Over 30 years ago we began spin balancing our car
tires which we quickly found was a vast improvement over static
Propellers are statically balanced during
overhaul and this is a good start, however, test-stand balancing
does not compensate for the unbalance caused by the prop spinner,
spinner bulkheads, starter ring gear and aerodynamic loading. Also,
since residual prop and/or crank
geometrical manufacturing errors are normally always present, these
slight errors introduce another chance for unbalance. For example,
the center of the prop mass is only one-thousand of an inch out of
alignment with the centerline of the crankshaft the result is a 0.3 IPS imbalance which amounts to a ~40 lb oscillating side-load at the
front engine bearing. Although the bearing oil pressure will prevent
metal on metal interference up to 1.2 ips, the vibration energy is
still transmitted throughout the engine and airframe.
All unbalance vibration amounts to wasted energy.
Well, not truly wasted since this energy will eventually work to cause
and component failures as well as premature avionics and instrument
failures. The repairs are
usually costly and since the band aid approach is temporary,
vibration continues to take its toll.
Vibration also exacerbates
pilot fatigue and can cause passenger discomfort and safety