There’s nothing about vibration that is good for aircraft, the pilot or the passengers. In 1973, the Army Air Mobility Research and Development Laboratory commissioned a study to examine the difference in reliability and maintainability of two groups of helicopters with distinctly different vibration characteristics. One group was fitted with rotor-mounted absorbers to reduce vibration, and the other group did not have the vibration-reducing absorbers. The study found that the failure rate for the helicopter group fitted with the vibration absorbers was reduced by 48 percent, and corrective maintenance was reduced by 38.5 percent. Because of the damage it can do, locating the source of vibration and eliminating it should be a high priority.
Propeller or rotor imbalance is the most common cause of vibration, and thanks to innovations like the DynaVibe prop and rotor balancer, dynamic balancing is quick and easy, and it usually resolves most vibration issues. However, sometimes the source of the vibration is more elusive and when that is the case, a vibration analyzer that does spectral analysis can pinpoint the source.
Vibration Symptom, Causes and Effects
A vibration issue isn’t always detectable by feel, because often owners and pilots have lived with vibration for a long time and have simply gotten used to it (see our Propeller Vibration Levels Guide for more information). There are always symptoms, however, such as: a panel that shakes, random instrument failure, fatigue cracks that appear or grow, cracked baffling, a compass that won’t “settle”, a rough RPM range and even tingling or numbness in pilot extremities. These symptoms, such as the fatigue crack pictured below, should not be ignored.
Beyond propeller assembly imbalance, the causes of vibration are many, and the list of possible sources includes: a weak cylinder, a loose or cracked intake hose, an alternator problem, belt resonance, gearbox issues, an oil-canning spinner, prop wash, and other causes. Because there are many possible sources, troubleshooting vibrations is often problematic, turning into an expensive, trial-and-error process of identifying the source based on someone’s best guess. The DynaVibe team has seen several forum posts from frustrated aircraft owners who have spent years and lots of their money trying to identify the source of a vibration.
The effects of aircraft vibration are all negative. When a vibration exists, some of the energy meant for propulsion is directed toward shaking the airplane. Even a seemingly small vibration can steal enough energy to cause a loss of horsepower. More troubling, however, is the damage vibration does by creating fatigue that reduces the life of the airframe, engine and instruments. Dennis Barker, president of Reynolds Aviation, knows well what damage vibrations can do to instruments: “Vibration can destroy those instruments, so balance is huge!” Barker uses a DynaVibe Classic to keep the Reynolds fleet of Cessna 172s in balance.
Identifying and resolving vibration is therefore necessary to extend the life of the aircraft and provide a smoother, safer flying experience.
Resolving Complex Vibrations
With the proper diagnostic equipment and approach, the source of a vibration is easily pinpointed. The approach is spectral analysis, which measures the frequency of vibrations, because vibration frequencies are associated with known causes or sources. When the frequency of a vibration is known it narrows the list of potential causes to few or one, eliminating the need for speculative maintenance actions.
Performing spectral analysis is accomplished using a vibration analyzer such as the DynaVibe GX2. By attaching one or two accelerometers and a photo-tachometer to the aircraft, the DynaVibe computer measures vibrations across the full spectrum of frequencies, producing a graph that displays the magnitude of the detected vibrations. For example here is a velocity mode spectral chart produced by the DynaVibe GX2 as the result of completing a vibration survey:
Vibration is measured in Inches Per Second (IPS), and this chart reveals a 1.0 IPS vibration at the half-per (H) frequency for this particular aircraft. The half-per vibration is an indicator of engine health, and in the chart above, the presence of this vibration indicates a combustion problem with one of the cylinders. To better understand what vibrations in different frequencies signal, view the recorded webinar, “Top 3 Vibration Causes” or contact a DynaVibe team member.
Brian Smith, owner of Stillwater Aircraft Services, gets value from the vibration survey capability of the DynaVibe GX2: “I can see a half-per vibration using the DynaVibe, and know that it’s caused by a combustion problem. That information eliminates other vibration sources and narrows it down to the specific problem I need to address. That’s a home run. When servicing an airplane, I don’t need to go down as many rabbit trails.”
A vibration analyzer like the DynaVibe GX2 does more than make locating vibration sources easy; it is an excellent preventative maintenance tool. Performing regular vibration surveys enables early identification of maintenance issues while they’re still relatively small, before the vibration does much damage, or most importantly, before it results in failure.
The DynaVibe GX2 is a dynamic prop balancer and vibration analyzer that is easy and economical to use. To learn about using DynaVibe to troubleshoot aircraft vibrations, enter your email address below, visit the RPX Technologies online store, or call: 405.896.0026.