A balanced propeller assembly is important not just for having a smooth flying experience, but also for the health and longevity of an airplane’s engine, instruments and airframe. Statically balancing a propeller can help reduce vibrations, but to reduce prop imbalance-induced vibration to the minimum, dynamic balancing is recommended.
This story of a Beechcraft Super Musketeer illustrates the importance of dynamic prop balancing, even after static balancing has been performed. This Beech had a 200 horsepower Lycoming engine and a constant speed prop. The airplane had long experienced a vibration problem, but the owner had never attempted to resolve it. The airplane’s propeller was last statically balanced in 2000.
Even though a prop has been statically balanced, once mounted, things can change because the entire propeller assembly comes into play, not just the perfectly balanced, ready-to-mount prop.
It takes just the slightest change in mass anywhere in the propeller assembly to change the balance and introduce a vibration. These numbers illustrate this reality: a perfectly balanced prop, offset by just .0005 inches during installation, can cause a vibration of .6 Inches per Second (IPS)! This scenario occurs more often than most pilots or owners know, because many props don’t have an indexing mechanism to ensure that during mounting, the prop it is perfectly centered with the crankshaft. The dynamic balancing process corrects these “coupling” errors that can result when mounting a prop.
In the case of the Super Musketeer, the prop imbalance was discovered when the owner brought it in for some maintenance. The mechanic had access to a DynaVibe balancing system and did a quick prop balance check. This check revealed a severe vibration of 1.5 IPS, well above the FAA specified maximum limit of 1.2 IPS for prop vibration. A vibration of this magnitude mandates removal of the prop for static balancing. The mechanic pulled the prop off and sent it to a prop shop for inspection and static balancing. The mechanic and owner were surprised to discover that during the previous static balance from 15 years ago, the weight had been added to the wrong prop blade, making the balance and vibration problem worse, not better. The owner had lived with this problem, simply assuming that some level of vibration is a product of combustion.
After the static balancing was complete, the mechanic remounted the prop on the Super Musketeer and dynamically checked the balance. The static balancing process had reduced the prop vibration by about half, to .67 IPS. Using the weight solution provided during the initial dynamic balancing check, the mechanic added the prescribed weight where indicated and did a second balancing run, which showed the vibration had dropped to .04 IPS, well below the level that is discernible by the pilot or passengers. The following report was generated by the DynaVibe GX2 and shows the results of the dynamic balancing runs, including the level of vibration detected and the weight solution calculated to resolve the detected vibration, and where to install it:
The owner’s feedback after dynamic balancing was that his Musketeer felt like a totally new airplane! Static balancing, followed by dynamic balancing, is a great way to get a smoother flying experience and take stress off of the engine, instruments and airframe.
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