Dynamic Propeller Balancing Basics
The most common source of aircraft vibration is the result of an imbalance between the propeller and drive train. The vibration created due to the imbalance can affect a wide range of aircraft systems, such as:
avionics, radio and indicator light failure
alternator or generator failure, attachment bracket cracks, thrown drive belts, flywheel cracks and bolt fatigue
vacuum pump failure
wiring harness failure, chafing, terminal end work hardening failure
fuel control failure
leaky oil coolers, cracked welded areas
cracked or broken exhaust components, heat muffs
cracked or broken carburetor air box
baffle, cowl and sheet metal cracks
broken or cracked engine/turbocharger mounts
Dynamic propeller balancing ensures that the aircraft propeller and other rotating components are rotationally balanced thus reducing vibrations and failures. Imbalances are detected using computerised diagnostic equipment and are corrected by the addition of small weights. In this way vibrations are reduced to an acceptable and safe level.
The dynamic propeller balancing procedure can be done on any aircraft in a flight-ready state. At DynaProp we use the best dynamic propeller balancing equipment available which is calibrated regularly by the manufacturer.The entire process usually takes 3-5 runs and an average of 3-5 hours and is far more economical than replacing the range of components that could potentially suffer damage as a result of vibration.
LAA article on Dynamic Propeller Balancing (April 2011)
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 than recommended. This average level can be noticeable to pilots and passenger. However, we often say that vibration is like cholesterol; its accumulation creeps up on you without your noticing. As it increases gradually, our bodies become accustomed to it and its increase goes unnoticed. As with cholesterol, it is advisable to have a “check-up” to see what your actual vibration level is at. We recommend having a “check-up” at least every 500 hours of flight.
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 0.15 inches per second (IPS) very easily. In a Dynamic Solution Systems study, the average level seen post-propeller balance was .039 IPS, however, our average is less than .024 IPS! Propeller mass imbalance dominates the vibration levels in a normally operating combustion engine. We do have the technology, however, to single out the engine combustion rate vibration values by cylinder to help determine the health of your engine.
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 prop balancing in the same way that an engine which only suffers from mass imbalance will. Evaluating ALL of the vibration data obtained helps us determine whether vibration is due to a propeller/engine mass imbalance or if there are other problems causing excess vibration.
If I have a new or overhauled propeller, is there any reason to have it checked?
When a prop balance is performed by the manufacturer or overhaul shop, it is a static (not rotating) balance, or "bubble balance," on the propeller only. Dynamic balancing is performed on the entire rotating assembly (engine, propeller, spinner, flange, extension, retaining bolts and nuts, washers, etc.) in its normal operating configuration while the engine is running. By using multiple vibration pickups, it is possible to balance the entire system on two planes (propeller and flywheel).
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 settling in time on a new or overhauled propeller before dynamically balancing to allow for grease leveling, seating of bearings, shim wear.
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 RAF and Royal Navy indicate that dynamic propeller balancing on turboprop aircraft results in 100% increase in Time Before Overhaul (TBO) of reduction gear boxes.
When should I have the prop rebalanced?
DynaProp 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. Also, if a large nick is reworked or the prop is removed and reinstalled for any reason, the dynamic balance should be re-checked as the location movement on the crankshaft flange can cause an imbalance level in excess of .3 IPS. We recommend that a dynamic balance check be performed at least every 500 hours maximum TIS.
Question for you:
Have you ever experienced a vibrating or shaky steering wheel when traveling on the motorway at a certain speeds?
The usual culprit of the above symptoms is a wheel that is out of balance likely to be caused by the loss or movement of one of the balancing weights that are applied to tyre when it was fitted. At high speeds, the effects can be quite dramatic and at times dangerous. Would you put tires on your car or truck without having a dynamic balance (spin balance) performed? probably not ...
Aeroplanes are far more sensitive to vibration. The propeller on your aircraft weighs as much or more than a car tire and can produce much higher vibration levels as it is directly mounted to your engine.
So, why not have the propeller dynamically balanced?