# What are the unknown sources of vibration in this Rotax 914 propulsion system

(This might be a little out of score for SE.Engineering, so close if appropriate.)

I have a drivetrain setup where I have been performing a vibration analysis.

Below is a diagram of the system. There is a 4-cycle 4-cylinder Rotax 914 engine which drives a gearbox which in turn drives a driveshaft which in turn drives another belt drive which finally drives the propeller:

Here is the order analysis for a constant 5600RPM engine/2100RPM propeller:

Most of the resonances make sense, but there are two sources I can't figure out: the ?? at 3.67 per and ??? at 1.67 per.

What could those relate to? My first thought was the alternator, but https://www.rotax-owner.com/en/912-914-technical-questions/7966-what-is-the-speed-of-the-external-alternator#p26065 suggests it should be 1.24:1 on the crankshaft speed, which would be 3.31, and there's no corresponding energy there.

Relatedly, there's an accessory output but it's 0.543:1 on the crankshaft speed, which would be 1.45. And there's nothing there, either.

So what is exciting the structure at these higher frequencies?

P.S. The graph is slightly mislabeled for the gearbox, it is 2.43:1, i.e. 51:21.

## 1 Answer

0.5 order is usually cylinder to cylinder IMEP variation (ie a misfire), not pistons.

Your unknowns are at 5/3, 11/3 and 20/3 of prop speed. How many teeth are on the belt pulleys? and the gear teeth 49/20?

• That's good insight that the unknowns are thirds of of the propeller speed. To answer your questions: the gearbox is 51:21. I will have to ask the manufacturer for the number of teeth on the belt and pulleys. Agreed that "cylinders" is the better word than "pistons". (In the Rotax 9xx engines, it's frequently due to a misbalance between the two carbs.) Aug 3 at 12:38