Rotax reduction drives, Rotax gear drives, Rotax A box, Rotax B box, Rotax C drive, Rotax E drive.

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How to select the proper Rotax reduction drive

When we first started using Rotax engines on our planes we had to use a belt drive system - as Rotax only supplied us with engines.

In the mid 80's Rotax came out with their A drive used on the Rotax 277, 377, 447, 503, and 532 points equipped engines.

The A drive uses a 4 bolt adapter plate which bolts to the engine and then the drive bolts to the adapter plate.

When they introduced the Dual CDI engines they modified engine, which required a new drive called the provision "8" B drive. This means the drive bolts directly to the engine using an 8 bolt configuration. At the same time they introduced the C drive and E drive.

The ABC and Es  of Rotax gear drives

The major difference in these drives are their "weight ratings." That is do you need a 1/2 ton truck, a 1 ton truck? An A or B drive can handle up to 3,000 kg cm2 of mass weight inertia. The C and E drives can handle 6,000 kg cm 2 of mass weight inertia.

So you have to find out how much your "truck load is" - Rotax Service Information 11 UL 91 explains the process - in real life I have found that a 3 blade IVO, 3 Blade GSC, 3 Blade Powerfin, 4 blade Ultra prop up to 66 inch can be loaded into the "1/2 ton" A or B box - with the Warp Drive 3 blades being too heavy.

Two blade props by the above manufacturers can be used up to 68 inches, if in doubt contact your  manufacturer!!!!

Another consideration is what type of plane you fly. If you fly a plane that uses a short prop like the Hornet, or older model Challengers, you will be spinning a smaller prop faster.

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!
The A and B drives ratios are 2.0:1, 2.238:1 and 2.58:1.

Prop selection - length vs pitch 2 blade vs 3 blade is another story!

The C and E drives

These drives can handle twice as much weight, and are available in ratios of 2.62:1, 3.0:1, 3.47:1 and 4.0:1. These drives are used with larger diameter props, or more blades.

For example a WW 1 replica fighter using a 582 engine could swing a very large two blade prop very slowly using a 4 to 1 reduction drive. This would be more authentic looking, be very efficient and quiet. But this combination would not work well on a very fast plane, or one with low ground clearance.

Or a trike could use a 6 blade prop, with less length and more pitch for the same reasons. While some in the industry may  argue that this allows the use of smaller engines to be used, with the larger ratio.

Real life has shown that a prop and its power are mated with the craft. That is while you may find some craft can fly on less power, using a larger redrive ratio - it is always better to have MORE power.

Type A gear boxes were used through 1989 on most UL series engines. On the twin cylinder engines, the type A gear box mounts to the crankcase via an adapter plate. The adapter mounts to the crankcase with 4 allen head bolts.

These crankcases are referred to as "provision 4." In 1983 and part of 84, a 3-bolt case was on the A drive this was later updated to the 4 bolt pattern, after shearing of the top bolt on the case was reported. On the 4 bolt adapter plates you can change the direction of the drive 180 degrees by rotating the adapter plate.

In the late 80's early 90's the new provision 8 series of engines and drives was introduced. The type B gear box which is internally the same as type A, attaches to the provision 8 case in either up or down configuration without the need of an adapter. They both dampen torsional vibration via a system of bevel washers and a dog hub/dog gear arrangement. Type A and B gear boxes are not interchangeable. "A" fits provision 4 and "B" fits provision 8.

Type C gear boxes are totally different internally and externally. They dampen torsional vibration via a rubber "Hardy Disc" rather than bevel washers. They also weigh about 10 pounds more and are more expensive.

Type E gear boxes dampen torsional vibration identically as the C unit. They are similar internally, but not identical. The functional difference is the addition of an electric start unit to the Type E. Type C and E fit provision 8 cases only. If you are going to be using the B drive and an electric start, or a C drive and an electric start you are better to buy the E drive.

For gearbox removal and installation instructions click here

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