 With his following design, the HP-17, Schreder developed a new, more cost-effective method for making wing spars.These spars featured a rectangular box section consisting of machined C-channels for the upper and lower caps, joined by shear web panels riveted to the flanges of the spar cap channels.These channels were again manufactured by Mazur, and were carefully made so that the floor of the channel tapers in thickness from 3/8" at the root to 1/16" at the tip.