Two pieces of information are needed to correct an imbalance: amplitude and phase. Amplitude is the severity of the vibration. Phase is the location of the heavy spot or imbalance in relation to a timing pulse that occurs during rotation. Once you have these two pieces of information, then the problem can be easily solved. To obtain the amplitude, or vibration, you need a vibration transducer. The most commonly used types are either velocity or acceleration transducers. These sensors have piezoelectric crystals that produce very small amounts of voltage when subjected to a vibration. The greater the vibration, the greater the voltage output.
Digital analyzers convert this voltage to amplitude readings and then calculate solutions that will lower the vibration level to acceptable limits. To take the reading, the transducers must be mounted to the aircraft. Using standard "L" brackets, mount one transducer 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.
You 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. The closer the sensor is to the source of the imbalance, the greater the amplitude readings will be. If you 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 you used a single sensor.
Once the sensors are installed and the cables routed away from hot and rotating components, a triggering device must be installed. Most analyzers use a phototach. The phototach is mounted behind the propeller on a bracket and emits a beam of light towards the rear of the propeller. A piece of reflective tape is installed on one of the propeller blades, in-line with the phototach. The correlation between the phototach and the front sensor will be shown as a phase and amplitude on the analyzer and a solution calculated.
Once a solution is obtained, a trial weight is installed. This trial weight is normally several large surface area washers, which are installed under the spinner retaining screws. When an acceptable vibration level is achieved, the weights are installed permanently in the starter ring gear or a hole can be drilled in the spinner bulkhead and the weight moved to this location. Some turbine-powered aircraft will have spinner bulkheads that are pre-drilled with balance weight holes facilitating ease of installation. After the weights are installed, the aircraft should be run once more to verify that the balance readings are still within tolerances.
Identifying an aerodynamic imbalance
Okay, so now you are at the point where you have performed a propeller balance and achieved a vibration level of .05 IPS, which is well below the .2 IPS limit. Yet, the owner of the aircraft still complains of a vibration in the airframe or instrument panel. In this instance, it could be that there is an aerodynamic imbalance in the propeller. In some cases, you will find that you chase your tail on a propeller with this problem. You may be able to lower vibration levels to a certain IPS level, but then it seems no further type of adjustment will give satisfactory results.
The way to detect this type of imbalance is to install reflective tape on each of the propeller blade tips in a manner that will distinguish one blade from the other. The aircraft is then operated at balancing rpm and you stand to the side of the propeller during rotation. The propeller tip path is then observed using a light source, if a difference in tracking of the blades is seen, then an aerodynamic imbalance is present. In a variable pitch propeller, the problem may be corrected by verifying the blade angles. In a fixed-pitch propeller, the only alternative may be to replace the propeller.
Any quality propeller balance job will end with a vibration survey performed on the engine assembly.
You have minimized the one-per-revolution vibration induced by an out-of-balance propeller. You have verified that the propeller has no aerodynamic imbalance. Now you need to look at the overall integrity of the rotating component to determine what is generating the vibration. How do you find it? You conduct a Vibration Spectrum Survey.
This spectral survey will show you all of the vibration levels and their frequencies (in RPM, CPM, or hertz) within the rotating component. Every moving part in an engine produces a vibration level at the frequency at which the component moves, or if the part is non-moving, it will vibrate by nature of its own natural frequency. Unfortunately, some of the components share similar or identical frequencies, which makes troubleshooting a bit more challenging. In the balance case example, you already have two vibration sensors installed on the engine so you can use these two locations to gather spectral data. Many digital balancers and analyzers have a "Spectrum" function.
Each engine and propeller combination, when healthy, will produce a normal spectrum display that is characteristic of that combination. The trick is to determine when this spectral display is not normal. When the spectral data is gathered, you have a digital display of these vibration readings, sometimes called a "signature," which then has to be interpreted. Each one of the spikes or peaks shown in the graph are representative of a rotating component or a multiple thereof. These multiples of the fundamental RPM are referred to as "harmonics" or "orders."
A half-order with readings considered to be abnormal may be the result of many different malfunctions. The first and foremost components to check for malfunction are the engine mounts. By design, the mounts dampen out lower frequency vibrations such as the half-order. If the integrity of the mounts has diminished, the result may be an increased level of vibration. If the mounts check out good, then it's time to go a little farther. Some of the malfunctions that could show a high half-order vibration are:
• Compression losses
• Fuel mixture
• Induction losses
• Improper valve lift
• Spark timing
• Plugged injectors
• Broken ring
• Bad magnetos
• Anything else associated with combustion
A normal half-order vibration reading should be in the range of .1-.3. Combustion problems will show an increase in the half order reading to levels between .3 and at times in excess of 1.0 IPS.
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