The Balanced Prop

The propeller must be in static and dynamic balance to reduce stress caused by vibrations and make for a smooth running propeller. Generally, a three-blade propeller runs more

smoothly than a two-blade prop; no matter how many blades there are it is important the propeller is in perfect balance.

If a two-blade prop has a 100-gram weight placed at the tip of one blade and 200-gram weight placed half way along the other blade, the prop would be in a state of static balance. A prop that is statically stable would not rotate on its prop shaft (if it were free to do so) from any position. The prop is then said to be in a state of static balance but it would not be dynamically balanced and would vibrate severely at high RPM. To be in dynamic balance, the prop has to be balanced evenly for the same reason a car tyre is balanced to prevent it from vibrating at high speed on the road.

There are at least two methods to balance the prop. One, the prop can be removed from the aircraft and placed on a workshop stand for balancing. This is a relatively simple method but not as accurate as using an electronic balancer, such as a Chadwick-Helmuth Vibrex Dynamic balancer. A small accelerometer is placed on top of the engine, which measures vibrations in inches per second at different RPM. The electronic balancer indicates where, and how much weight is to be placed on the prop to balance it. Again, this is similar to balancing a car wheel. A correctly balanced propeller will have the vibrations reduced to around 0.04 inches per second. A smooth running prop makes the ride more enjoyable and less fatiguing for the plane’s occupants.