There are two kinds of prop balancing: static and dynamic. Static balancing is done at the prop overhaul shop. In a static balance the prop is literally balanced by filing it until both ends of it weigh the same. Lycomming recommends that my prop be overhauled (static balance is part of an overhaul) at engine major overhaul or 2000 hours. Dynamic balancing is done with the prop on the plane. The entire rotating mass (all of the rotating parts that contribute to a total rotating mass). This includes the prop, crankshaft, spinner, spinner back plate, ring gear etc. The A+P who does my dynamic balancing (Mike Ricard; Lawrence, Mass, 978-682-1738) says that the prop should be dynamically balanced any time the engine or prop is overhauled.
My prop has been dynamically balanced twice (since I have owned the plane). The first time was two years ago after a major overhaul and the second time was recently after an engine teardown (crankshaft AD) and a major prop overhaul.
The first step is to put the plane in a location where you can safely run the engine. Next put some chocks under the wheels to protect the mechanic. After that a piece of reflective tape needs to be placed on the prop. (see picture)
The purpose of this piece of reflective tape is to allow the mechanic to see the position of the prop at a certain time using a strobe unit. (see picture).
A velometer is then bolted to the top of the engine. A velometer measures velocity in inches per second. (see picture)
The strobe unit and the velometer are connected to a spectrum analyzer plotter so that the vibrations measured by the velometer can be visualized. The plotter plots velocity in inches/second in the y axis (using log paper) and frequency in the x axis. In other words, you can see how much amplitude the vibrations have at each frequency (you can see harmonics of the frequency that the prop is turning at). (see picture)
The next step was for me to run the engine at 2000 rpm (indicated) while Mike does the measurements. He uses the strobe to visualize what the position of the prop is when the heaviest point is at the top of the engine (most amplitude detected by the velometer). With the engine running at 2000 rpm the strobe will fire when the heaviest part of the rotating mass is at the top (where the velometer is positioned).This will cause the prop to appear to "freeze" in place because of the visual strobe effect. This is the position that the prop is in when the heaviest part of the rotating mass is at the top. I then shut down the engine. Mike repositioned the prop to where it was when the velometer detected the heaviest point. Knowing that the heaviest point was at the top dead center tells you that the lightest point is exactly oposite that. In otherwords, with the prop in this position, the bottom of the spinner is the lightest point that needs some weight added to counter balance the heaviest point oposite this. Mike added a small washer to the outside of the spinner. This weight is positioned on the outside temporarily during testing. After the correct weight is determined then the spinner is removed and the weight is put on the inside instead. (see picture)
Mike used the plot to determine how much weight (experienced guess based on what he saw on the plot) to decide how much weight needed to be added. In my case not much was needed. According to him my prop was already pretty well balanced. This time he only needed to make two sweeps. After the second plot he determined that the small weight that he added was all that was needed. In the picture of the plot below the red lines (highest) reflect the original vibrations and the blue lines (lowest) are the remaining vibrations after the weight was added and the balancing was finished (as good as it gets).
Here is what the washer looks like on the inside of the spinner (back plate I guess). As you can see that there are normally washers there. He substituted one of the normal ones for a slightly larger one (with a flat side so that it will still fit). It weighs the same as the test washer that was added to the outside of the spinner during testing. (see picture)
I have also included the plot from my previous balance. You can see that at the fundamental frequency of the prop the vibration started out more that time. Since then I have had the prop overhauled. It is noticeably quiter (more of a smooth whirring sound) and it feels a little smoother. I had it overhauled at 2100 hours. (see picture).
Since I was running the engine at exactly 2000 rpm indicated during the balancing you can see that you can fairly accurately determine how far off the tach is. As you can see from the plot my tach reads about 100 rpm low. This is very common with "grummans".
The dynamic balancing of a propeller is considered by the FAA a major airframe alteration (unless the procedure is described in the factory maintenance manual). Mike gave me a form 337 for this to keep with my records. Of course I did a test flight without a passenger and noted in my engine logbook that everything was fine. It is amusing to note that and engine overhaul (or the teardown that I recently had) is not considered a major alternation or major repair. No test flight is required before taking a passenger (you can bring a friend during the initial engine breakin flight). However, you cannot bring that friend during the testing of the small washer pictured above. Pretty funny! By the way, the cost of this prop balance was $185. This is in the North East where things seem to cost more for some reason. I hope this article has been interesting.If anybody has any corrections or additions or comments you can email me at: firstname.lastname@example.org and I will make the changes.