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Old Jul 17, 2006, 12:29 PM
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We move on to the central airframe (drawing airframe).
For this you'll need
- two pieces of cf tube, O.D. 8 mm, I.D. 6 mm, with lengths of 35 mm and 120 mm
- two pieces of cg tube, O.D. 6 mm, I.D. 4 mm, with lenghts of 25 mm and 70 mm
- one piece of cf tube, O.D. 5 mm, I.D. 3.5 mm, length 290 mm
- one piece of cf tube, O.D. 4 mm, I.D. 2.5 mm, length 230 mm
- one piece of aluminium or brass tube, O.D.6 mm, I.D. 5 mm, length 20 mm
- one wheel collar, I.D. 6 mm
- two wheel collars, I.D. 8 mm

Epoxy the 6 mm cf tubes into the front end of the 8 mm cf tubes. Now comes the only tricky part in building the Minimum: you have to drill a 5 mm hole and a 4 mm hole at angles of 82 and 70 into the reinforced 8 mm cf tubes. For the location of the holes have a look at the plan. Before you drill the holes put some duct tape or similar around the cf tubes. First, predrill the holes for the rotor mast with a fresh 4 mm wood drill, you know these drills with a pointy bit in the middle and a flat outer edge. Then enlarge the holes to 5 mm, using normal drills. Finally, drill the holes for the support strut, using a 4 mm wood drill again. Remove the duct tape. If you do it this way, you'll minmize the likelyhood of the cf tubes splitting up during the drilling. Now print out the airframe plan and loosely assemble the cf parts on the plan (first picture). Use a round file to sand off the top of the 4 mm support strut so that it fits onto the 5 mm rotor mast. You may have to enlarge the slanting holes to make everyting fit, but that doesn't really matter. Once everything is aligned, take the airframe parts apart, apply expoy at the relevant positions and reassemble. Put some 1.5 mm balsa pieces under the rotor mast and some 2 mm balsa pieces under the support strut and hold everything down while the epoxy is curing. Then slip the aluminium tube over the top of the rotor mast and epoxy in place. This tube is to keep the end of the rotor mast from splitting under strain.

The wheel collars are used to keep the pivot joint, the motor spar and the tail boom in their positions. For the 6 mm wheel collar holding the pivot joint I suggest a version with two grub screws opposite each other. If you can't find one, make one yourself. Drill a 2.5 mm hole opposite the original one and cut a M3 thread into it. Slip this wheel collar over the top of the rotor mast and epoxy it in place, about 5 mm from the top of the mast. Now, using the wheel collar holes as a guidance, drill 2.5 mm holes through the cf of the mast and extend the wheel collar's M3 threads right through the cf.
If you follow this procedure, you'll take a minimum of material out of the wall of the mast and leave it as strong as possible. Epoxy one 8 mm wheel collar over the end of the motor support and repeat the thread-cutting procedure. Put the last wheel collar aside for now.

To be continued.

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