Vibration Control in the 21st Century - By Norman S. Serrano
Vibration has long been known for its role and negative impact with both fixed-wing and rotor-wing aircraft. It significantly reduces the performance of the aircraft, reduces the lifespan of parts and equipment as well as physical impact to the pilot, aircrew and passengers.
The key is identifying the mechanical condition of the source causing the vibration and dynamically balancing it. We have a tried and true method to gather collected vibration data and interpret it. It does all this in such a manner, that the time spent on smoothing the vibration is significantly reduced. Reduction equals maintenance savings and faster return to service for the airframe.
Let’s look at an example: Some aircraft technicians would believe that “magic” is required to track and balance a helicopter's rotor or dynamic balance an airplane's propeller. Bear in mind, to develop proficiency in dynamic balancing a technician would have had gone through a school of Hard Knocks (on the job blunders). Unfortunately, some of those “on the job blunders” would lead the technician down the wrong path in understanding the simplest principle in dynamic balancing. That is why many experienced technicians will contradict each other on their own intuitive, “magical,” theories of dynamic balancing.
Let’s look a little deeper - since only a few moving parts exist on an airplane, the propeller and engine assembly are usually the main sources for vibration. A static balanced propeller usually made the vibrations feel more tolerable – just like the balanced tires on your car made the vehicle ride more smoothly. Before dynamic balancing became a common practice, if vibrations still felt intolerable, aircraft manufacturers would simply replace the propeller; one after another until achieving what was considered to feel normal. Of course, this practice is expensive. Some of the higher frequency vibrations aren’t even noticeable to the pilot; these types of vibrations would only be felt close to their source. This un-apparent vibration would still cause damage to the aircraft. Aircraft manufacturers would then design engine mounts, isolators, dampers, etc.; to help absorb the vibrations caused by the propeller and engine assembly.
While this example focused on an airplane, it applies equally to helicopters, racecars, and any device that turns at a high rate of speed.
The principal idea that I want to communicate is that vibration can be controlled. The second issue I want to get across is that you can avoid costly replacement of parts that aren’t bad or damaged.
Even today, with all of our technological advances, it is not just the aircraft manufacturers that replace parts, hoping to eliminate the vibration. This happens in virtually every hangar. Aircraft will always experience vibrations in one form or another. Remember, vibration will either be correctable or uncorrectable. Correctable vibrations will come from rotating parts that have mass or aerodynamic differences between them which a mechanical adjustment can correct. Vibrations that are uncorrectable will normally relate to the aircraft's design – no mechanical adjustment is possible.
Since aircraft structures will always possess some flexibility, oscillatory vibration will either be linear or nonlinear. For linear vibrations, mathematical techniques and treatments can become well developed. Techniques for treating nonlinear vibrations are very difficult to apply. Be aware, depending on the aircraft's flexibility, a correctable vibration can become nonlinear, by increasing the level of vibration.
Electronic balancers are used to measure correctable vibrations that respond linearly to balance adjustments. The vibration measurement itself should represent the unbalance condition of the rotating assembly. With this measurement, it is now possible to calculate a balancing solution to correct the unbalance condition.
The worst paradigm the aviation industry has falling into is that the vibration responses of each balance adjustment are linear. Linearity will change without warning. This does not mean that some vibrations can not be solved, we just have to reconfigure to the new responses of each balance adjustment. In today’s technology the industry has become dependent upon computer models to calculate a solution to smooth out rotor or propeller vibration. If the rotating system does not mach the linearity of the computer model the calculated solution will not smooth the rotating system – the more balance adjustments involved in the computer model the more likely the computer model will not match the actual rotor or propeller system.
There is no education that teaches theory of dynamic balance. Any type of dynamic balance training teaches only how to operate the electronic balancer. This leads to many problems when trouble-shooting rotor or propeller vibrations. For helicopters, if the calculated solution adjustment does not smooth the rotor the mechanic could only rely on what they can see, such as blade tracking, to make any conclusions if bad components exist within the rotor system. In fact, the rotor system may be perfectly fine; the rotor just may not match the computer model. The mechanic has no choice but to blindly change components within the rotor system in trying to solve the vibration problem. Actually, the mechanic is unknowingly changing the real rotor system to match the computer model.
A new paradigm will began to form with experienced mechanics, as they learn that the complex group of solution adjustments does not always work, they began to pick and chose adjustments out of the calculated group of adjustments. The problem gets worse; first, this puts the mechanic and the aircraft, or electronic balancer, manufacture at odds with each other – due to the fact the mechanic did not accomplish all the calculated adjustments. Secondly, the rotor system may happen to match the computer model close enough to smooth the rotor; but a smooth rotor could not be achieved because the complete group of adjustments was not accomplished. Quite simple, frustration forms and the mechanic would rather stay away from track and balance altogether.
To cure that track and balance frustration, first proper training in dynamic balance theory or a computer balance model that is able to change or conform to each real rotor system. Secondly, if the calculated solution can not smooth the rotor system, due to a component fault, a mechanical faultfinding method should be available to find bad components within the rotor system. Currently, if the calculated solution does not smooth the rotor system, the mechanic does not know if it is because of a bad component or if the computer model does not match the actual rotor. Remember that the mechanic is not even trained to consider if the computer model is the problem.
Finally, current technology in measuring the rotor vibration may be inadequate in measuring actual oscillatory vibration movement of the rotor system; therefore, vibration analysis will become inadequate. Current technology only measures vibration in single separate dimensions. The rotor system vibrates in multiple dimensions.
Reference: Norman Serrano, “Basics Principles of Dynamic Balancing”, P.O. Box 464, Pleasant Grove, Utah 84062. Copyright 2005. All rights reserved.