propbalance

THEORY: Propeller and Rotor Balancing

By Mark Lester

The purpose of balancing

One of the most important applications of vibration analysis is the solution of balancing problems. An unbalanced propeller, rotor or driveshaft will cause vibration and stress in the rotating part and in its supporting structure. Balancing of a rotating part is therefore highly advisable in order to accomplish one or more of the following:

Increase quality of ride.
Minimize vibration.
Minimize audible and signal noises.
Minimize structural stresses.
Minimize operator annoyance and fatigue.
Increase bearing life.
Minimize power loss.
Imbalance in just one rotating component of an aircraft may cause the entire machine to vibrate. This induced vibration in turn may cause excessive wear in bearings, bushings, shafts, gears, cabling, hoses, cowlings and exhaust systems substantially reducing their service life. Vibrations set up highly undesirable alternating stresses in structures which may eventually lead to structural failure. Aircraft performance is decreased because of the absorption of energy by the supporting structure.

Rotational unbalance

A rotating body will not exert any variable disturbing force on its supports when the axis of rotation coincides with one of the principal axes of inertia of the body. This condition is quite difficult to achieve in the normal process of manufacturing since due to errors in geometrical dimensions and non-homogeneity of the material, some irregularities in the mass distribution are always present.

As a result of the above, variable disturbing forces occur which produce vibrations. To remove these vibrations and establish safe and quiet operation, balancing becomes necessary. The importance of balancing becomes especially great in the case of high speed machines. In such cases the slightest unbalance may produce a very large disturbing force.

Vibrations due to irregular mass distribution occur at a frequency that is related to the rotating machine's speed of operation and therefore measuring such a vibration requires that the balancer utilize a filter that isolates the vibration that occurs at the machine's speed of operation.

Centrifugal force

A rotating body having an uneven mass distribution or unbalance will vibrate due to the excess centrifugal force exerted during rotation by the heavier side of the rotor. This unbalance causes centrifugal force, which in turn causes vibration. When at rest, the unbalance exerts no centrifugal force and does not cause vibration to occur. Yet, the actual unbalance is still present. Unbalance, therefore, is independent of rotational speed and remains the same, whether the rotor is at rest or is rotating (provided that the part does not deform during rotation).

Centrifugal force, on the other hand, varies with speed. When rotation begins, the unbalance will exert centrifugal force tending to vibrate the rotor and its supporting structure. The higher the speed, the greater the centrifugal force exerted by the unbalance and the more violent the vibration. Centrifugal force increases proportionally to the square of the increase in speed. If the speed doubles, the centrifugal force is quadrupled, etc.

Measuring unbalance with a vibration analyzer

As mentioned above, the relationship between unbalance of a rotating body and the vibration produced is highly dependent upon rotor speed and other operating conditions. When making vibration measurements for the purpose of determining and correcting unbalance, the operator must operate the machine in a consistent and repeatable manner to insure that repeatable measurements can be made.

To measure the vibration due to rotor unbalance, a vibration transducer (sensor) is attached to the vibrating body when measuring imbalance. The vibration sensor converts this mechanical motion into an electrical signal that corresponds to the body's motion in space. The vibration analyzer is then used to sample this electrical signal and make various calculations based on the electrical signal's properties.

In addition to the vibration measurement, a tachometer (tach) signal is collected. A tach sensor such as a photo tach or magnetic pickup is used to detect the position of the rotating body with respect to time. As in the case of the vibration transducer, the tach sensor converts this information into an electrical signal which can then be sampled by the vibration analyzer and used in various calculations.

The operator will use the Analyzer to collect a series of narrowband vibration readings called Peak Phase measurements since each reading is composed of a peak value and a phase reading. The peak reading (amplitude) is proportional to the amount of mass imbalance in the rotating machine. The phase reading (phase angle) provides information about the location of the mass imbalance.

A Balance Solution (corrective weight) is computed by the Analyzer based on the amplitude and phase angle of the vibration reading. The corrective weight is then applied to the machine and the measurement process is repeated. The balancing job is finished when the vibration is reduced below an acceptable level.

A complete balancing job will usually consist of a number of Peak Phase readings along with the weight changes made which were made. For more information see the section on Balance History.

What is dynamic propeller balancing?

Dynamic Propeller Balancing is the process whereby an electronic balancer is used to measure the vibration produced by the aircraft power plant. Small trim balance weights are added to the propeller/crankshaft assembly to correct for errors in mass distribution and to reduce power plant vibration due to mass imbalance to the lowest level practical.

How is it done?

The engine/propeller combination is balanced right on the aircraft, in a flight ready state. A small vibration sensor (accelerometer) is attached to the engine in a location where vibration due to mass imbalance is maximum. A small tach pickup (photo-tach) is also mounted to the cowl or engine to produce a propeller tach signal. The engine is operated and the vibration and tach signals are processed by the electronic balancer.

The balancer produces a vibration level (magnitude) which corresponds to the amount of mass imbalance which exists. The balancer also provides a "phase angle" which corresponds to the location of the mass imbalance on the propeller disk. The vibration level and phase angle are used to compute a balance solution (weight amount and location). The balance solution is added to the propeller and the measurement is repeated until the vibration level is found to be acceptable.

My aircraft runs fine. Why should I have my prop dynamically balanced?

By all means, have your propeller balance checked. The average airplane which has not had a Dynamic Prop Balance has a vibration level due to propeller/crankshaft mass imbalance of about .450 inches per second (IN/S) velocity. This level is more than four times higher than what is considered to be an acceptable vibration level for propellers and represents a significantly higher level of wear and fatigue on engine components and accessories. This average level is usually very noticeable to the pilot and occupants. Yours may be higher or lower than the average but only a mechanic with a vibration analyzer can tell for sure. On the average, 19 out of 20 fixed-wing aircraft can benefit from Dynamic Prop Balancing yet many will never have it done.

My engine and prop were overhauled recently. Should I have my prop dynamically balanced?

Both new and used components need to be dynamically balanced. In fact, the best time to dynamically balance a propeller/engine combination is right after overhaul when components are fresh. In the DSS study, no appreciable difference was found between the vibration levels of recently overhauled and longer time propeller/engine combinations. Even brand-new aircraft with zero time engines and propellers need to be dynamically balanced.

What level of propeller vibration is acceptable?

In most cases, the vibration level due to mass imbalance can be brought down to under .100 IN/S very easily. In the DSS study, the average level seen post-propeller balance was .039 IN/S. When propeller vibration levels are this low, the operator will generally see a significant reduction in component wear and fatigue and will find that the aircraft "feels" like a completely different machine.

In addition to that the mechanic now knows what all of the other engine vibration levels are (Prop/crankshaft mass imbalance usually dominates them) and can utilize that information if additional work is needed.

But won't having my propeller dynamically balanced "mask" other engine problems?

No! An engine with an internal problem which results in unusual vibration will not respond to balancing in the same way that an engine which only suffers from mass imbalance will. A qualified mechanic will use ALL of the information available to make a judgment about your engine, including the vibration response.