then glassing it and then hollowing out the foam, is this right or am I missing a step?

Once I have a good grasp on everything and made a design I will start my project with a thread then you guys can laugh at what I do wrong!!!

thanks everyone

Kim
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Old Sep 24, 2011, 08:16 PM
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Kim

can't really answer your questions about CAD programs, since I do everything the old fashioned way...........pencil and paper. by the time I learn CAD, I could have 3 or 4 plans drawn on paper. LOL.

as far as fiberglassing goes, it depends what you have underneath. in other words, what type of material as your structure. for example, if it's a balsa build, you don't need to glass for strength, so 1oz cloth is good enough. for foam, you are going to need at least a 2oz cloth, since it's going to be adding some strength and durability to the airframe. or if you have 1oz cloth, do two layers of cloth and lay them up in a criss cross "X" pattern for bi-directional strength. two 1oz layers done this way is actually stronger than a single layer of 2oz cloth, so something to think about. for a fast 90mm plane, I would do 3 layers of 1oz cloth. one in a 45 degree angle, then one along the length of the airframe, then the last layer to create the "X". this will give you the most strength for the least amount of weight. keep your layups light by using peel ply on the outer edge. this allows you to squeegee out excess resin and keep the saturation to the bare minimum needed. hope that helps for now.

for formers, every 6 or 8 inches is fine if it's a big model. if it's smaller, size accordingly. for wing ribs, every 4 or 5 inches should be fine. again, if it's smaller, size accordingly.

to answer your question about foam and glass............I do see some people doing what you suggested and then hollowing out the foam, or disolving it. this is the "lost foam" method. I believe there are cases where this is needed or even necessary, but for the most part, I would say avoid this. why?, because the foam actually adds strength if it's designed right. let's say you are going to hotwire a fuselage out of a block of foam. you make your templates, then get them straight on your block and cut the outer edge. then you possibly cut your fuselage in half and using your templates, cut the inner edge to give you a thin walled fuselage. perhaps a wall thickness of 5mm or so. then you glass the inside and put the two halves back together and glass the whole exterior. using 1oz cloth on the inside and outside, you create a 3 layer composite sandwich, which is incredibly strong and light, yet you didn't have to somehow do anything with the foam except use it as structure. there are also good points about cutting a fuse out of foam and then simply glassing over the entire surface, then melt the foam out with a solvent, ;eaving you with a glass shell. if you are going to use this method, I would suggest the glassing method described above, using three layers of alternately layed up layers of cloth. it's up to you which method you choose, but I prefer leaving some foam there for the added strength it provides. remember that the more sandwiches of material you have, the stronger the overall structure will be.

I don't think anyone on here will laugh at you once you get going. just remember to take your time and ask what needs to be asked. even if you think it's a stupid question. the only stupid question is the one not asked. if you aren't sure, ask. if you don't know, ask. even if you think you know, ask anyway. I've been modelling for 30+ years and I still ask questions, because I like to hear other people's perspectives too. I'm still learning as much as I am teaching. if you go slow, step by step and ask questions before you complete a step, you won't go wrong and you'll end up with a completed plane. everyone on this forum will see that you get where you want to go. good luck and we'll see you in a build thread, real soon.

Rich
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Old Sep 24, 2011, 08:36 PM
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Rich

You are the man, I see you in everyones threads helping out, you are an asset to the community and you have told me things in this one response that I might never have found on my own.

Thank you so much

I'm going to go and buy some materials, the only problem is that since i'm 'new school' i think i will need to figure out the cad programs to start building ha ha!!!

Kim
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Old Sep 24, 2011, 08:48 PM
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any time Kim. what kind of plane do you have in mind? I'm assuming by "fast 90mm", you are looking to do something along the lines of a sport jet, like the BVM Bandit?

I'll make a suggestion, if you don't mind me doing so.............for your first scratchbuild, I wouldn't be scratching your own design, simply because there's so much to do, without trying to figure out the build aswell. if it were me and it was my first scratchbuild, I would choose an existing design, that already has a plan available, then use it to cut your own parts and build the plane. this will give you some experience with how an why something goes together the way it does and this will translate into ideas for your own design. plus, you can design yours while you build the other, which will also translate into the way you design your own. for example, if you see something that you don't like on the current build, you can say to yourself, "I don't like how this is done and I can do it better on my design by....................." know what I mean? if you are looking for a sport jet, Steve Shumate has a parkjet sport jet that looks identical to the BVM Bandit and it's on this link at the bottom of the page. just download, print and build it.

http://www.parkjets.com/free-plans/c...umate-jetset44

see what you think anyway.

Rich
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Old Sep 24, 2011, 08:53 PM
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I was actually thinking of using an electra as a template but after seeing those plans i think i will start there,

I guess one of my main concerns is taking this 2d shape and converting it into a 3d object, i suppose i just start going then shape it how I think it will look good?!

Kim
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Old Sep 24, 2011, 09:02 PM
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that plane flies very well. if you want it larger, simply scale up the plan to the size you want. you can always change certain aspects of the plan to suit your own needs. for example, if you know the wings are built as a flat plate on a surface, then add wing ribs to form a curved upper surface, I would change that when building a larger model. I would choose a symmetrical airfoil that's the same thickness and hotwire a foam core wing based on the size required. things like that.
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Old Sep 24, 2011, 09:04 PM
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remember that thinner wings fly faster, since they have better penetration, but they don't lift as well as a thicker wing. what you'll have to do, is measure the total thickness of the designed wing at the size you want, then match a symetrical foil to that thickness and hotwire that airfoil into a wing panel. it's as simple as that.
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Old Sep 24, 2011, 09:20 PM
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ok cool, so if i go to symetrical foils then I basically make the top half twice and stick them together right, but if the model is bigger then i need thicker to carry the weight!?

Kim
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Old Sep 24, 2011, 09:52 PM
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if you scale up the plans, we need to know how thick the thickest part of the airfoil is, based on the length of the airfoil section. that's its thickness. so if the airfoil is 100mm long and the thickness is 10mm, the thickness is 10%.

remember, we're not talking about the wingspan here, just the length of the actual airfoil shape, or the length of the root chord from front to back and the tip chord from front to back.
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Old Sep 24, 2011, 09:52 PM
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I'm betting it's probably around 8 or 9% thick, give or take.
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Old Sep 24, 2011, 10:16 PM
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awesome, that makes total sense, scale up in 3d when i scale up in 2d.



Thank you

Kim
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Old Sep 25, 2011, 12:06 AM
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What glue do you use for depron?

thanks

Kim
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Old Sep 25, 2011, 07:49 PM
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depends on how fast you want to build. I personally like Elmers spray glue, foam safe CA, Gorilla glue and Epoxy. use Eopxy sparingly though, cause it's heavy as hell Gorilla glue is amazing, but you've got to stay with it while it cures, cause it foams out of the joint and you've got to constantly wipe off the foaming glue. it gives a bond equal in strength to Epoxy at half the weight. for general construction, I use Elmers and foam safe CA. the CA of course bonds instantly, so it's great for structure and Elmers can be used for just about anything too. if you use Elmers, make sure you let it tack up a bit first before you bond the pieces. another glue you can use is UHU POR. it's available online and is supposed to be excellent for foam. I can't comment specifically though, since I've never used it.
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Old Sep 26, 2011, 02:17 AM
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cool I am going out tomorrow night for supplies so i will find some glue at home depot, probably end up being elmers.

I'm going to blow the plans for the Steve Shumate super bandit, I want to make a 90 or 120mm, i'm not sure which yet or how much I will have to blow up the plans, I guess I will measure it all tomorrow and figure it out from there.

Do you know what size of edf the plans are supposed to be for?

thanks

Kim
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Old Sep 26, 2011, 11:13 AM
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I believe that at 100% scale on the plan, it's for a 70mm fan. so for a 90mm, you may have to scale it up 25% I'm guessing. you need 20mm more space in the fuselage, but more important are the size of the inlets. you need 100% FSA or slightly smaller if you want to go fast. if you are trying for speed, a good combo of inlet to exhaust is 85% FSA inlets - 100% fan (obviously) - 85% exhaust. if you can scale up the plan and get somewhere close to that, you will be going fast. of course, you can always scale the inlets down a bit if you need to go bigger on the plan, but those are sizes to shoot for. just remember that the smaller you go on the inlets and exhaust, the more room you'll need for taking off. bigger means more static thrust and smaller means more dynamic thrust for top speed. 85% on the exhaust is a good compromise between both and somewhere between 85-100% is a good compromise for both on the inlet side. something to keep in mind.

Rich
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