Check out this great video, a description and explanation of how the Atlas project team installs “biscuits” or bulkheads in our tubular rotor spars. The biscuits are used to prevent thin-walled bucking, in which the spars can crush like a pop can under twisting and bending loads. They”re small balsa and foam sandwiches, installed with an extremely long pusher tool and tacked in place with wood glue. Pretty unique as far as construction methods are concerned!