Step 6: Adding the Servo Axis
Start with a long, thin beam of balsa - the longer the better, up until it starts bending with the motors. Mine is about a foot long. Next, cut two struts of balsa, about 2", and glue little cylinders of plastic to the end of each. Slide the long beam through the cylinders and keep it there. Now, build little cradles out of balsa to hold the motors on the end of the beam -- you can see these in the overhead shot . They should be a tiny bit less wide than the motors, so that the motors are gripped firmly once in place. Glue one strut to each side of the frame, at the back -- see the picture. Once those are secure, glue little knobs of balsa onto the beam, on the inside of the plastic cylinders, so that the beam can't move side-to-side from its centered, stable position.
Once the glue has dried, it's time to add the shaft mechanism. Get a 2" beam of balsa and glue a tiny piece ( 1 cm ) of coffee-stirring straw to the end. Whittle down a bit of balsa until it fits through, then slip it through the straw and cut it down so there's a little bit sticking out each side. Glue each end to a 1cm piece of balsa, oriented perpendicular to the straw and the shaft, and glue each piece to the long beam below. Let it dry. If you did this right, you should have a shaft fixed to a joint on the beam; when you push and pull the shaft, the beam should rotate one way and then the other. This is really important: make sure that the shaft is fixed to the top part of the beam when the motors are correctly oriented, facing away from the main blimp frame.
Finally, the servo. Create a 'cradle' that fits around the servo like the one seen in the picture, and make sure the bits above and below the servo are made of plywood or another hard wood; I glued them to a third balsa piece. Drill one hole into each of these strong sections, then thread a thin wire through each hole and though the mounting holes on the servo. Twist the wires tight so that the servo doesn't move during operation. Glue or otherwise affix a little piece of wire to the end of the shaft mechanism that's nearest to the blimp frame, then fit the wire into one of the small holes that come pre-drilled in the servo arm. It should fit snugly but be free to rotate. Finally, glue the wooden servo mount to the main frame, and you've finished the servo assembly.
Here are a couple videos of the servo mechanism in action: