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Building Tips and technical articles. => Building techniques => Topic started by: Balsa Butcher on August 13, 2011, 10:11:19 AM
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I'm working on a Shark 35 wing. The plans show 1/4" spars with 1/16' webbing in between each rib. The orientation is not shown on the plans. Any opinions on how the should be attached - Top to bottom, parallel to each other? or Top to bottom, bottom to top which would make them roughly perpendicular? Does it make any difference?
Plans show them attached only to the rear of the spar. Putting one rear and one in front of the spar in an "X" pattern would add strength at the cost of weight so I'd prefer to avoid that option. Another would be to leave them off completely but with wing gear, the webbing is probably worth the slight weigh penalty. BTW: this is an RSM kit and it is excellent! Thanks in advance. 8)
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If the wing has leading edge sheeting, then the spar webbing will make an immense difference to the torsional stiffness of the wing -- the sheeting plus the webbing make a closed tube (hence the term "D-tube").
Normally you put the webs with the grain running vertically. If you're even more insane than I am about detail you could make up some 1/16" balsa plywood from 1/32" sheet (Ambroid makes a good glue, but you have to double-glue everything). Then you could make the shear webs from the sheet.
But I've never had a shear web fail -- I always see them break off of the spar at the glue joint.
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Thanks Tim, I call that technique "boxing the spar". I have used it often, sometimes cutting lightning holes in the vertical boxing pieces. I have considered it for this project as well. What I am referring to is 1/16" x 1/4" pieces joining the two spars together, sort of a reinforced "C" tube as opposed to turning it into a "D" tube wing.
BTW, It too like to fabricate "balsa plywood". Usually I make fuselage bulkheads out of 3 laminations of 1/32" balsa with the grain going in opposing directions. Lighter and stronger than the usual 1/8" that is usually used. 8)
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So the webbing makes the spar into sort of a truss structure, with the ribs as verticals and the 1/4 x 1/16 the diagonals?
I'd run them up, down, up, down, as it would tend to cancel out any oddball tendencies from running them all one way or the other. But I'm not sure if that's best or just personal bias.
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That's the techinque - I know which ever way I do it it will work OK, alternating sounds the best to me as well. 8)
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"Boxing-in" the leading edge, with a wing having top and bottom spars and sheeting from spars to leading edge, is best done with 1/32" balsa, grain vertical, and only needed on the rear of the spars. No need to use thicker balsa. I often omit the shear webbing past the 1/2 point of wing span, The bending forces out there are much less than toward the center of the wing.
F.C.
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This has been workin well for me. Grain going the long way, 1/16 thk.
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Thanks Greg, that's what I had in mind. Very nice workmanship. 8)
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The balsa ply as suggested by Tim is a very good idea if you have to cut holes to clear a bellcrank but that's the only place I use it.
For the rest of the wing cross bracing in both directions for the middle third of the wing is good and single braces like Greg has shown is all that's required for the rest.
If you want to use full shear webs connecting the spars and the ribs, set it with the grain vertical.
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I go with vertical webbing in the middle of the wing, out to one bay past the LG, with diagonals going to the tips from there. Weight gain is negligible and the extra strength is quite noticeable. I have also gone to adding shear webs (1/32nd") to the TE sheeting.
Big Bear
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Thanks all, went with sheer along the spar - with 1/4" spar compression strength is not an issue but the shear should help the twist resistance. Rear sheeting is boxed in per Bill's technique. I've found this to be a good idea on all stunters. 8)
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Somewhere on here is a shot of Tom Morris' Melinium Wing. Almost like what Randy Powell does. More work, but looks very substancial. The angled bracing like trusses does save weight by a few grams.