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FAQ: Dynamic Propeller Balancing
What is Dynamic Propeller Balancing?
Dynamic propeller balancing is the process whereby an electronic balancing computer is used to measure the vibration produced by the aircraft propeller and powerplant. Small trim balance weights are added to the propeller/crankshaft assembly to correct for errors in mass distribution and thus reduce the measured vibration due to mass imbalance to the lowest level practical.
How is it done?
The engine/propeller combination is balanced right on the aircraft, in a flight ready state. A small vibration sensor [accelerometer] is attached to the engine in a location where vibration due to mass imbalance is at the maximum, at the crankcase spine as close as possible to the propeller flange. A small tach pickup [photo-tach] is also mounted to the cowl or engine to produce a propeller tach signal. The engine is operated and the vibration and tach signals are processed by the balancing computer. The balancing computer measures the vibration level [magnitude] produced by the accelerometer which corresponds to the amount of mass imbalance which exists. The balancing computer also provides a "phase angle" which corresponds to the location of the mass imbalance on the propeller disk. The vibration and phase angle are used to compute a balance solution {weight amount and location]. The computer "learns" the reaction to the weight/location [trial weight] installed and compensates each additional run process until the measured imbalance is found to be at an acceptable level.
My aircraft runs fine. Why should I have my propeller dynamically balanced?
The average aircraft which has not had a dynamic prop balance has a vibration level due to propeller mass imbalance of about .45 inches per second velocity [IN/S], This level is over four times higher than what is considered to be an acceptable vibration level for propellers and represents a significantly higher level of wear and fatigue on engine components, accessories, airframe structure, avionics and electrical components. This average level is very noticeable to pilots and occupants. The manufacturing industry subjects products to vibration and shock destructive testing to determine life-times before failure; the less vibration an aircraft or product is subject to, the longer the lifespan between failures. Vibration redirects energy from the intended purpose into bearing area friction; friction robs horsepower! More available horsepower to the propeller can produce more thrust. Your vibration level may be significantly higher or lower than the average, but only a mechanic with a vibration analyzer can tell for sure. On the average, 19 out of 20 fixed-wing aircraft can significantly benefit from dynamic propeller balancing.
What level of propeller vibration is acceptable?
In most cases, the vibration level due to mass imbalance can be brought down to a value under .10 inches per second (IPS) very easily. In a Dynamic Solution Systems study, the average level seen post-propeller balance was .039 IPS, our average is less than .024 IPS! Propeller mass imbalance dominates the vibration levels in a normally operating combustion engine, it is possible to evaluate engine combustion rate vibration values to help determine if the engine is running smoothly.
Won't having my propeller dynamically balanced "Mask" other problems?
No, An engine with internal or external problems which result in unusual vibrations will not respond to balancing in the same way that an engine which only suffers from mass imbalance will. A qualified, experienced mechanic will use ALL of the information available to make a judgment about your engine including the vibration response during the balance procedure. With proper equipment and technique, it is even possible to evaluate vibration values produced by each cylinder to help determine if the engine is running smoothly.
If I have a new or overhauled propeller, is there any reason to have it checked?
Balance performed by the manufacturer and overhaul shop is static (NOT MOVING) or "bubble balance" on the propeller only. Dynamic balance testing is performed on the entire rotating assembly (engine, propeller, spinner, extension, etc) in its normal operating configuration. Constant speed propeller blades are centrifugally loaded and seated in the hub only under operating conditions. The result is a true balance under all operating conditions. We recommend at least 5 hours of time on a new or overhauled prop to allow for grease leveling, seating of bearings, shim wear etc, to occur.
Since I have a geared engine, is there any reason to have a dynamic balance performed?
Geared engines have a greater need of dynamic propeller balance because the propeller mass is greater compared to the reduction gear box mass. Studies done by the U.S. Navy indicates that dynamic propeller balance on turboprop aircraft results in 100% increase in TBO of reduction gear boxes.
When should I have the prop rebalanced?
I recommend checking the dynamic balance at the next annual or 100 hour TIS (Time In Service) to indicate the wear/imbalance pattern over time of your prop/engine system. (This would be the optimal solution) If a large nick is reworked or the prop removed and reinstalled; the dynamic balance should be checked as the location movement on the crankshaft flange can cause a imbalance level in excess of .3 IPS. I recommend dynamic balance check of previously balanced props at 500 hours maximum TIS.
Would you put tires on your car or truck without having a dynamic balance (spin balance) performed?
The propeller on your aircraft weighs as much or more than a car tire and can produce much higher vibration levels and is directly mounted to your engine...why wouldn't you have the propeller dynamically balanced?
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