Rotax B drive, Rotax B gear box, Rotax B reduction drive parts for Rotax 447, 503 and 582.
The Rotax B drive reduction gearbox was used on the Rotax Rotax 447, Rotax 503 twin cylinder two stroke aircraft engines, and Rotax 582 twin cylinder liquid cooled 65 HP engine. It is available in several gear ratios, and can be used in both pusher and tractor configurations. It has also found it's way into Hovercaft and Airboat applications.
The Rotax B drive is referred to as a Provision 8 gear box. The reason for this is that it has 8 mounting bolts holding it onto the engine. This came about when Rotax introduced the Rotax 447 single CDI, the Rotax 503 Dual CDI engine and the Rotax 582.
With the introduction of these engines Rotax made new engine molds, and changed the mounting system for the drive, making it stronger and eliminating the need for an adapter plate.
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Rotax B Drive gearbox ratios
The drives ratios are 2.0:1, 2.238:1 and 2.58:1. The 2.58 to one ratio was generally used in ultralight applications, when spinning props from 60 to 68 inches in length. The 2 to 1 and 2.238 to 1 ratio was used more on air boats and hovercraft when a smaller prop was used.
This is generally less efficient and noisier. While if you are flying anything using a 2.58 to 1 reduction drive you will be flying on 60 to 68 inch props. Spinning them slower, quieter, and more efficiently!
Generally the maximum prop diameter for a model B gearbox is 68" for two blade propeller, and 64" for a three blade. As you get into more blades you have to reduce their length and pitch if using the same power and gear ratio. Trying to use the same lenght with more blades will result in the engine not being able to produce the power and thrust it was designed to.
The weight of the propeller is called "Mass Weight Inertia," you can find your props weight using the Mass Weight Intertia formula.
If you do not know what reduction drive ratio you have you just need to pull the engine over by hand, counting the number of times you engine turns over versus the number of times the prop turns. On a one to one ratio your engine will turn one complete revoltion per prop revolution. You can also count the teeth on your drive gear and your main gear and divide them into each other.
There is usally a number stamped into the gear box casing that tells you the ratio, as long as the ratio hasn't been changed!
Example: 49 teeth on the large gear divided by 19 teeth on the small gear = a 2.58 to 1 ratio.