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I am starting to build a new redrive for my coot, the original redrive I used when I first started flying the coot was a 3.17 to 1 Ross but it left me walking home to many times in the first 400 hrs so I went to a 2.95 to 1 cog belt and there was a loss in performance which I am now trying to gain back.
In the testing stages of the cog belts I tried a 2.25 ratio which did not work at all, also tried a 2.8 which was barely acceptable and then a 2.95 which was a compromise to the 3.16 Ross but the cog belt has not left me walking home in 900 hrs but its just not practical to get any farther than a 2.95 with a cog belt so I am now going to a planetary.
Here some shots of the Dodge gears I am working with, the gear ratio is a 3.21 to 1 and they come from the overdrive of the trany that is behind a Cummins turbo diesel and by an eyeball engineering standpoint if it can hold up to a one ton truck pulling a 12,000 lb trailer it should hold up to a weeny little rotary.
There is a big difference in the sun gears compared to the C-6, first of all the gear cut is 15 degrees rather than 25 degrees in the C-6 which means less loading on the thrust bearings compared to the
C-6, second of all the cut is in the opposite direction which means the thrust is put against the planet and the planet gear set comes with a thrust plate that slides onto the sun gear and it comes with a thrust bearing but do to the high RPM of the rotary it will take a double stack of thrust bearings, at 7.000 rpm the prop shaft and the planet will be turning 2180 rpm so the differential is 4820 and the speed limit is 4400 rpm so it will definitely take two gearings especially if if an RX-8 engine were used which I plane go to in the future.
To do a double I will half to toss the thrust plate in the trash can and make a new one that will except two thrust bearings rather than one and I may toss the Dodge bearings that are pressed all together and replace them with off the shelf bearings and thrust plates.
Another thing I like about this gear box setup is there is that the sun gear is splined on one end and I can rework the clutch gear hub and bolt it to the pilot shaft and the sun gear will slide on and engage without the hassle of drilling, pinning or welding the sun gear to the pilot shaft.
Also the ring gear there are dogs on the outside that will help lock it into the housing although about half of the ring gear will be cut of and tossed into the trash can to save weight.
With this setup the advantage is that I will be able to unbolt a set of bolts that hold the gearbox onto the bell housing and then and pull the gear box off and then just pull out all the gears and replace them if needed.
Ken Welter
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