December 2005- One of the more overlooked and certainly misunderstood areas of aircraft maintenance may be prop balancing. While most aircraft owners would never drive their car with the tires out of balance (and the steering wheel shaking in their hands) they don’t think twice about operating their airplane with the prop out of balance.
I flew with a buddy a week or so ago in his nicely restored Cherokee. It flew perfectly with hands off, but at cruise power there was an obvious vibration and it was clearly evident in the compass and in the artificial horizon, which was vibrating at a high frequency.
Prop balancing is not black magic or rocket science. There are computerized tools that attach to the engine, read the engine vibration and suggest a suitable fix. The application of balance weights to the inside of the spinner back plate is a relatively simple process. The advantages are many. Other than the obvious (passenger comfort), there are legitimate maintenance issues associated with a properly balanced prop.
An unbalanced prop can cause spinner and spinner bulkheads to crack, oil cooler cracks, hard fuel and oil line failures, alternator bracket failures, case half chafing, premature gyro and instrument failures, as well as airframe issues like broken or cracked cowl and cowl fasteners, fairings and landing/taxi/nav and strobe light bulb failures.
Some avionics issues can be attributed to poor prop balancing. If you want to read a fascinating article about the subject, go to maxcoombs.com/balance-education.htm. Max is a neighbor of mine, and a very educated A&P/IA mechanic. The article you will find at the web address was published by a company that manufactures prop balancing equipment, and it explains in detail, and in layman’s terms, some of the intricacies of prop balancing. It is certainly worth reading.
The process itself is not complicated. A vibration sensor and a photocell are attached to temporary brackets on the engine, and a piece of reflective tape is placed on the back of the spinner or propeller to act as a timing mark for the photocell.
When the engine is run at various power settings, the computer to which the sensors are attached, reads vibration in inches per second, or IPS. It also tells you which direction relative to the timing mark in the “o’clock” format. So, a typical reading might be .38 IPS at 2:30. That translates to .038 of an inch at the two-thirty position.
The computer will also offer a fix typically by adding some weight 180 degrees out of phase. So a fix for the above example might be “+ 16 grams at 8:30”. The weights are attached to the spinner and another run is performed. This sequence is repeated until the vibration is reduced to the lowest possible number.
On the aircraft we are working on right now, the initial run was .58 IPS. The engine shook visibly and there was a pretty good vibration visible to the eye on the instrument panel. When we were done, we had reduced the vibration to .06 and that is about as good as it gets (.06 represented a nearly 10 times reduction in vibration). The engine felt smooth and the instrument panel had no visible vibration.
This is even more evident in a multi-engine aircraft. In this situation, second- and third-order harmonics become a real issue. A vibration harmonic can be described like this: imagine throwing a rock into a pool of water. The ripples fan out in concentric rings around the impact site. These ripples represent a vibration. Now imagine throwing a second rock into the water near the first. It too sends out ripples or vibration. Where the two ripples meet they clash. This impact is a harmonic.
You can easily hear when a twin does not have its engines in sync; the distinct “whah, whah, whah,” sound is caused by the two engines first-order harmonic not being synchronized. Second- and third-order harmonics happen at longer intervals and are much harder to hear. The human ear (and body) is much better at detecting low frequency vibration than high frequency vibration.
Just for interest, go look in your logbooks and see what type of maintenance you have had done in the last 300-400 hours. If you have replaced gyros, oil coolers, “smoking” rivets, cracks in the cowling or fairings on what appears to be a more-frequently-than-expected basis, the cure might be in balancing the prop. The cost to accomplish a prop balance is generally less than $300, or about the same as an overhauled attitude gyro.
Michael Leighton is a 3,600 + CFIIMEI/ATP as well as an A&P mechanic and former F.A.A. Accident Prevention Counselor. He operates an air charter company in South Florida. You can reach him via e-mail at av8tor0414@aol.com.
