DynaVibe Blog

DynaVibe Helps Warbirds Fly Smoothly

Monday, July 06, 2015

Background

Aircorps Aviation, specializing in the restoration, maintenance, and rebuilding of vintage WWII aircraft, recently purchased a DynaVibe GX2 to perform dynamic prop balancing and vibration analysis on the warbirds that come through its shop.  RPX Technologies co-founder Matthew Dock visited Aircorps Aviation to demonstrate how the DynaVibe GX2 works. 

During the visit, Bruce Olson’s T-6 was analyzed and balanced with the DynaVibe system.  The initial reading revealed a propeller vibration of 0.45 Inches Per Second (IPS), indicating a prop imbalance.  Making some adjustments over the course of a few runs, the vibration was reduced to 0.06 IPS, reducing stress on the airplane and providing a smoother ride.

Outcome

The T-6 owner, Bruce Olson, found the DynaVibe GX2 demonstration interesting beyond just the dynamic prop balancing that was done.  “It made it clear to me that there are certain, other areas of that airplane that showed no vibration whatsoever.  It gives me an awful lot of confidence that the engine is in good shape.  Now we have a baseline for the engine, so if we come back next year and put it on the test stand, and something changes, we’ll know that there’s something going on in the engine.”

The preventative maintenance value of the DynaVibe GX2 analysis was not lost on Olson:  “After having a baseline and basically finding out if there is anything wrong with my airplane, it’s like having a full physical of your own body.  You now feel comfortable that everything’s okay, and there’s nothing I have to worry about… it lets us see if there’s anything going on inside that airplane that you could catch well in advance of a failure.”

Olson summarized the experience of having his T-6 dynamically balanced and getting a full-spectrum vibration analysis: “I’m very happy with what the guys did, it’s better than it was before.”

To learn more about DynaVibe and how to dynamically balance your prop, use the form below and enter your email address, visit the RPX Technologies online store or contact us directly: 469.708.8779 or sales@rpxtech.com.


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Prop Balancing Myth Busting

Friday, July 03, 2015

We were chatting not long ago with an aircraft owner whose propeller we had balanced during a DynaVibe demo session.  He related a conversation he had with a prop shop tech about dynamic prop balancing.  The tech’s position was this: “when we come out with a prop, it's balanced, it's perfect.  By putting weight on it [during the dynamic balancing process], you're just fixing a problem somewhere else in the engine."

This statement merits some scrutiny and analysis to uncover the truth.  There’s a myth that needs busting here, so let’s dissect this statement and examine it more closely:

When we come out with a prop, it's balanced, it's perfect.” 

This may sound like pride of craftsmanship, but that’s okay!  Airplane owners want professionals who stand by their work to take care of their airplanes.  Regarding the truth of the statement, it’s highly likely that it is true.  Shops as a matter of course statically balance props before they’re mounted.  The ability for a shop to ensure that a prop is perfectly, statically balanced requires no stretch of the imagination.

What’s wrong with the statement is the assumption that nothing changes once that propeller is mounted.  In fact, many things can change, because now the entire prop assembly enters the picture, not just the perfectly balanced, ready-to-hang prop.  Even the slightest change in mass anywhere in this assembly can throw the balance off, introducing vibration.  Here are some numbers to illustrate:  a perfectly balanced prop, if offset by even .0005 inches during installation, can cause a .6 Inches per Second (IPS) vibration!  How common is this scenario? It happens more often than most pilots or owners realize, because most blades don’t have an indexing mechanism to ensure that a prop is perfectly centered with the crankshaft when it is mounted. 

For these reasons, dynamically balancing a statically balanced prop is still recommended, because the dynamic balancing process corrects “coupling” errors that result from mounting the prop.

“By putting weight on it, you're just fixing a problem somewhere else in the engine."

The second part of the statement we’re dissecting simply reveals a misunderstanding about the need for, and benefits of, full-spectrum vibration analysis.  One of the frustrations with troubleshooting vibrations is the many potential sources of those vibrations.  The prop is often the culprit, but not always, and we listed many of these sources in a recent blog post.  Fortunately, the science of vibration analysis comes to the aid of frustrated pilots, owners and mechanics that have spent time and money trying to chase down the source of a complex vibration.  Different frequencies of vibration point to different sources, and when the source is not the prop, putting weight on the prop isn’t the solution to stopping the vibration.  The best approach is to address the root cause of the vibration.  The DynaVibe GX2 is a prop balance and vibration analyzer that tells you what the source or sources are, so you can fix them.

As a case in point, the RPX Technologies team worked with the owner of a Grumman Widgeon (pictured) where each engine exhibited a vibration.  The DynaVibe analysis was able to pinpoint different vibration sources in each engine, and the propeller was not the cause of either one.  Full-spectrum vibration analysis creates certainty about vibration causes and sources, preventing speculative service work.

If you’re experiencing a vibration problem, there’s no need to guess where it’s coming from.  The DynaVibe GX2 can perform a full-spectrum vibration analysis and tell you exactly where to focus your efforts to fix complex vibration problems.


Dynamic Prop Balancing: how often to do it?

Monday, June 29, 2015

How often should your prop be dynamically balanced?  There are differing opinions on this subject, and we wanted to weigh-in with ours.

Almost everyone in the flying community agrees that where balancing is concerned, once is not enough.  You should not assume that once a prop is in balance, it stays in balance.  There are simply too many ways a propeller can achieve an imbalance.  Here are some conditions under which you should dynamically balance your prop:

  • If your prop has only been statically balanced.
  • At each annual or 100-hour time-in-service inspection.
  • If the prop has developed a nick since the last balance, even if you’ve filled it in with JB Weld.
  • Anytime the prop is removed and reinstalled.
  • Anytime the spinner is removed, and then not replaced exactly as removed.
  • Every time you re-torque your prop bolts.

Anytime any of the conditions listed above occur, the balance of your prop can change and dynamically checking the balance is advised.

Wood propellers require special attention because of variations in humidity.  Typically, wood props should be retorqued and rebalanced every 25 hours, or anytime the aircraft experiences a shift in ambient humidity.

For more information on why to balance your prop, or on how to do it, enter your email address below to be contacted by a member of the DynaVibe team.    The DynaVibe GX2 is a second-generation prop balancer with full-spectrum vibration analysis capability available for purchase online.  The GX2 “learns” as you balance, making balancing quick and easy by telling you precisely how much weight to add and where to add it.  It will balance your propeller with a minimum number of runups!

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Going on a Vibration Wild Goose Chase

Wednesday, June 24, 2015

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:

  • Change the rubber engine mounts
  • Check the condition and installation of the rubber carb mounting flanges
  • Make sure there is no leakage between the float chamber and carb body
  • Check to see if the filters are clean
  • Make sure the carb vent lines are running properly
  • Replace or check the “O” rings under the rubber carb flange
  • Balance the carbs
  • Replace the needles and jets
  • Inspect the gearbox as a potential source of the vibration
  • Check the vent tubes for cracks and replace if necessary

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.

Case Study: Complex Vibration Troubleshooting on a Rotax 912

Sunday, June 14, 2015

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.

The Problem

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.

The Analysis

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.


Infographic: Why Balance?

Thursday, June 11, 2015

Here at RPX Technologies, we believe that planes shouldn't shake themselves apart, and therefore propeller balancing should be easy.  Read the "Why Balance Your Prop?" webpage for a complete discussion of reasons to balance your prop.  This infographic summarizes why it is so important, and how simple it is to dynamically balance your prop with Dynavibe:

Infographic - reasons for dynamically balancing airplane propellers

The DynaVibe Classic is the most affordable dynamic balancing solution on the market.  For dynamic prop balancing and vibration survey capability, use the DynaVibe GX2.  Buy the right DynaVibe for your needs by visiting our online store.  If you have any questions, please call us at 469.708.8779 or simply enter your email address in the form below.

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Addressing the Root Cause of Fatigue

Saturday, June 06, 2015

Aircraft fatigue is the result of stress, and it can seriously erode the longevity of your airframe.  There are no good outcomes to leaving fatigue unaddressed.  “Fatigue damage and its consequences are the most serious structural design and maintenance issues that have to be addressed.”*  From an engineering perspective, fatigue is caused by vibration and the number of vibration cycles.  The greater the magnitude of the vibration, the more stress it produces.  When a propeller that isn’t in balance is producing the vibration, you can’t eliminate the cycles, or prop rotations, but you can reduce – even eliminate – the vibration associated with each one.

An out-of-balance propeller produces a vibration with each rotation. This stress often manifests itself as airframe fatigue cracks, as pictured in the photo above.  There is good news:  stress that is the result of propeller imbalance is easy to address.  Dynamically balancing your propeller can eliminate almost all of this vibration, and it’s important that you do so, as there’s a direct relationship between stress and the number of prop rotations.  Unresolved, each prop rotation produces stress that steadily produces fatigue cracks and then propagates their growth.  Pilots and owners don’t have to live with prop vibration that stresses the airframe.   

If you have fatigue cracks you’ll still need to stop-drill them, but dynamically balancing your prop will reduce or eliminate the source of the stress, arresting fatigue crack propagation and preventing new ones from forming.  Using the DynaVibe Classic, a dynamic prop balancer, it’s easy to reduce or eliminate this damaging vibration (the DynaVibe GX2 allows you to identify all sources of vibration, not just prop imbalance-induced). It’s a great investment in extending the life of your aircraft, and a balanced prop produces a smoother safer ride! 

If you have fatigue cracks on your airplane and you’d like to know about propeller balancing and vibration resolution, contact us for a consultation at sales@rpxtech.com or by calling:  469-708-8779.



JindÅ™ich Finda, Andrew Vechart and Radek Hédl. “Prediction of Fatigue Crack Growth in Airframe Structures”, pg. 1, 2012 http://www.phmsociety.org/sites/phmsociety.org/files/phm_submission/2012/phmce_12_013.pdf

AirCorps Aviation Open House

Friday, June 05, 2015

DynaVibe is at Bemidji for AirCorps Aviation's Open House. Ran in to Bernie Vasquez. Balancing a T-6 and a Mustang!


Balancing props using the DynaVibe GX2.  The P-51 has a V-1650-7 engine with some interesting vibration ratios.


Common Signs Your Prop Needs Balancing

Wednesday, June 03, 2015

Aircraft vibration is more than just annoying.  Left untreated it has the potential to damage all parts of the aircraft:  engine, airframe and instruments. The reality is that piston engines vibrate, but when there is vibration, most of the time it is the result of propeller assembly imbalance and not from combustion.  Here are common signs that your airplane is suffering from vibration that could easily be resolved by dynamically balancing your propeller:

  • Your arms, legs or feet get numb during a cross-country
  • Your compass “jiggles” or won’t settle down
  • Instrument needles vibrate
  • Avionics randomly fail
  • There is an RPM range that is “rough”
  • You experience a “shiver” in your airplane as you reduce rpm prior to landing
  • Rivets loosen
  • Fatigue cracks appear or existing ones grow
  • Visors shake

Even the slightest misalignment of any part of the rotating assembly, such as the spinner, propeller, prop mounting hardware, starter ring, crank or any other rotating component can cause damaging vibration.  There are many potential sources of vibration, which you can read about on our “Why Balance” web page.

Left unchecked, vibration has some nasty side effects, like producing fatigue that reduces the longevity of the airframe and avionics that are continually stressed and fail prematurely.  Fortunately, the solution is easy: dynamic propeller balancing using the DynaVibe Classic or DynaVibe GX2.  The prop balancing process with DynaVibe is simple, as this short video explains.

The difference dynamic prop balancing makes is often dramatic.  Many pilots feel like they’re flying an entirely different aircraft, as these testimonials share.  To learn more about dynamic propeller balancing, visit the RPX Technologies website or contact us by email:  sales@rpxtech.com


Identifying and Addressing Rotax Engine Vibration

Friday, May 29, 2015



RPX recently helped a RV owner with a Rotax 912 engine diagnose a complex vibration issue. Sometimes, dynamic prop balancing isn't enough to resolve a vibration. Isolating the source of the vibration was done using the DynaVibe GX2 to perform a full-spectrum analysis.  We've produced a short video to explain the steps in diagnosing complex vibrations on this engine.  If you'd like to learn more about this, or to get assistance from the RPX team in diagnosing a vibration issue, please get in touch with us!



Questions? Contact us at sales@rpxtech.com