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Old Dec 03, 2014, 04:27 AM
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Joined May 2014
53 Posts
Question
Wing Spar Length

Hi, what is the typical length of wing spar vs. wingspan? I am going to build twin boom made of 5mm depron, and I am trying to save as much weight as possible. The wingspan will be 1.5m and I will be flying it casual mild acrobatic.

Sorry if my question had been asked before, I tried to find the answer but none so far.

Thanks.

Martin.
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Old Dec 03, 2014, 04:54 AM
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Joined Apr 2013
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A lot depends on the wing planform, wing loading and spar type.
And you inevitably push the limits and end up pulling 6 Gs or more.
If the spar is CF tube, they are so light it doesn't pay to skimp, and I'd go with at least 3/4 of span.
For a tapered wing there's going to be less stress outboard, so the spar can be shorter (like 2/3 of span).
You can even get away with 1/2 of span if you don't mind a bit of "floppy" control feel and are careful not to pull hard Gs.
If you've got extremely light wing loading you can get away with very little spar (or even none).
Having said all that, I just finished a 65 inch (1.65 meters) span rectangular wing with spar of about 60 inches. It's very nice to see that wing not flex at all at any speed/G.
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Old Dec 04, 2014, 12:46 AM
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When using flat sheet wings, I prefer to run the spar full span, or as close to it as possible while positioning the spar on the projected cg. It seems like the wingtips always get bent over time if the spar ends short. Stress from "rough" landings gets concentrated at the point where the spar ends.
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Old Dec 05, 2014, 07:40 PM
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Joined May 2014
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Thanks for the inputs, guys. The spar is 2 x 8mm OD cf tube. I will try 3/4 and see how it goes.

Thanks again.

Martin
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