Reciprocating engines are hard to diagnose because there are numerous components in the engine that have the same fundamental frequency. For instance, the propeller and the crankshaft share the same rotational frequency. The only way to distinguish one from the other is to use two sensors. Using standard "L" brackets, one transducer is mounted at the front of the engine as close to the propeller as possible and a second transducer (check transducer) at the rear of the engine. The closer the sensor is to the source of the imbalance, the greater the amplitude readings will be.

I recently spent the afternoon working on a Mooney. The owner complained of a vibration in the airframe. The first thing I did was to mount dual sensors. I verified that the propeller was in balance (.12 IPS), but the rear of the engine was reading .85 IPS. Using the Model 2020 ProBalancer Analyzer I used the rear engine readings and solved to reduce them. I found that as I started to lower the rear engine vibration levels, the front readings were increasing. Obviously there was a problem with the crankshaft in the airplane. I would have never been able to come to this conclusion with only one sensor. Ultimately I had to return the aircraft to its original weight configuration and notify the owner of my findings.

Remember that it only takes a few extra minutes to install a second sensor, doing so can save many engine cycles and improves the ability to accurately troubleshoot vibrations.