For those that don't know who he is, Eric Tucker is the founder of the Rotax Flying and Safety Club, which is the organization approved by Rotax for training technicians and service/repair centers (including overhaul) in the Americas. He often visit Austria for the latest on BRP Powertrain's upcoming changes, as well as works closely with the engineering and service teams at Rotax.
Before anyone asks, I did inquire about the low RPM issue. First, you need to know that the overload clutch is NOT designed to absorb prop strikes, but as a side effect of the design, it does. What it is actually designed for was the 912S (ULS, and by extension, the 912iS) series engine harmonic issues. Because of the high compression in the cylinders, it was creating idle problems at low RPM, and creating engine harmonics at higher RPM. Rather than design the engine from the ground up, Rotax engineers first sought to see if they can eliminate the problems in the existing design. This is what the overload clutch does. When it was realized the overload clutch could protect the engine in the event of a prop strike, the idea stuck.
Part of the design was not just the addition of the clutch. It also changed the design of the dog gears. The dog gears have a ~30 degree slippage in newer produced engines (some other amount in older engines) to help cushion and absorb the pulsing at idle, and smooth out the idle run. Remember, this engine is designed for HIGH RPM operation for long periods of time, and to save costs and engineering, that's what it is centered around. The engine is not designed, nor as thoroughly tested, with low RPM.
So, what happens at low flight RPM? The firing impulses are spaced further apart, and the resulting pulsing resonates and excites the dogs and causes them to chatter. The dogs are the only thing transmitting the torsional motion from the prop gear to the prop shaft, the prop gear is freewheeling. Anyways, this chattering is amplified by the propeller, which is basically a giant flywheel on the front of the engine. These cause prop shaft harmonics which are further amplified by the large diameter of the prop shaft gear, and transmitted to the teeth of the gear, causing accelerated wear. Depending on the aircraft and prop combinations, some aircraft are more sensitive than others.
The 912 UL and 914 do not experience this problem as severely, because the dogs mesh more closely and they have a lower compression.
Before anyone asks, I did inquire about the low RPM issue. First, you need to know that the overload clutch is NOT designed to absorb prop strikes, but as a side effect of the design, it does. What it is actually designed for was the 912S (ULS, and by extension, the 912iS) series engine harmonic issues. Because of the high compression in the cylinders, it was creating idle problems at low RPM, and creating engine harmonics at higher RPM. Rather than design the engine from the ground up, Rotax engineers first sought to see if they can eliminate the problems in the existing design. This is what the overload clutch does. When it was realized the overload clutch could protect the engine in the event of a prop strike, the idea stuck.
Part of the design was not just the addition of the clutch. It also changed the design of the dog gears. The dog gears have a ~30 degree slippage in newer produced engines (some other amount in older engines) to help cushion and absorb the pulsing at idle, and smooth out the idle run. Remember, this engine is designed for HIGH RPM operation for long periods of time, and to save costs and engineering, that's what it is centered around. The engine is not designed, nor as thoroughly tested, with low RPM.
So, what happens at low flight RPM? The firing impulses are spaced further apart, and the resulting pulsing resonates and excites the dogs and causes them to chatter. The dogs are the only thing transmitting the torsional motion from the prop gear to the prop shaft, the prop gear is freewheeling. Anyways, this chattering is amplified by the propeller, which is basically a giant flywheel on the front of the engine. These cause prop shaft harmonics which are further amplified by the large diameter of the prop shaft gear, and transmitted to the teeth of the gear, causing accelerated wear. Depending on the aircraft and prop combinations, some aircraft are more sensitive than others.
The 912 UL and 914 do not experience this problem as severely, because the dogs mesh more closely and they have a lower compression.
