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general erosion remove material. Painting and re-painting add material. Over time, these subtle additions and subtractions to each blade changes its original contour. This throws off the balance thus increasing vibration.
Extreme additions to a prop blade are too much paint or an uneven paint job. Extreme subtractions come from over-zealous filing by an inexperienced mechanic, or the necessary correction of a big divot. If yougo in and out of dirt strips, the risk of rock divots goes with the territory. If you get one, it will be filed out at the next Phase. These extreme situations can aggravate prop imbalance and increase vibration.
Bad Vibrations
There are some telltale cracks on a King Air that point to out-of-balance props. Just remember that turbulence and the expansion/contraction of pressurization are going to take their toll, so the development of some cracks is normal on any airplane. All noticeable cracks should be documented and monitored.
That said, if I see cracks in the exhaust stacks, engine cowlings or nacelle inspection panels it gets my attention and I will look more closely. If I see cowlings starting to crack at doublers or stringers, or nacelle panels cracking at screw holes, those are signs the props may be out of balance and I start thinking about dynamic prop balancing. The same holds true with exhaust stack cracks, especially when they keep cracking around previous repair areas.
Another important concern regarding prop vibration is wear and tear on the bearing inside the engine fuel control input shaft. Being much smaller than the rest of the bearings in the engine, this bearing is the most susceptible to prop vibration. Failure of the fuel control can cause the engine to run away and an over-torque situation could result; if this happens, the engine needs to come off for inspection and repair. The point in mentioning this is that the risk of premature failure of engine components can be reduced by dynamically balancing the props.
The Balancing Act – Static versus Dynamic
Static balancing is done by prop shops after repair or overhaul. Where one blade is a tad heavier or lighter than the others, weights are added or subtracted as necessary to bring the prop into balance with itself. Dynamic balancing is done after the props are reinstalled on the aircraft. It is performed, one side at a time, with the engine running normally at takeoff rpm. We may call it prop balancing, but it is the entire prop-and-engine combination that is being balanced.
Diagnostic equipment is required to measure the vibrations so that the strategic placement of stainless steel balancing weights can be determined. The equipment measures vibrations in inches per second (ips). Although the maintenance manual lists 0.3 ips as the goal, the
20 • KING AIR MAGAZINE
OCTOBER 2021