FAQ: Dynamic Propeller Balancing
What is dynamic propeller balancing?
Dynamic Propeller Balancing is the process whereby an electronic balancer is used to measure the vibration produced by the aircraft power plant. Small trim balance weights are added to the propeller/crankshaft assembly to correct for errors in mass distribution and to reduce power plant 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 maximum. 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 electronic balancer.
The balancer produces a vibration level (magnitude) which corresponds to the amount of mass imbalance which exists. The balancer also provides a "phase angle" which corresponds to the location of the mass imbalance on the propeller disk. The vibration level and phase angle are used to compute a balance solution (weight amount and location). The balance solution is added to the propeller and the measurement is repeated until the vibration level is found to be acceptable.
My aircraft runs fine. Why should I have my prop dynamically balanced?
By all means, have your propeller balance checked. The average airplane which has not had a Dynamic Prop Balance has a vibration level due to propeller/crankshaft mass imbalance of about .450 inches per second (IN/S) velocity. This level is more than 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 and accessories. This average level is usually very noticeable to the pilot and occupants. Yours may be 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 benefit from Dynamic Prop Balancing yet many will never have it done.
My engine and prop were overhauled recently. Should I have my prop dynamically balanced?
Both new and used components need to be dynamically balanced. In fact, the best time to dynamically balance a propeller/engine combination is right after overhaul when components are fresh. In the DSS study, no appreciable difference was found between the vibration levels of recently overhauled and longer time propeller/engine combinations. Even brand-new aircraft with zero time engines and propellers need to be dynamically balanced.
What level of propeller vibration is acceptable?
In most cases, the vibration level due to mass imbalance can be brought down to under .100 IN/S very easily. In the DSS study, the average level seen post-propeller balance was .039 IN/S. When propeller vibration levels are this low, the operator will generally see a significant reduction in component wear and fatigue and will find that the aircraft "feels" like a completely different machine.
In addition to that the mechanic now knows what all of the other engine vibration levels are (Prop/crankshaft mass imbalance usually dominates them) and can utilize that information if additional work is needed.
But won't having my propeller dynamically balanced "mask" other engine problems?
No! An engine with an internal problem which results in unusual vibration will not respond to balancing in the same way that an engine which only suffers from mass imbalance will. A qualified mechanic will use ALL of the information available to make a judgment about your engine, including the vibration response.
Where can I find a mechanic to balance my propeller?
Dynamic propeller balancing is done my many prop shops, repair stations and FBO's. For a local referral to a maintenance shop which provides dynamic propeller balancing services, talk to Dynamic Solutions Systems.
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