-This is a procedure I learned about in the book: Composite Basics by Andrew Marshall for sizing a composite spar. It seems to give too light structures by a factor of 10 but their could be errors in my calculations. The units seem to work out but are these relatively simple calculations nonsense as I suspect?

-800lbs gross, designed for 10Gs, 22' span, 2.75' cord, .4125'=4.95" thickness

-weight center at 5' from wing root

-M=5' * 800lbs/2 * 10Gs = 20kft.lbs

-estimate thickness of cap to calculate average spar depth: about .017' or .204" for one side: .4125'-.017'=.3955'

-20kft.lbs / .3955' = 50.6klbs / 2 since each cap takes 1/2 load = 25.3klbs

-Rutan lay-up gives 48ksi in tension and 40ksi in compression

-layer of Rutan 7715UNI or 7725BID is .0075" thick

-multiply by 1 for root, by .56 for 3/4 from tip, .25 for 1/2, .06 for 1/4

-25.3klbs/48ksi=.53sq.in for lower cap at root/3" wide cap/.0075"/layer=24 layers; 14 at 3/4(from tip), 6 at 1/2; 2 layers at 1/4

-length from root to determine weight of lower spar cap = 2layer*11' + 4layers*5.5'+ 6layers*2.75' + 10layers*1.25' = 22'+22'+16.5'+12.5'=73'*12"/'* 3" * .0075"= 19.7cu.in * 1.08oz/cu.in=22.3oz for lower cap

-25.3/40=.63.sq.in for upper cap at root=29 layers; 16 layers at 3/4; 8 layers at 1/2; 2 layers at 1/4

-length from root to determine weight of top spar cap = 2layer*11' + 6layers*5.5'+ 8layers*2.75' + 13layers*1.25' = 22'+33'+22'+16.25'=93.25'*12"/'* 3" * .0075"= 25.2cu.in * 1.08oz/cu.in=27.2oz for upper cap

-shear stress (15ksi, about half tensile strength)

-4000lbs/15000psi=.267sq.in

-.267sq.in/4.95"(spar depth=total wing thickness since spar caps carry some shear)=.054"/.0075"/layer=7.2 layers so 8 total layers at root, 4layers at 3/4 from tip, 2 layers at 1/2

-length from root to determine weight of shear web = 2layer*11' + 2layers*2.75' + 4layers*1.25' = 22'+5.5'+5'=32.5'*12"/'* 3" * .0075"= 2.9cu.in * 1.08oz/cu.in=3.16oz for shear web

Brock