Replies to This Discussion

Permalink Reply by David on January 25, 2022 at 10:06pm

I would like to know as well.

Ive seen a lot used engines with a newer gearbox, so clearly there was an issue at some point

• ▶ Reply

Permalink Reply by Matt Pope on May 11, 2022 at 7:16pm

This is an old post, but since nobody answered...

I recently attended Rotax's 912-series service course, hosted by Rotech (in Canada) and taught by a guy with 30 years experience working on them. He had a detailed answer for every question I could think of, so if you have a Rotax engine and enjoy learning about this stuff, a course like that is a good investment. 

I don't know about the other engines, but the 912's gearbox is pretty interesting.

The overload clutch requires a huge amount of torque to slip - I think it's around 700 Nm (516 ft-lb), and it's only designed to protect the engine during a prop strike. You're likely to bend the airframe or engine mount if you don't brace the engine properly before checking the overload clutch torque (but it doesn't need to be checked, except at overhaul). So the overload clutch doesn't absorb any vibration. 

Torsional vibration is dealt with in a few different ways. There's a set of angled dog gears with 30 degrees of slop between them, and when they make contact with each other, they try to ride up on each other's sloped surfaces. That converts the torsional pulses into compression, which is absorbed by a pair of belleville washers and stored as potential energy, which is then released after the power pulse ends (the belleville washers act as a spring to absorb and release those compressive shocks and smooth out the power pulses). The washers also slip past each other to absorb that 30 degrees of slop between the gears, and there's a specified amount of friction between them. In a way, it's kind of a clutch that slips back and forth 30 degrees to absorb the power pulses - it's often called the "slipper clutch," but I don't think that's an official Rotax term. Checking the torque required to make the washers slip is part of the annual inspection, and it's pretty easy. Older 912 gearboxes had only 15 degrees of slop between the dog gears, and the oldest 912 gearboxes had none at all (only the belleville washers absorbed the vibration). I'm not sure when the design changes happened.

Also to minimize torsional vibration, the minimum idle RPM is 1400. If you hear clattering, that's not great for the gearbox. For gearbox longevity the recommended idle setting is more like 1600 for a low drag airplane and 1800 for a high drag airplane (opinions vary, of course). You also want to minimize the time at idle. Above 2000 to 2500 RPM, the power pulses are at a high enough frequency that everything smooths out and there's minimal wear in the gearbox. 

I know that the newer 915iS engine has a redesigned gearbox to handle the 41% power increase compared to the 912 ULS and 912iS engines, but I don't know anything about it beyond that. 

• ▶ Reply