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Old 04-04-2012, 09:38 AM
BMatthews's Avatar
Join Date: Oct 2002
Location: Chilliwack, BC, CANADA
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Default RE: Scratch Designed/Built Sailplane

Nice thread! ! ! ! And a very sleek looking design as well.

I've got a couple of structural suggestions for you to consider.

[ul][*] Sheet the lower center section as well. Closing the structure will greatly stiffen the center section and thus support the tips from twisting to a good degree. The weight penalty of that much balsa is peanuts. If you were worried about weight you could have saved it by doing the whole fuselage with the planked idea instead of using all that 1/2 inch sheet.[*] It looks like you're depending on the carbon tubes for the center section dihedral bracing. Not a wise idea. The carbon rod that is close to the main spar is the only one which is really doing much. But at the same time it is up near the upper side of the wing where it's again not in the right spot to give the sort of support it should be doing. In flight a reasonably hard pull out will simply pull apart the relatively weak butt joint between the lower spars when the rather small carbon rods flex. And the ribs and sheet will flex enough that your rear one isn't really doing anything at all. But you can save the day by using a Dremel and a large diameter wood saw blade to cut some slots in the lower spar and join the two sides with "bicuits" of carbon fiber flat strip laid into the slots with epoxy. Otherwise the only thing holding them together is the butt joint. And under the sort of tensile loading you'll have it'll soon split. The slots should be cut spanwise into the spars and extend about 1 inch to either side of the middle joint. I'd go with two such slots and then glue in the carbon fiber strips to span the joint. [*] Before you sheet the bottom of the center section I'd double up the webbing by doing the front of the center section spars. Again because the tips pass all their loading to the center you can't go wrong by making the center section as strong as practical. When wings fail it's always in near the center so a few extra grams to aid in strengthening the center section is never bad.