Hi all,
The dummy rotary on my DH2 spins on a ball race on the propshaft. The prop only turns at 4200 (still too quick for scale, but what the hell) but the drag, or slip, of the bearing, combined with the aerodynamic drag of the dummy motor itself, slows the Rotary down to I guess about 1700 rpm, about right. I would be slightly hesitant to spin a Williams kit at anything like this speed, as they are reasonably heavy when completed. Mine is constructed around a Paxolin 'spider' (with 9 legs!) onto which the bearing mounts. The cylinders are cast in polyurathane foam, to get the basic shell and fins, and attached to the spider using expanding foam to fill the cylinder and secure the spider. Each finger of the spider is drilled to accept a couple of cross pins, which once surrounded in squirty foam is very secure. The crankcase is made from blue foam, covered in paper for smooth finish. Again, the crankcase is crossdrilled and pegged to the spider, secured this time with epoxy. The key to the success of the rotary is the fact that it is very light, the heaviest part of the whole thing is the bearing and housing. It spins very easily, although no matter how fast you spin it by hand, it wont do more than 10 or so revolutions, due to the drag of the cylinders. When designing the system for the DH2, I intended to have the dummy engine bearing on the housing for the shaft, and not on the shaft at all, feeling that the motor would spin too fast, with dire results. I was going to blow the motor round with the exhaust gasses, as I had used that system before on a smaller Fokker, but the exhaust on the Fokker was right next to the dummy, so it was easier. Doing it this way means there is definately no extra load on the model engine, and the rotary engine can be sped up or slowed down dependant on how close the exhaust is.
The exhaust is much further away on the DH, and there is a former, speed reducer and shaft in the way, so it was not to be done the way I had before. Another problem with a coventional plane with a rotary at the front, is balancing. It is usually neccessary to move the model engine back a bit, to make way for the dummy in front. This, on most WW1 planes, takes the heavy bit closer to the Cg which means you need more weight at the front to keep the balance point in the same place, so the model will be quite a lot heavier, no matter how light the dummy rotary is. A compomise is to only have the front half of the rotary, and using a model engine with the bits at the back, so it can be closer to the front. I am due to have the motor out of the DH quite soon so I will post pics of the whole 'kit and kaboodle' when I do.
Hope this helps, Ian.