No Bad Vibes
Propeller Balancing and Vibration Analysis in Reciprocating Engines
By John Sharski
So, your aircraft has a brand-new propeller fitted on a fresh, just-overhauled engine and, by the way, the propeller was statically balanced, so why bother balancing it again? You wouldn't think of buying a new set of four-hundred-dollar tires for your car and then pull out of the garage without having them professionally balanced first, would you? Think about that the next time you or a customer spends thousands of dollars on an overhauled engine and/or propeller. All components of an engine assembly, from the crank all the way out to the spinner, are manufactured to strict tolerances, but until the power train and rotating components are balanced dynamically, as an assembly, the job is only half done.
Propeller balancing equipment, like any other tool, serves a purpose. The purpose, of course, is to minimize vibration levels in the rotating components to a level that will ensure the longevity of the components and a longer service life of the propeller. Increased component life is not limited to just the rotating components. Firewalls, instrumentation, and even aircraft structural members reap the benefits of low vibration levels.
It has long been known in the helicopter world that vibration levels must be minimized for this reason. A helicopter has a rotating airfoil with diameters as large as 50 feet. Such a large rotating mass coupled with the human body's ability to easily sense low frequency vibrations, can make a relatively small amount of vibration feel as though it's going to shake your fillings loose if it's not minimized. The same repercussions occur in fixed-wing aircraft.
A general aviation aircraft is normally flown with the propeller speed of somewhere around 2,300 rpm. This rpm is higher in frequency than that of a helicopter rotor and is not conducted or sensed well by the human body. Normally, by the time the body can start to feel the effect of a vibration at this rpm, the damage has long since started to take its toll on the engine and airframe. The side effects of a high frequency vibration are normally a "buzz" in the pedals or yoke and at times may cause the pilot's feet to fall asleep. The industry standard for an acceptable level of vibration is .2 IPS (Inches Per Second). Digital balancers and analyzers have refined the balancing procedure to the point that most technicians can balance down to levels around .05 IPS.
Types of imbalance
There are two types of imbalance that may be present in a propeller.
1. Mass imbalance
2. Aerodynamic imbalance
A mass imbalance is nothing more than an imbalance in a rotating component, normally the propeller, that is located away from center of the rotating mass. The farther from the center of rotation, the greater the imbalance and its destructive force. It's similar to when a washing machine is so out of balance that it bounces off the walls of the utility closet. If we don't turn it off and redistribute the load, then naturally something is going to be damaged. Fortunately, the washing machine isn't flying at 2,000 feet AGL (Above Ground Level), but the same concept applies.
An aerodynamic imbalance, although not common in propeller aircraft, happens when a blade pitch variance occurs from one blade to another during the rotational cycle. If one blade is grabbing more air than any one of the other blades, a vibration will be felt. This can and will occur even though any mass imbalance may have already been corrected.
Correcting a mass imbalance
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.
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.