Aircraft Maintenance: Myths and secrets of propeller balancing

Many years ago, I purchased my first aircraft: a Grumman AA–5 Traveler. As a new aircraft owner, I was eager to learn everything I could about my airplane and made it my mission to become an expert in its maintenance and operation. And so, I was surprised when a very experienced Grumman instructor went flying with me and commented about the excessive vibration on the ground and in flight. Not having flown many other airplanes recently, I just thought it was normal. I consulted my A&P at the time and we reviewed the engine mounts, exhaust, and other systems, finding no issues. At his suggestion, we performed a dynamic balance on the propeller, and the results were astounding. For the first time, the instrument panel was rock steady on the ground with no vibration. Startups and shutdowns didn’t shake the aircraft, and cruising was smoother than I ever thought possible. This simple task changed my entire flying experience and gave me a new perspective on the things we accept as normal in the aircraft we fly.

I concur with this, having seen the effects of vibration-induced wear on nearly every component of the airframe, engine, and even avionics. This is especially true for electrical components with potentiometers such as fuel senders, dimmers, and VOR/LOC/GS needles, among others. I’ve also noticed dramatically less fatigue following long flights in aircraft with well-balanced propellers.

All this said, myths still run rampant when it comes to dynamic propeller balancing. The most common misconception is that a new, or newly overhauled, propeller does not need to be dynamically balanced. This is entirely false. Propeller shops statically balance propellers, but that is no substitute for a dynamic balance while installed on the aircraft and spinning at cruise rpm.

Another myth is that a propeller will maintain its balance over the course of many hours or years. The fact is that wear (external and internal), corrosion, spinner installation, lubrication, and even moisture (for wood props) can cause changes to the balance of a propeller. The best solution is to have your propeller balanced annually, or at least after any propeller maintenance or removal. If it’s been more than two years, chances are that you’ll see a noticeable benefit to having a dynamic propeller balance performed.

The process for dynamic balancing is fairly simple, thanks to the magic of modern technology. Personally, I use an RPX DynaVibe GX3 analyzer for balancing, so I’ll use that as an example of the process:

• The first step is to remove any pre-existing weights from the spinner or propeller. Intelligent propeller vibration analyzers rely on good input data, so it’s critical to start from a clean slate.
• The system is installed using an accelerometer mounted vertically, as close to the propeller as possible. The most common mounting point is on the top case half bolts closest to the propeller. The DynaVibe includes adapters to facilitate this.
• An optical tachometer is located on the cowl or engine, facing forward, and a piece of reflective tape is applied to the face of one prop blade (the part of the blade that you see from the cockpit).
• The unit steps you through a series of questions about the engine, propeller, spinner, and potential weight locations, and then leads you through the test process of running the engine and analyzing the results.
• After the test is complete, the DynaVibe gives specific instructions on how to install weights to balance the propeller. It even advises how to “split” the weights at different available bolt locations to achieve the weight distribution goal.
• The testing and analysis procedure repeats with subsequent recommended adjustments to the weights to refine the solution until you achieve the best result possible.

One of the most amazing things about the process is that it results in a complete vibration spectrum analysis report (VSAR) for your propeller and engine, as measured by the accelerometer. In addition to documenting the entire series of analysis and adjustment runs, the final report contains a graph of the vibration spectrum showing the frequency and amplitude of the vibration, as measured in inches per second (IPS). This graph not only shows vibration signatures in terms of rpm, but all in terms of propeller rotation (labeled as “1-per” on the graph). This comprehensive analysis can help identify vibration issues caused by other components on the engine or airframe. For example, vibrations at the 0.5-per point could be caused by cylinder issues, vibrations at the 2- or 3-per point would indicate normal aerodynamic pulses as the prop passes around the cowl, and vibrations in the 6,500 to 7,000 rpm range could indicate alternator balance issues. I have found that the folks at RPX are a wealth of knowledge when evaluating VSAR results and are always willing to help.

It’s also important to evaluate the starting point of the analysis. If a propeller’s initial analysis run indicates a major 1-per propeller balance result above 1.25 IPS, then the propeller may need to be evaluated by a prop shop. You can try rotating the installation of a two-blade propeller 180 degrees (or 60 degrees for a three-blade) to see if that helps, but there is a good chance it needs to be evaluated for static balance or blade tracking/angle issues. You can also try rotating the spinner to see if that reduces the balance weight requirements. The ultimate goal of the process is to reduce propeller vibration to at or below 0.07 IPS. In my experience, it’s often possible to get as low as 0.03 IPS or better using the DynaVibe GX3 if you are patient and methodical.

Here are a few final tips for success:

• Always ensure that you are viewing propeller rotation (for balance weight installation) as viewed from the cockpit.
• Use the accelerometer as your zero-degree point and mount it vertically, and as close to the propeller as possible.
• Avoid direct sunlight that can reflect off the prop and confuse the optical tach. Keeping the sun off to the side helps.
• Perform the balancing in very calm wind to get the best results.

Dynamic propeller balancing should be a part of your routine maintenance regimen. Think of it as an investment in the health of your aircraft, and in your personal flight experience. As Sennett likes to say: “You don’t think twice about the importance of balancing tires on your car, so why would you skip balancing your prop?” Until next time, I hope you and your families remain safe and healthy, and I wish you blue skies.