Issue link: https://www.e-digitaleditions.com/i/1197609
22 JANUARY/FEBRUARY 2020 SPORT ROCKETRY You can use the alternative blade at- tachment method as shown in the draw- ing where the blade arms are attached on the bottom of the blades and are centered in line with the center of the 3/4" wide blade tips. However, this method moves the blade arm attachment location toward the blade's trailing edge, and will require more room when the folded unit is placed in the body. Note that aligning the blade arm with the centerline of the 3/4" width of the blade's tip gives the tip zero pitch. If the arm is positioned closer to the trail- ing edge, the blade will have more negative incidence overall and will spin up fast, but lose lifting efficiency at the outer region of the rotor. If the arm is positioned further from the trailing edge, positive incidence at the tips will create more lift in the outer region, but will have a harder time initiat- ing rotation after ejection, and if extreme, will stall the tips and not rotate. With either blade arm attachment method, attach the hinge halves by wrap- ping thread through the holes in the hinges and around the blade arms. The distance between the holes in the hinges is conve- niently very close to the width of the blade arms, which properly aligns the hinge on the blade arm. Before attaching, make sure the hinge knuckles are oriented correctly to join the hinge halves on the hub. Use thin CA to reinforce the hinge/blade arm joint and thread. Drill a small hole on a 15 degree angle through the top side of the blade arms and put the original hinge pins in the holes, leaving enough pin exposed to hook the rubber band on. Cut off any excess on the bottom side and use thin CA to secure in place. Join the hinges back together using .025 music wires as the replacements for the original pins, and bend the wire ends 90 degrees to secure them in place. Hook up the rubber bands and adjust the dihe- dral by sanding/filing off the ends of the blade arms where they meet the upper hub plate. If too much is removed, thick CA can be used to build it back up. Making the Central Shaft Assembly The backbone of the rotor unit is a 12" long x .06" (1.5mm) diameter carbon rod. Photo 5. Jig used to build the S9 hub. Photo 6. A hub being built on the jig.