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11-25-2010, 12:11 PM | #1 |
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I am: Trevor B
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Wing Spar Question
I'm starting a new plans-built project with a 72" WS. The plans specify a solid balsa fore and aft spar for the non D-Tube wing. The intent is that the ribs, LE and TE will be the only things showing through the covering.
I have a concern about the spars warping at some point in the future and messing up the wing. I'm more familiar with building wings with upper and lower spars with shear webs. Has anyone else run into this? I'm thinking of building the spars by laminating several thinner pieces of balsa, or else redesigning the ribs. Low airframe weight is a factor. Any other suggestions are greatly appreciated.
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11-25-2010, 05:13 PM | #2 |
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Re: Wing Spar Question
Hi Wymbly,
Just a comment on shear webs. A few years ago my daughter did a Science Fair project on strength of wings using/not using shear webs. It was a fairly extensive project (The project was chosen to got to the Canada wide Science Fair). The one thing I remember is I will never build a wing without using shear webs (with the wood grain going up & down obviously). The increased strength versus weight added was almost unbelievable, something like 2-500% stronger. The strength test involved supporting the wing at each end & adding weight in the middle until the wing fractured somewhere. If weight is a real concern, use lighter spars, but do use shear webs! Even on a 72" ws, 1/8 balsa would be fine for shear webs. Another way of keeping light is to get a scale from CTC etc. & measure each sheet or stick of balsa used. Then pick & choose. And use the right glue in the right amount. Rcjetguy.
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11-25-2010, 07:56 PM | #3 |
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I am: Trevor B
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Re: Wing Spar Question
Hi RCjetguy,
Thanks for the great story about your daughter Congrats to her for exploring that one. Science in action always gets me excited. What did she end up studying? Oh yes, I'm fully aware of the the science behind shear webs. The main bending forces applied to a "beam" (in this case the wing), are transferred parallel into the spars as tension and compression. The main purpose of the shear web is to hold them apart. A consequence is that you can make the wing even stronger with the same materials just by having the spars farther apart and a taller shear web. Ever seen a wood I floor joist? Same concept; lighter and span farther than a comparable solid joist. The plans call for a single rectangular hole in each rib so that the spar can be fed through. I'm trying to avoid having to make two smaller rectangular holes (even though I know that is the best solution). I guess if I had the ribs laser cut, the difference in effort would be minimal. PS, now that I look at my own answer, it all seems like a silly question. I'm building from plans; everything is lots of work; what's a few more cuts, lol. I value the experience from the builders out there
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MAAC #77168 Director - Lakehead Aeromodellers Gravity doesn't Discriminate Last edited by wymbly1971; 11-25-2010 at 08:48 PM. |
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11-27-2010, 07:59 AM | #4 |
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Re: Wing Spar Question
That sounds like one of the older designs one sees around. Goldberg wings were commonly like that. It is obviously a poor design. That being said, it is poor because it does not address the main area were strength is needed, not because Goldberg models would fall out of the sky. The place that needs the most strength is at the top of the spar, the part in compression. The second most strength is needed on the bottom where the spare is in tension. Most materials used to make models are stronger in tension than compression so most wing failures are at the top where the spar is compressed.
In this case, although it may not be the most efficient (i.e. lightest) way to make a wing, I don't see why it would not be working after a few years, assuming it works now. Why would it change. The best spar is an I-beam shape with shear webs. As far as covering the front part of the wing with balsa to make a D shape structure depends on the requirements of the design. |
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