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arms (blade arms) as extensions of the balsa blades, which also act as the dihe- dral stop when deployed. Commercially purchased #118 DU-BRO small nylon model airplane hinges are used to link the blade arms to the hub core. 0.625" dental rubber bands put around the hub's alumi- num tube spindle are stretched out and at- tached around pins on the top of the blade arms to deploy the blades. A spacer be- tween the lever arm and the hinge creates the correct dihedral when the blades are deployed into position. Sort the hinges so the knuckles are all arranged the same way. This way the blades (also all made in the same direc- tion) are interchangeable. Trim the hinge half that will be mounted in the hub core, cutting the corners off so the three hinges will fit together on a balsa hub plate and have space in the center of the plate for mounting the aluminum tube spindle. Use a drop of medium or thick CA to glue the hinges on the hub plate. Glue the top plate on with the grain running in a dif- ferent direction as the bottom plate. Care- fully add thin cyanoacrylate glue (CA) to the top and bottom surfaces of the hub plates as reinforcement, but don't let the thin CA get into the moving part of the hinge or you will have some extra work to free them up! Photo 5 shows the hub jig that makes alignment easy. This jig uses one half of the hinge to align the other (trimmed) half of the hinge onto a hub plate. The photo shows a black tube underneath for placing the aluminum tube into for its perpendic- ular alignment in the hub's center. Photo 6 shows a hub being built on the jig. After the hub is complete, I use a Dremel cut-off wheel to grind off the flattened end of the hinge pins so they can be removed. Save them, these will be used as the pins in the blade arms that the rubber bands will be hooked on. Consistency in building rotor units is a must. I strongly recommend using a jig for attaching the blade arms to the blades. The blade arm attachment jig in the full-page drawing is used in two steps. The first is to mark the cutout slot in the root end of the blades. The other end of the jig will align the blade arm in the cutout slot for gluing. This method attaches the blade arms close to the center of the blades' root ends, and when the unit is folded and placed in the 40mm body it easily fits without touching the tube walls except at the blade tips in the tailcone. 20 JANUARY/FEBRUARY 2020 SPORT ROCKETRY Cutout for blade attachment Alternative blade attachment does not utilize this cutout. Centerline of 40 mm mandrel during Blade is constructed by wetting balsa mm aluminum mandrel then wrapping Root Example Blade SCALE Blade is attached to spruce lever arm on high pressure side. Hub is parallel to the pitch angle of the tip. Alternative Blade Attachment View SCALE 1 : 1 1/32 The alternative blade attachment method is likely easier to construct, but will likely result in poorer airflow near the blade root. A jig is still recommended for this method. To use this attachment method, attach the blade at the blade tip's centerline.