It's all about how we do the measurement - almost all prop shops and
prop balancers will use a single accelerometer to survey your
vibration. We prefer to ALWAYS use two - that way we can see what's going on at the back of your
engine in response to our balancing weights on the front. This is really
important in order to arrive at the lowest possible vibration in your
aircraft and to provide a full vibration survey report to you.
We must use two sensors to distinguish the
propeller from the crankshaft since both of these components turn at the
same rpm and the only way to tell one from the other is to use two
sensors. If we balanced a propeller down to a .05 IPS using the
front transducer and at the rear of the engine, the sensor is reading a .6
or .7, the culprit is really the crankshaft. This would never be
discovered if we used a single sensor.
In a situation like this, where we reach a very low reading at
the front only to discover a high value at the rear we re-balance using
both sensors to reach a compromise value, where the front and rear
readings are made equal. The pilot will still notice a very significant
reduction in vibration in the cockpit and the engine will experience a
lower torsional vibration throughout.