Something to use instead of elastic in the arms to mount the feathers onto?
Hmmm, this design requires something at least
similar to elastic to work I'd say no, and since elastic is so flat & stretchy I still recommend it, but if you find something else (maybe bungee cords & bend the feather shafts around it, pinching it in place?) give it a shot & let me know how it goes.
And do you mean the math that determines the length of the feathers?
It's shown in
Realistic Wing Tutorial - P.1 , but the "measurements" come from the actual lengths of feathers from wild birds, photographed & catalogued in the
Feather Atlas, the ones I made of crow's feathers were in centimeters, I just changed centimeters to inches and tried it out, and they looked/worked great.
Hope that answers your questions~