These vibration readings are normally an indication of a mass imbalance in the propeller, or, as described earlier, a damaged crankshaft. To reiterate, the only way to make the determination between propeller and crank is through the use of two vibration sensors. The general rule is that the amplitude from the sensor at the rear of the engine should not show readings with higher amplitudes than that of the front sensor.
Second-order readings are not so cut and dry. In the event you have a two-bladed propeller, you will have an inherent two-per-revolution vibration level that is characteristic of a two-bladed prop. This inherent vibration is referred to as the end-per-rev. The problem could be an internal mass imbalance in the rods and pistons, but it can't be seen because of the presence of the end-per-rev. The best bet is to contact the engine OEM or obtain technical support from the equipment manufacturer. If the aircraft has a three- or four-bladed propeller, then the problem would be a little easier to identify because the end-per-rev would show up at three or four times the one-per-rev. In this instance, the problem could be identified as being in the piston and rod area.
The end-per-rev vibration can be so uncomfortable that in some helicopters, there will be what is called a vibration absorber to help dampen this vibration. The absorber is suspended by bushings to allow for movement. Weight is then added or removed until the absorber bounces at the same frequency as the end-per-rev. This helps to mask to vibration to improve crew comfort. Unfortunately, in fixed-wing aircraft, we may have to live with the end-per-rev.
To help with the understanding of vibration analysis it important to understand that anything of mass, when struck or excited by an outside force, will vibrate at a certain frequency based on its material and structural makeup.
Out on my front porch, I have a wind chime. It is made up of cylinders of thin-walled metal. Each of the cylinders is of a different length. When the length is changed the natural frequency changes. The end result is when the wind blows the chime, there are numerous different notes and tones excited from the force of the wind. In a reciprocating engine, each of the components gives off its own note or tone, you just need equipment a little more sophisticated than your ears to denote and examine the differences.
The key to accurate vibration analysis is to establish a predictive maintenance program. Once data is collected, the most effective way to manage the data is to build and maintain a database that allows for future trending of the engine and propeller combinations of the same type. These types of programs are available commercially and have features that simplify the data comparison effort.
For example, say you have a regular customer and after completion of each of the 100-hour inspections, a vibration signature is acquired on the aircraft engine and propeller assembly. Over a period of time, any changes in vibration magnitude in any of the rotating components can be compared to the vibration levels obtained from previous inspections Findings from this data can be as dramatic as predicting impending failure of a specific component.
In some databases, numerous vibration signatures can be overlaid in what is referred to as a waterfall plot, resulting in easily viewed increases or decreases in vibration levels that are displayed over a period of time. As a component begins to wear, the vibration levels begin to rise in the frequency output of a particular component.
In conclusion, a properly balanced propeller reduces the overall fatigue in the airframe and components. An unacceptable level of vibration affects the entire aircraft in a negative manner. Reducing mass imbalances is the first step to a healthier airframe. But, the implementation of a predictive maintenance program consisting of nothing more than acquiring a vibration signature at specified intervals will reap even greater returns. The cost and labor involved is minimal and the end result is happier customers with healthier aircraft.
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