mrbigg

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i'm building an 86" wing span ultimate profile bipe. powered by a zdz80 or brison 5.8. the spars are 1/2 inch square. each wing will have a 1 1/2 inch aluminum wing tube 48 inches long also. now for the question- what should i use, spruce or balsa. i have spruce now, but when i pick them up all at once they sure feel heavy. can i get away with using balsa without the wing exploding? there will be shear webbing also.

BMatthews

With GOOD webbing you could easily replace the 1/2 sq spars with 1/2x1/4 laid flat.

By good webbing I mean vertical grain balsa that completley fills the gaps between the spars rather than glued on sheet on the back and front. In fact I'd stake my sad reputation on the idea that with fully infilled webbing as described and if you can find nice straight grain spruce that you could use 1/8x1/2 caps and it would be pleanty strong enough. This is what we use in sailplanes when carbon isn't used and such spars have wthstood launch line pulls of up to 80 to 100 pounds. And these are much thinner sections than I'm sure you are contemplating. As the spar gets deeper the abilty to withstand the forces goes up by the square or the cube or something of the spacing. The wood will withstand an amazing amount of tension and compression but it MUST be supported to prevent buckling. The webbing is the key. Don't skimp on the webbing.

PS: I wouldn't use balsa spars for this....

mrbigg

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I was going to put the webs on the outside- the lazy way. you dont recomend this at all- even if i end up using the 1/2 x 1/2" spruce spars?

Jimmbbo

We had a rather spirited discussion of shear webs a few months back... do a search to read what the "best minds" thought..

Cheers!

Jim

mrbigg

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thanks man.

SoCal GliderGuider

1/2" x 1/2" is over kill. Do as Bruce says use 1/8" x 1/2" spruce with proper webbing. It is more than worth the trouble. Saves a considerable amount of weight too. Depending on the all up weight you could use 1/8" x 3/8" spruce.

As to your wing joiner: 1 1/2" diameter by 48" long??? Maybe in a full size plane! I use a 1/2" OD x 1/16" wall x 16" total length steel tube in my 118" sailplane that can break a 300 pound test tow line. The tube extents into the carbon tube/spar in each carbon and foam wing about 7". No, I don't bend it either. Do you have a sketch of your plans? need to post an image here so we can see what your structural needs are.

mrbigg

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this plane will be doing alot of 3D style moves. it will be powered with a 95cc gas engine. that's why i need the strength. the plans are from Sig's .40 size ultimate funfly that were doubled in size. with a total of an 86" wingspan- 42" for each wing panel, total of four, and 2" for the fuse width(profile fuse). the .40 size uses 1/4" square balsa for the spars, but it only weighs 5 lbs. since the plans were enlarged, the spars are now 1/2" square. some giant scale guys told me to use spruce.

mrbigg

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i forgot- the total weight should be about 22lbs.

BMatthews

So that's where the size came from. Doubling the plans seldom means you need to double the wood sizes. Just pay attenton to the outlines and re-design the internals to suit your needs. For example the former spacing may not be appropriate to the new size. You should probably plot out more fuselage formers to better support the side from collapsing under load. Here again I'm assuming that you would not be using 1/2 sq for the longerons or double thick sheeting for the sides and stuff like that.

For that weight and if you insist on using the "easy way" then stick with 1/2 x 1/4 top and bottom laid flat and web with 1/16 ply with the outer face vertical grained and glued on front and back out to the wing struts and HARD 1/16 balsa from the struts to the tips. Use clamps to ensure a strong thin glue line and use carpenters glue for the ply to spruce joints. At least that way you'll get as much as you can out of it.

But really for a constant chord wing it's not that hard to rig up a jig and find someone with a table saw and cut out all the 1/2 wide balsa fill in webbing.

And then there's the idea of getting rid of the spruce all together and go for double laminations of .028 x 1/2 carbon. Use two layers on each cap for a total of .056 laminated with slow set epoxy. If you choose this method then you WILL need the 1/2 inch balsa infill webbing right out to the tips but MAN would it be strong. No surprises with a setup like that. If you choose such a method then do NOT drill into the caps for strut mounts. Instead add flanges that tie in under the caps and use appropriatley thinner infill at those points. The flanges can be the same carbon strip with balsa fill or spruce if there will be some compression from screws.

You're building a serious airplane and you need to approach it seriously. Just doubling everything could well lead to problems. Some things will be rediculously heavy in section while other aspects will not be up to the new task.

CafeenMan

I'm not going to give specific advice as large planes aren't my realm. What I do know is that biplanes can have much weaker wings than monoplanes if the wings are tied together (interplane struts).

Shear webs are never a bad idea and if you're willing to do the work to make them right (as BMatthews said) then you can significantly lower the weight of the wing because they strengthen the spars many times over.

mrbigg

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bruce, i tried to tell you last night but my compeuter crashed. i have already notched the ribs for the half inch spar. what do you think about this- 1/4x1/2 inch balsa and spruce laminated together with the shear webs, with the alumimum tubes. this will lighten it up some. what do you think?

BMatthews

I forgot about the aluminium tubes. I'm assuming that those are joiner tubes? So the 48 inch spec is a mistype? I sure hope so. There's certainly no need for 4 feet of aluminium tubing in there.

On the front and back webbing thing see my comments about ply webbing above. DON'T SKIMP ON THIS! ! ! It's a heavy model and will need the ply webbing. The 1/4 inch depth should be OK to ensure a good glue joint but it won't be as good as the full width infill method. With the front and back type you're depending on the crush resistance of the plywood AND the shear strength of the glue joint. With the infill method the glue is just there to fill any minor gaps and keep the infill webbing from moving around. In that case it's strictly the crush resistance of the 1/2 wide end grain balsa. (yeah, I know I sound like a broken record... )

And a reinforcement to be sure to use the yellow aliphatic resin glue for the webbing. CA often does not like the sap residue or other factors in spruce and plywood and can cause problems in the joint strength. Aliphatic glue won't suffer from that but it will require a even wide spread clamping pressure to squeeze the glue joints to a tight fit. Not a crushing fit but firmly in contact. You'll need a bunch of small clamping pads made from something like 3/8 or 1/2 MDF board that just fit in between the ribs. A small C clamp tightened to about the same torque as you would tighten a 4-40 engine mount screw will provide the right clamping pressure. This is all standard wood working but not all modellers have any cabinet making experience so I hope I'm not being too elementary.

Even with 1/4 x 1/2 caps you need to closely study the grain for bad runout. If the grain runs from side to side or face to face in less than 8 to 10 thicknesses then you risk the spar failing. A good test would be to grab the ends of your 3 foot stock and bow it up and down so it arcs about 6 to 7 inches to the flat side. You'll feel it when it really tightens up, take it to that point and hold it for a second or two. If it fails suddenly with a short angled snap then it was no good for you in the first place. Study what the grain lines look like in that case and avoid any further stock that shows the same sort of pattern. THIS IS CRITICAL! Seriously, I've seen spruce spar stock that snapped like a Ritz cracker under the same treatment. I used that wood for mixing epoxy after that (and it broke even then... ).

mrbigg

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i reread the plywood web instructions from earlier. don't put it beatween the spars right? before and after right? i was going to use a 48" wing tube. too much? how about 36"? what for the root ribs-1/8" or 1/4" aircraft ply. i have another question for you to that's kind of off topic- the font half of the fuse is sheeted with a plywood doubler. i will upload a pic for you. this doubler will from the front to just past the lower wing root rib.

mrbigg

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here's the pics. this will give you an idea of how big it is. your right this is a serious plane and i really need the help of you pros out there. thanks for all your help!

BMatthews

I didn't realize it was a giant PROFILE model..... [X(]

Even a 36 inch tube for the joiner is way too much. The loading only happens right at the joint and into the tube about 2 or3 diameters to either side. Beyond that it's only extra weight. 8 to 10 inches into each panel is plenty. Call it two or three rib bays, whatever fits.

If you have two telescoping sizes then what you need to do is use the larger as the reciever tubes and the inner as the load tube. The reciever tube is boxed in between the spars and the joint between the tube and the spars needs to be "bondo'd" in with a mix of epoxy and microballoons or chopped fiber to truly bond the receiver tube to the spars. It's this high strength bonded joint of the tube to the spars where the bending loads at the inner joiner tube are converted to tensile and compresive forces in the spar caps. The joint MUST be solid. Glider flyers know this and typically fill that void up as mentioned above and then bind the root joint with kevlar thread as well to prevent the high bending loads from splitting this critical joint. Obviously for this to work the tubing has to fit between the spar caps. If your tubing is larger than that then I don't see how it can work. The bending loads MUST be placed between the spar caps for this to work. If you try to put them out to one side and transfer them through to the spar then it'll take some fancy engineering. Something like thick plywood root ribs would be a MINIMUM and even then I'd have serious doubts about the loading for such an aggresive use large model.

Now diameter, wall thickness of the actual joiner and type of alloy is what will be the big concern here. It's a 22lb model that's going to be pulling some 10 to 13 G's when pushed hard so the tubing needs to be pretty sturdy. That's something like 220 to 300 lbs split 4 ways. So you need tubing that can withstand about 60 pounds without bending. Adding in a safety factor of say 2 and that means the tubing needs to withstand at least 100 lbs of bending stress without any deformation. You may want to try that out before commiting to what material you're using. To support the tube for a test drill a close fitting hole through a couple of bits of 2x6 crossways so the side grain supports the round shape of the tubing. Bolt down one of the 2x6 pieces to a fence or arrange some other way to hold it so it's firmly held with a one foot stub of your wing joiner tubing held 1/2 in the block. Now put the other block over the stub and press down on the center of the "paddle" with increasing weight up to at least 80 lbs. If the tubing fails before that it's not the right stuff. If it holds but it's bent after the test then it's not the right stuff either. It needs to take the load and spring back without any problems. If it won't then you need to find some other tubing or otherwise change how you'll join the panels.

Something to think about. This IS a biplane so you have an option. If you don't make the interplane strut funtional or if it only has minimal functionality then the wings will bend under load just like a regular monoplane wing but they'll do it together. But *IF* you added truly functional wire rigging then the two wings are joined together into a SYSTEM where the wings themselves act like the top and bottom caps of a single spar and the wire X rigging and support struts between them act like the webbing we've been discussing. With that format the rigging wires and struts carry much of the G loading. Also when you work it that way the bending loads on the joiner tubes are minimized and the forces become primarily shear loading trying to scissor the tubes.

The fuselage-

It's a good thing the plywood goes back that far. The vibration and side loads from a 95 cc engine are going to be extreme. Frankly I have my doubts about the ability of the fuselage to withstand such loads. It's built now and I'm sure you'll go through with the project but please be sure to avoid any runups while in front of or to one side of the engine for quite some time.

Remember how we discussed how the spacing of the spar caps is related to the loads the spar can withstand? The wider the spacing teh better. With a profile the sides (spar caps) are not that far apart. The loads they will have to withstand are going to be very high. It's possible that you won't have a problem at first but the vibration may cause it to develop issues later. At the very least a daily check for vibration cracks, loose joints or new levels of flex in any part of the nose or wing joint areas is a must.

If this was my model I'd use center line side strakes similar to what the latest depron parkflier 3D models are using to help stiffen the fuselage very effectively while not really adding that much weight. Such side strakes would be easy to add at this point in the construcion phase and must might save you a lot of "embarassment" later. Buld them using the same method and material you used in the fuselage. In case you haven't seen this idea before here's a pic....



and you can get a better idea of it at [link=http://www.hobby-lobby.com/field_shockflyer.htm]THIS PAGE[/link]

It sounds like I'm being alarmist in all this but you have to realize that when you double up a model you don't only just double up the loading. In particular you went WAY more than double the available power for this project. Probably more like 10 or 15 times what the original model has for a power loading. On a positive note it looks like a great project. Hopefully what's coming out in this thread will help rather than hinder.

mrbigg

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the 95cc wll be max. if it's light enough maybe a 70cc engine will work. so you don't think i should use the light ply for doublers? get all the strength i can get right? i like the way the shock flyer is built. what if i take. the plywood doublers just like before, but add lets say about a 2 to 3 inch wide strip down the center of the fuse, all the way to the end, just below the stab. perhaps some carbon fiber strips on the top and bottom. the fuse with the foam and balsa sheeting is 1 2/16" thick. think the carbon will help? how thick should it be?

BMatthews

To my mind even a 70 CC engine is a big heavy lump that shakes like a paint mixer. Lite ply is poplar and poplar tends to be brittle and does not have many longer fibers to carry vibration or structural loads well from the stuff I've seen. For example if you break it there's not a lot of splintering or fiberousness to the break. Do the same with birch ply and you'll find it has a very ragged break.

I gather you'll be adding the ply OVER the balsa? That'll help spread the stresses from the ply to the balsa but what I'd use is 3/32 birch ply with graduated small to larger lightening holes cut in back near the rear. It'll not be that much heavier but would be MUCH stronger than solid lite ply. I'd start the lightening holes about 6 inches back from the engine with only few small ones and by the time it gets to the pylon the holes could be quite large. The gradually spreading and tapering "web" of wood is what would spread out the vibration evenly to the structure. "Hard" transitions is what you want to avoid. Those just form stress risers where a crack will happen.

Now I'm not that big on big models but it would seem to me that the side spines can be say 1/2 inch thick and about 3 inches wide per side? Or 2 inches if a 6 inch width would bother your senses. I wouldn't go less than 2 though. The simple way out would be to make them from 1/2 inch sheet balsa with a 1/4 x 1/2 spruce outer cap. Plane the cap down to taper to nothing at the end of the 3 foot length and just leave the balsa spine to run to the tail. Arrange the splice joints so thee joints don't end at the end of the spruce. Also they should be tapered over an angle of about 20 degrees. Nothing steeper or you just have a butt joint that isn't as strong and will futher produce a stress point. Built up would be that much lighter but you'd need to make it with diagonals and stuff to properley web the parts together. Sort of like this this sketch below. In this the flat upper is 1/2x1/4 balsa on edge, the outer curved part is the same in spruce and the left side fill is 1/2 sheet at the nose. To lighten the rest where it's not so critical I've shown a criss cross of 1/4 sq balsa. This Warren truss arrangement will ensure the most stiffness with least weight. The top and bottom could then be sheeted or just covered. Sheeting would be best of course. And if it was sheeted then the Warren truss could be replaced with just straight up and down pieces of 1/2x1/4 on edge. Or even a piece of 1/2 inch foam. But do NOT miss out the balsa infill at the forward end. There the vibration will be highest and if you just use foam the foam would probably self powder itself.

Here's a quick sketch of what I'm suggesting....

mrbigg

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yes the ply will go over the balsa. can you sketch me your recomendation of the lightening holes. thanks.