>The forces on the balsa shear web are not acting on the end-grain >of the balsa as you suggest. They are shear forces that are acting >through the neutral axis across the grain. Actually, you're both sorta 50% correct. Shear stress in the web material is the same in any direction. Counterintuitive perhaps, but it's true. So for withstanding shear stress, endgrain balsa is as good as lengthwise-grain balsa. The great advantage of endgrain balsa is that it is vastly better at restraining the sparcaps from buckling. The endgrain web is also necessary if the spar is wrapped with a +/-45 deg cloth, which puts a large compressive load on the web if the cloth's compression fibers buckle. To belabor the point... A shear web can also be considered to play the same role as the diagonal members in a truss beam -- it prevents "parallelogramming" of any given chunk of the beam. In a truss, the parallelogramming is restrained by the diagonal members, which are either in compression or tension depending on whether they are angled at +45 or -45 degrees. In a spar, the parallelogramming is restrained by filling the spar with a homogeneous web material (e.g. balsa), which is then in compression along one diagonal, and in equal tension along the other diagonal. This equal and opposite tension/compression along the diagonals is what's commonly called "shear". This interpretation of shear loads suggests that an ideal shear web material is balsa "plywood" oriented at +/-45 degrees, so that the diagonal tension/compression loads associated with the shear are along the grain, just like in a truss. Such a web is indeed about 2.5x stronger in shear than an endgrain web of the same weight (which I've verified by busting spar samples). However, if such a web is pierced by ribs like on the Allegro-Lite, all advantage of the diagonal grain is lost, since the ribs will fail in shear well before the web. A plain endgrain web is the easy and appropriate solution in that case. - Mark RCSE-List facilities provided by Model Airplane News. Send "subscribe" and "unsubscribe" requests to [EMAIL PROTECTED]
