Case Study: Complex Vibration Troubleshooting on a Rotax 912

The RPX Technologies team recently helped a Kitfox owner with a Rotax 912 engine troubleshoot a complex vibration problem.  

This episode illustrates how to isolate the source of complex aircraft vibrations.

The Problem

This Kitfox owner has struggled over the past two years trying to locate the source of a troubling vibration. The symptom was an engine that ran rough in the mid-RPM range, producing a lot of vibration. Trial-and-error service procedures failed to correctly guess the vibration source. In this case, the owner had the gearbox rebuilt because it was a suspected vibration source, but this remedy was ineffective. The owner was unwilling to continue having speculative service work done to fix the vibration, because these procedures were cost prohibitive.

The Analysis

A friend of the owner recommended analysis using the DynaVibe GX2, a full-spectrum vibration analyzer and prop balancer.  DynaVibe was able to pinpoint the vibration source, and the graphs that follow are the actual reports generated by the DynaVibe during this analysis.

To perform this analysis, two DynaVibe accelerometers were placed on the engine, and readings were taken at various RPM levels, as illustrated in the following DynaVibe GX2 charts showing readings from the accelerometer mounted on the gearbox:

Step 1. The initial reading, shown above, was taken at an engine RPM of 2270 and a prop RPM of 1000. Here, a 2 inches per second (IPS) vibration peak, the highest on this chart, is seen at the 1.2-per interval. This is indicative of a carb imbalance. 

A peak at the 1-per interval is also present, but is difficult to see because as it is eclipsed by the 1.2-per vibration peak.  A second reading was then taken at a slightly higher RPM to see what might change:

Step 2. This second reading, shown above, was taken with the engine running at 3000 and the prop turning at 1350.  

The prop vibration, shown at the 1-per interval on the chart, seems to be increasing, which is indicative of a propeller mass imbalance, while the carb imbalance is decreasing.  

These readings show the complexity of vibration analysis: changing RPM levels can simultaneously cause one vibration to increase and another type to decrease.  A third reading was taken, at still a higher RPM setting:

Step 3. This third reading, shown above, was taken with the engine running at 3632 RPM and the prop turning at 1600 RPM. 

The chart from this reading shows that the prop vibration at the 1-per interval continues to increase, while the carb vibration at the 1.2-per interval continues to decrease.  A fourth and final reading was taken to confirm these vibration trends:

Step 4. A final reading was taken at near-cruise RPM, and it confirms that there are indeed two vibration issues:  a propeller vibration of almost 1 IPS which appears at higher RPMs, and a carb vibration, which is present only at lower RPMs.  Note that the carb vibration has almost disappeared at this RPM level.

A key benefit of the DynaVibe GX2 is the ability to monitor the vibration spectrum in real-time, allowing the RPM to change while watching the change in vibration.  This real-time vibration review is critical for diagnosing complex problems. 

With this data, this Kitfox owner can confidently address the vibration issues because the sources are known.  The right approach is first to balance the prop, and then address other vibration sources, in this case, the carburetor balance.

If you have similar, hard to pinpoint vibration issues, contact the RPX Technologies team to discuss how DynaVibe can provide an accurate, full-spectrum analysis to locate the source.