No Bad Vibes: Propeller Balancing and Vibration Analysis in Reciprocating Engines

No Bad Vibes

Propeller Balancing and Vibration Analysis in Reciprocating Engines

By John Sharski

November 2000

So, your aircraft has a brand-new propeller fitted on a fresh, just-overhauled engine and, by the way, the propeller was statically balanced, so why bother balancing it again? You wouldn't think of buying a new set of four-hundred-dollar tires for your car and then pull out of the garage without having them professionally balanced first, would you? Think about that the next time you or a customer spends thousands of dollars on an overhauled engine and/or propeller. All components of an engine assembly, from the crank all the way out to the spinner, are manufactured to strict tolerances, but until the power train and rotating components are balanced dynamically, as an assembly, the job is only half done.
Propeller balancing equipment, like any other tool, serves a purpose. The purpose, of course, is to minimize vibration levels in the rotating components to a level that will ensure the longevity of the components and a longer service life of the propeller. Increased component life is not limited to just the rotating components. Firewalls, instrumentation, and even aircraft structural members reap the benefits of low vibration levels.
It has long been known in the helicopter world that vibration levels must be minimized for this reason. A helicopter has a rotating airfoil with diameters as large as 50 feet. Such a large rotating mass coupled with the human body's ability to easily sense low frequency vibrations, can make a relatively small amount of vibration feel as though it's going to shake your fillings loose if it's not minimized. The same repercussions occur in fixed-wing aircraft.
A general aviation aircraft is normally flown with the propeller speed of somewhere around 2,300 rpm. This rpm is higher in frequency than that of a helicopter rotor and is not conducted or sensed well by the human body. Normally, by the time the body can start to feel the effect of a vibration at this rpm, the damage has long since started to take its toll on the engine and airframe. The side effects of a high frequency vibration are normally a "buzz" in the pedals or yoke and at times may cause the pilot's feet to fall asleep. The industry standard for an acceptable level of vibration is .2 IPS (Inches Per Second). Digital balancers and analyzers have refined the balancing procedure to the point that most technicians can balance down to levels around .05 IPS.

Types of imbalance
There are two types of imbalance that may be present in a propeller.
1. Mass imbalance
2. Aerodynamic imbalance

A mass imbalance is nothing more than an imbalance in a rotating component, normally the propeller, that is located away from center of the rotating mass. The farther from the center of rotation, the greater the imbalance and its destructive force. It's similar to when a washing machine is so out of balance that it bounces off the walls of the utility closet. If we don't turn it off and redistribute the load, then naturally something is going to be damaged. Fortunately, the washing machine isn't flying at 2,000 feet AGL (Above Ground Level), but the same concept applies.
An aerodynamic imbalance, although not common in propeller aircraft, happens when a blade pitch variance occurs from one blade to another during the rotational cycle. If one blade is grabbing more air than any one of the other blades, a vibration will be felt. This can and will occur even though any mass imbalance may have already been corrected.

Correcting a mass imbalance