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.