This webinar was part of the EAA webinar series and was originally broadcast on February 24, 2016. It features RPX Technologies co-founder Matt Dock as presenter.
To learn more about using DynaVibe to dynamically balance your prop, or to conduct a vibration survey to pinpoint more complex vibrations, please call us at 405.896.0026 or by email: email@example.com. If you prefer, enter your email address or phone number using "Contact Us" at the bottom of this page and we'll contact you! Shop for DynaVibe by visiting our online store.
There's increasing talk about vibrations surveys - what they are, why and how to do them - for good reason: vibrations damage aircraft. A vibration survey will allow you to pinpoint the source of a vibration so that you don't have to pay for speculative maintenance in an effort to resolve it. We've written a lot recently in this blog about vibration surveys, and now we've done one better: we can show you the process in our new video!
We set up an RV-12 with a Rotax 912 engine for balancing and vibration surveying with the DynaVibe GX2, our dynamic prop balancer that also has vibration survey capability. In just over three minutes, the video will show you the process sequence and sensor placement for collecting a vibe survey. You can watch it here:
If you are new to the concept and practice of vibe surveys, please review some of our older posts that discuss this subject. Even better, we welcome your inquiries on how and why to perform vibe surveys, and how to interpret the results you're seeing. Feel free to call us at 405.896.0026, send email to firstname.lastname@example.org or simply fill your email address using the "Contact Us" button below. To shop for a DynaVibe system, visit out online store.
For any operator, particularly the ag aviation operator, having an engine go out-of-service unexpectedly is not only frustrating, but potentially revenue-impacting, or worse, life-threatening. It’s not possible to predict all engine failures in advance, but it is possible to detect many of them through the use of a full-spectrum vibration survey.
A full-spectrum vibration survey, or vibe survey for short, is a procedure that uses engine-mounted sensors to measure the frequency and magnitude of vibrations emitted by an aircraft engine. Certain frequencies of vibrations map to specific problems or potential problems. In other words, a vibe survey can tell you a number of things about what’s going on inside your engine based on the presence of vibrations. Identifying these vibrations early, when they’re relatively small, allows the operator to address them before they result in a failure. For this reason, a vibe survey is an excellent preventative maintenance tool.
Conducting a vibe survey doesn’t take long, and most of the time is spent setting up the airplane with a phototachometer, reflective tape and an accelerometer to measure the vibration. Once the airplane is set up, the engine is run up to operating RPM and with the push of a button, the vibe survey data is captured. The photograph below shows the set up on a Grumman Ag-Cat with a Garrett TPE331 engine and a 3-blade prop:
Vibe surveys measure vibration in Inches Per Second (IPS). In general, the maximum, acceptable level of vibration is .15 IPS. However, vibrations at this level probably escape the notice of the operator, but a vibe survey can detect a vibration that an operator can’t yet feel. Vibrations of .50 IPS are substantial enough that, over time, they can damage the engine, airframe or instruments.
Most of the time, vibrations are the result of propeller assembly imbalance, a condition corrected by dynamically balancing the prop. Typically, a vibe survey is done as part of dynamic propeller balancing. The table below shows the actual result of a series of prop balancing runs performed using a DynaVibe GX prop balancer and vibration analyzer on the Ag-Cat.
The second column in the chart above shows the vibration magnitude for each successive prop balancing run. Subsequent columns show the recommended weight and location for adding that weight to balance the prop. At the end of the third balancing run, the vibration was almost eliminated, and was at a low enough level to not require further balancing runs.
Sometimes vibrations are more complex, and the value of a vibe survey is revealing the source of the vibration, particularly if the cause is something other than the prop. In the case of this particular Ag-Cat, the post-prop balancing vibe survey looked like this:
After prop balancing the only significant vibration remaining is the 3-per vibration (100 Hz) caused by the 3-bladed prop passing near the ground. This vibration is not a concern as it will go away once in the Ag-Cat is in the air.
Vibe surveys are a great way to do engine trend monitoring. Conduct a vibe survey on a new engine or anytime an engine has been overhauled. This baseline vibe survey then serves as the benchmark for comparing future vibe surveys done at regular intervals. The approach allows detection of vibrations when they’re relatively slight, before the operator can feel them. The operator is alerted to their presence and can continue to monitor them and address them before they grow to the point where they can damage the engine, or worse, result in a failure.
Matthew Mitchell, who operates Mitchell Flying Service based out of Perry Municipal Airport (F22) in Oklahoma, has been flying since 1990 and puts about 250 to 300 hours per year on his Grumman Ag-Cat. Mitchell has two TPE-331 engines that he rotates as needed. Anytime Mitchell puts an engine together, he likes to have a baseline vibe survey done. This allows him to monitor the health of that engine while it is in use. Regular vibrations surveys can help operators like Mitchell to identify problems before they cause a shutdown or result in a failure.
The DynaVibe GX system is the most affordable way to do dynamic propeller balancing and vibration surveys for engine trend monitoring. To learn more, click the yellow "Contact Us" button below, or call the DynaVibe team at: 405.896.0026. You can also enter your email address in the form below if you want us to contact you. If you'd like to shop for a DynaVibe GX2, visit our online store.
Click here to watch a short video showing how to conduct a vibe survey with DynaVibe.
Much has been written in this blog about vibration analysis: what it is, how it works and the benefits. The DynaVibe GX2 is the solution from RPX Technologies that simplifies performing a vibration analysis, or vibration survey as they are often called. If you’ve never seen the output of a vibration survey, you might find it helpful to see an example.
The following graph is the actual output of a vibration survey taken using a DynaVibe GX2 of a Rotax 912 engine:
This Velocity Mode chart shows the vibrations that were detected, measured in Inches Per Second, during the vibration survey. We’ve annotated the chart so you can easily see what the presence of these vibrations indicate:
The threshold of tolerance for a vibration is about .2 IPS. Anything greater should trigger some action to identify and resolve the vibration(s). With the information a vibration survey provides, resolving these vibrations becomes much easier because there is certainty about their cause. Let’s look at each of the vibrations this survey identifies in more detail.
The 1-per Vibration
A 1-per vibration is caused by propeller imbalance. Dynamically balancing the prop should eliminate a 1-per vibration. Dynamic prop balancing is always a good place to start when resolving vibrations, because it eliminates the most likely source of vibration. Because this particular vibration survey reveals multiple vibration sources, dynamically balancing the prop will help, but other vibrations remain.
The 1.2-per Vibration
The presence of a 1.2-per vibration can mean different things. On a standard engine, this vibration would appear on the survey as a half-per vibration, but since the Rotax engine in this example is geared, this vibration manifests as a 1.2-per. Typically, this vibration indicates a combustion problem. It can also indicate that the carbs are out of balance. The way to know for sure is to complete at least one more vibration survey at a higher RPM. If the 1.2-per vibration stays consistent as you vary the RPM, then a cylinder combustion problem is the likely cause. If, however, the intensity of the 1.2-per vibration varies as RPM changes, a carburetor imbalance is the most probable cause. Out of balance carbs tend to show the most pronounced vibration at mid-RPM levels. In this example, a second vibration survey (not shown) confirmed the suspected carb imbalance, so balancing the carbs would resolve this vibration.
The 2-per vibration
The 2-per vibration in this example measured 1.7 IPS, a very significant vibration. A 2-per vibration is related to something that rotates with the prop. Since this engine had a 1-per vibration, it is known that the prop was out of balance. The recommended approach is to first dynamically balance the prop and then complete another vibration survey to see if the intensity of the 2-per vibration (or any others) has changed. In this case, the 2-per vibration persisted even when the prop was balanced. Since the Rotax 912 is a geared engine, attention fell on the gearbox as the vibration source. In this example, testing the friction torque of the gearbox revealed that it was well over the manufacturer’s maximum, which led the mechanic to contact the manufacturer for a resolution.
The 2.43-per vibration
The most significant vibration shown on this survey, the 2.43-per, is probably caused by the difference in length of the intake manifold. This engine is designed with the front cylinder having a longer flow path than the back cylinder. This difference in front-back airflow causes a torsional vibration in the engine, and the DynaVibe GX2 measure this vibration by utilizes two accelerometers mounted in different places on the engine. This vibration typically diminishes almost to zero as RPM is increased to standard cruise levels. This assumption was easily verified by doing another vibration survey at cruise RPM, proving the assumption true. For this reason, the presence of this engine’s 2.43-per vibration was not a concern.
Here’s what you should take away from this vibration survey example and discussion:
Vibration surveys are also an excellent preventative maintenance strategy. Doing vibration surveys at regular intervals can provide an early warning of maintenance issues while they’re relatively small, before they’re noticeable or even better, before they cause a failure. You can purchase a DynaVibe GX2 by visiting our online store, or if you wish to learn more, just call us at 405.896.0026 or click on the "Contact Us" tab near the bottom of this page and we’ll contact you.
RPX Technologies co-founder Matthew Dock contributed an article in the December 2015 edition of Sport Aviation - the Experimenter section (pg. 22) of the magazine: "When Prop Balancing Isn't Enough."
In the article, Matt discusses what to do when dynamic prop balancing isn't enough to eliminate a vibration. Click the cover image above to read the article! If what Matt describes sounds like a problem you're having, DynaVibe can help detect a vibration's source, so you can eliminate it.
If you'd like to learn how the DynaVibe GX2 can help you troubleshoot complex vibrations, contact RPX Technologies by entering your email in the form below or simply call us at 405.896.0026.
Pilots, aircraft owners, builders and mechanics are increasingly discovering the benefits of vibration analysis as a way to identify and resolve complex vibrations. Spectral analysis, vibration analysis or vibrations surveys are very helpful in pinpointing the source of an elusive vibration or simply doing engine trend monitoring as a preventative maintenance approach.
The DynaVibe GX2 is a prop balancer and vibration analyzer. When you use it to perform vibration analysis, it detects and reports the various vibrations that are affecting an airplane. For example, DynaVibe might detect a vibration that is occurring once per prop rotation – what is commonly referred to as a 1-per vibration. There are also half-per, 1.5-per, 2-per and many other vibrations that can occur. What do these vibrations indicate?
One way to find out what a vibration means is to contact the DynaVibe team. We’re always happy to help customers understand how to use DynaVibe for spectral analysis and explain the results of a vibration survey. We have also put together the following vibration table using the excellent material originally published by John Schwaner of Sacramento SkyRanch. We hope you find it helpful, with the following disclaimer: this data is for educational purposes only. If your engine is exhibiting symptoms of vibration, properly rated professionals using diagnostic equipment should check it.
This table is a good place to start in understanding the cause of vibrations detected through analysis with the DynaVibe GX2. If you would like to know more about vibration orders and causes, we can help. Just reach out to us using the contact information or form below.
The DynaVibe GX2 is he most affordable full-spectrum vibration analyzer on the market. To learn how to use DynaVibe for vibration analysis, enter your email address in the form below to be contacted by a DynaVibe team member. To purchase a DynaVibe, visit our online store, or simply call: 405.896.0026.
Matthew Mitchell operates Mitchell Flying Service, an agricultural aviation operator based out of Perry Municipal Airport (F22) in Oklahoma. Mitchell has been flying since 1990 and puts about 250 to 300 hours per year on his Grumman Ag-Cat, for which he has two TPE-331 engines that he rotates as needed. The rigors of agricultural aviation have shown Mitchell the value of keeping the prop on his Ag-Cat in good balance, which is done using a DynaVibe GX2.
Anytime something changes with the prop, such as maintenance or mounting the prop after an engine change, Mitchell likes to have a dynamic balancing performed. “It is essential to have the prop balanced,” states Mitchell. Doing so eliminates vibration caused by propeller assembly imbalance, which reduces wear on the engine, airframe and instruments. Dynamically balancing a propeller is an excellent preventative maintenance approach that can help eliminate future engine problems that might otherwise ground an operator.
Another significant benefit to using the DynaVibe GX2 is the vibration survey capability it provides. “DynaVibe checks other vibration ranges besides the propeller,” says Mitchell. “It’s a good way to keep an eye on the engine, because you may not feel a vibration that DynaVibe can detect.” Anytime Mitchell puts an engine together, he likes to have a baseline vibration analysis done. This allows him to do trend monitoring on the engine with DynaVibe to monitor the health of that engine while it is in use. Regular vibrations surveys can help operators like Mitchell to identify problems before they cause a shutdown or result in a failure.
The DynaVibe GX2 is a dynamic prop balancer and vibration analyzer that can improve performance and maintain engine health. To get your own DynaVibe GX2, visit the RPX Technologies online store, enter your email address below or simply call us at: 405.896.0026.
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.
If you do a Google search on the term “aircraft engine vibration problem” or use the make of your aircraft or engine in the search, the search results page will invariably include a forum posting like this one or this one. If you take the time to read the posts, what you’ll find are essentially pleas for help with a complex vibration problem. The initial posts in the thread describe the conditions under which the vibration occurs, and list the things the owner has already done to troubleshoot it. These initial posts conclude with statements like “Any ideas would be interesting” or “Any help would be appreciated.”
What follows in these posts are suggestions and statements of sympathy: “Welcome to my world! I have been fighting a vibration problem for over half a year now.” What’s clear from reading these forum posts is the elusive nature of solutions to vibration problems. Many offer suggestions by posting ideas, things to try or what’s worked for them. In one post, the following suggestions were made to the owner with a vibration problem:
One poster was convinced that the vibration was a design feature of the engine. Another poster who was experiencing similar vibration problems shared plans to pull a new prop off and put the old prop back in hopes that might eliminate the vibration. Still another poster theorized that the source of the vibration could be “rubber engine isolators, engine mount bolt torque, bad carb setup or poor gearbox shimming.”
What’s troubling is that these vibration problem forum threads don’t always have a happy ending. What’s also clear is that most of the advice provided is speculative and well meaning, but those who post in response don’t have certainty about how to address complex vibration problems
These forums that chronicle the frustrations of trying to diagnose complex vibration problems reinforce the value of full-spectrum vibration analysis. The DynaVibe GX2 is a vibration analyzer, identifying the frequency of vibrations and thereby directing the user to the source of that vibration. It eliminates the guesswork in troubleshooting complex vibrations. This video case study illustrates how quickly and precisely the DynaVibe GX2 can identify the source of a complex vibration.
Sometimes, there are multiple sources of vibration, as depicted in this case study, making full-spectrum vibration analysis even more critical as a diagnostic tool.
It's wise to be persistent in tracking down and eliminating vibrations, because left unresolved, vibration damages engine components, instruments, the airframe and it make passengers uncomfortable. Most of the time, dynamically balancing the propeller eliminates vibration. When balancing doesn't help, then a full-spectrum analysis with the DynaVibe GX2 vibration analyzer will get to the source of the problem.
The RPX Technologies team recently helped a Kitfox owner with a Rotax 912 engine troubleshoot a complex vibration problem.
This episode illustrates how to isolate the source of complex aircraft vibrations.
This Kitfox owner has struggled over the past two years trying to locate the source of a troubling vibration. The symptom was an engine that ran rough in the mid-RPM range, producing a lot of vibration. Trial-and-error service procedures failed to correctly guess the vibration source. In this case, the owner had the gearbox rebuilt because it was a suspected vibration source, but this remedy was ineffective. The owner was unwilling to continue having speculative service work done to fix the vibration, because these procedures were cost prohibitive.
A friend of the owner recommended analysis using the DynaVibe GX2, a full-spectrum vibration analyzer and prop balancer. DynaVibe was able to pinpoint the vibration source, and the graphs that follow are the actual reports generated by the DynaVibe during this analysis.
To perform this analysis, two DynaVibe accelerometers were placed on the engine, and readings were taken at various RPM levels, as illustrated in the following DynaVibe GX2 charts showing readings from the accelerometer mounted on the gearbox:
Step 1. The initial reading, shown above, was taken at an engine RPM of 2270 and a prop RPM of 1000. Here, a 2 inches per second (IPS) vibration peak, the highest on this chart, is seen at the 1.2-per interval. This is indicative of a carb imbalance.
A peak at the 1-per interval is also present, but is difficult to see because as it is eclipsed by the 1.2-per vibration peak. A second reading was then taken at a slightly higher RPM to see what might change:
Step 2. This second reading, shown above, was taken with the engine running at 3000 and the prop turning at 1350.
The prop vibration, shown at the 1-per interval on the chart, seems to be increasing, which is indicative of a propeller mass imbalance, while the carb imbalance is decreasing.
These readings show the complexity of vibration analysis: changing RPM levels can simultaneously cause one vibration to increase and another type to decrease. A third reading was taken, at still a higher RPM setting:
Step 3. This third reading, shown above, was taken with the engine running at 3632 RPM and the prop turning at 1600 RPM.
The chart from this reading shows that the prop vibration at the 1-per interval continues to increase, while the carb vibration at the 1.2-per interval continues to decrease. A fourth and final reading was taken to confirm these vibration trends:
Step 4. A final reading was taken at near-cruise RPM, and it confirms that there are indeed two vibration issues: a propeller vibration of almost 1 IPS which appears at higher RPMs, and a carb vibration, which is present only at lower RPMs. Note that the carb vibration has almost disappeared at this RPM level.
A key benefit of the DynaVibe GX2 is the ability to monitor the vibration spectrum in real-time, allowing the RPM to change while watching the change in vibration. This real-time vibration review is critical for diagnosing complex problems.
With this data, this Kitfox owner can confidently address the vibration issues because the sources are known. The right approach is first to balance the prop, and then address other vibration sources, in this case, the carburetor balance.
If you have similar, hard to pinpoint vibration issues, contact the RPX Technologies team to discuss how DynaVibe can provide an accurate, full-spectrum analysis to locate the source.