The guys I'm working with did use some planetary gears, but gave it up due to torsional vibration issues. This was after doing a COMPLETE custom gearbox. They designed a beautiful pendulous damper, (like the ones used on the big radial engines only smaller), and using that they could keep the torsional vibration issues to a minimum. The problem was it simply cost too much money. The complete engine gearbox damper system worked great, but cost as much as any regular aircraft engine. You can be sure by the time you get to those cost levels most buyers, (99.9%) are going to choose a lyc. or continental. They found that all the commercial gear sets were either too heavy or not durable enough. So no planetaries. The advantage of the gears we are using is they can be adjusted for backlash. That is another problem with the planetary sets. I am including a couple pictures of what I'm doing. If you think you can do it yourself, please feel free to run with it. Little or no money will be made from this, I am just hoping to cover the costs of my own. The other feature of this system is that the engine is on an offset centerline. (about 2-1/2" lower) On a rotary this is very helpful because it allows you to easily run the intake tubes over the top of the engine without raising the cowl line. The lack of space on concentric shafted gearboxes is why some people went to all the trouble of running the engine in the spark plugs up configuration. If you run a dry sump plugs up makes sense, but it is parts intensive and costly. Not to mention the need to re-engineer the drain passages for oil control. This internal ring gear configuration works well for the rotary/wankel/Mazda engine, and allows you to run a conventional sump if you want to. The spur gear design can be set to almost no clearance since the ring gear expands just a bit faster than the pinion gear. On the aircraft dyno we have been using the total amount of backlash on a prop of 71" diameter amounts to as little as .06" at the end of the prop and that is when warmed up by running for an hour! The set up is a torsionally "stiff" model and all the harmonics are above the normal operating RPM of a 2 rotor. All this stuff looks simple, but it really isn't if you want the thing to last.

Bill Jepson