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Building Tips and technical articles. => Building techniques => Topic started by: charlie on March 18, 2007, 11:29:56 PM
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Some of the classic design stunters have 2 1/8th square spars halfway between the mainspar and the trailing edge but through the rib rather than on the surface of the wing. Two examples are 1. Bob Palmer's Mk 1 Thunderbird (has no cap strips on the ribs) and Lew McFarland's Ruffy (has got cap strips on the ribs). Can anyone tell me what function these spars are meant to serve? I should add that I built a Thunderbird many years ago without putting in those additional spars and it never failed and is still operational (Albeit rather tatty these days).
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I believe the purpose of these secondary, buried spars is to provide vertical location and some stiffness to the rear of the (sometimes rather thin) rib section. If capstrips are installed, the 1/8" sq. buried spars are just an annoyance. With no capstrips, they do provide a bit of stiffness, but you can do better with a shear web of 1/16" balsa.
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Charlie, I believe those spars were to help stiffen up the wing. The idea was to make it a bit more twist resistant. In most cases, this was a carry over from earlier days, and was not really needed.
John Miller
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Thanks chaps, It was a mystery to me.
Charlie
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The Carl Goldberg kit-VooDoo had a 1/4" square "alignment spar" toward the rear of the airfoil.
It didn't do much for structure, being on the neutral axis.
Most scratch builders (in the absence of die cutting & lazers) just omitted it.
None the worse without it.