# 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.

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.) Commented Aug 3, 2023 at 12:38

Initial look makes me think it's a function of modulation between the engine and propeller, since the unknown frequencies are the sum/difference of the engine and propeller frequencies. The simplest way to test this but may not bring a whole lot of answers would be to perform operational changes such as varying load on the propeller or varying engine RPM to see if the modulated frequencies shift in predictable ways.heterogeneous?

Editing because I do not have enough reputation to reply to comments:

I do not believe heterodyning and modulation are necessarily the same thing. I could be wrong as my experience is all in underwater acoustics so I cannot confirm if things perform differently or are worded differently in atmospheric acoustics. However, modulation is a phenomenon we see when two components affect each other with different frequencies (propeller to engine, engine to propeller) and form side-bands or additional components that show up at the sum and difference of those frequency components.

Heterodyning is usually intentional and produces a significantly more "clean" output at the desired final frequency. It is correct that you wouldn't be seeing the same sideband effect with it than in natural unintentional frequency modulation.

• Heterodyning doesn't show up as such in the frequency domain. For instance if you have 20 hz and 21 hz tone heterodyining, although 1 Hz is audible, there is no signal at 1 hz. Synthesize two sine waves and check it out. Commented May 26 at 22:58
• @GregLocock I know you are right about the heterodyne, aka beat frequency, effect, but is it possible that this beat frequency causes resonation in the structure and thus emerges as a true frequency? Because Trey is dead on right about the unknown frequencies being the sum/difference of engine and propeller. Commented May 27 at 13:19
• Not in a linear system, but if you have backlash and so on anything is possible. Commented May 27 at 22:44