Ercoupe 415C Dynamic Propeller Balancing the McCauley 1A90/CF7146 on the C85-12F
Foreword
I had been interested for some time in balancing my propeller to reduce the vibration from my engine so, as you might expect, I was excited when offered the opportunity to borrow a Dynavibe G3X. This article describes my experience balancing the propeller and, I hope, will offer insight and encouragement to others in the club who have an interest in this beneficial procedure.
The balancing procedure I used is 3 Run Single Plane Balancing. After mounting the balancing sensors, the first run is made to read the initial existing imbalance. Next, a trial weight is applied and a second run is made. The second run imbalance reading is taken and the information from these first two runs is used to calculate the weight and location to apply a correction weight. The correction weight is applied and a third run is made to measure the resulting residual imbalance. Single Plane Balancing means that only imbalance in the plane in line with the propeller path will be measured and adjusted. The Dynavibe G3X has an automatic procedure, which I used, that walks the user through each step and generates a report when finished.
Propeller balancing as described here will not be effective if your propeller is bent, the engine mounts are loose or broken, or your engine is not in good running condition. I have recently had my propeller overhauled, my engine mounts replaced, and I checked the tracking of my propeller as part of the balancing process.
Preparation
Since this procedure involves running the engine on the ground, I exercised extreme caution. I kept all onlookers and helpers at a safe distance and I turned off the mags and put the key on the dash where I could easily see it when I needed to handle the propeller. I am especially mindful that a balance weight can come loose and be thrown from the spinner so that area in line with the arc of the propeller must always be clear.
The following equipment and parts were used for the procedure:
Dynavibe G3X Vibration Analyzer/Balancer
A digital scale for measuring correction weights and fasteners
Brackets for mounting the accelerometer and the laser tachometer
Balance weights and fasteners of various sizes
McCauley Propeller Balancing Procedure
I used the Dynavibe GX3, shown here. This instrument is designed for dynamic balancing propellers and has programs with on-screen prompts to help the operator through the balancing process. This instrument is capable of displaying the vibration spectrum of engine. The Dynavibe has an easy to use printed manual which describes the operation, capabilities, and procedures for using the instrument. Dynavibe also makes a basic model, the Dynavibe Classic, which can be used for balancing propellers. The Classic model will balance just as well as the GX3 but lacks the capability of displaying the spectrum.
These items were checked before starting the balancing procedure:
Propeller timing. I checked that my propeller was indexed to the proper location on the crankshaft flange as described in the article from Coupe Capers, March 2015.
Propeller tracking. My propeller tracking is within +/- 1/16 inch, see Figure 2.
Propeller and spinner clean. I checked my propeller and spinner inside and out for any deposits which could affect the balance.
Engine mounts. My engine mounts were replaced 2 years ago and are in good condition.
Setting up for the balance procedure
I first removed the cowl and mounted the accelerometer to the engine. I removed a forward crankcase bolt and replaced it with a longer one to provide additional length for the accelerometer mounting bracket, see Figure 3. I mounted the accelerometer vertically and as close to front of the engine as
possible.
The photocell was mounted to the top of the nosebowl using painters tape. Reflective tape was applied to the propeller. The photocell will project a light to aid in lining up the tape. I marked the spinner screws at the backplate numbering them 1 through 8 clockwise, while standing in front of the aircraft, starting at the top, just to the clockwise side of the propeller. The cables from the accelerometer and the photocell were routed to the cockpit and connected to the balancing instrument. Finally, I secured the cables with zip ties and tape to keep them away from the engine as shown in the next images. I ran the engine with the cowl off.
Performing the 3 run balancing procedure
Run 1
I set the Dynavibe up and entered all the required information for the automatic balancing routine. With the engine warmed up and in a safe location, I took the first reading. I ran the engine at full throttle for just long enough to take the reading. The Dynavibe GX3 used the information entered and the vibration reading taken during the first run to calculate a trial weight, in grams, and a location in degrees and screw number, to install on the spinner.
Run 2
I Installed the trial weight on the spinner screws as shown in Figure 6. For this run I split the weight equally between the nearest two screw locations on the spinner. I did not want too much weight on a single screw.
I added 9.6 grams on each screw at 45 degrees and 90 degrees. I needed to use longer screws for mounting the trial weights so I included the additional weight in the calculation. For the second run, I was not trying to reduce the imbalance but to change the imbalance to a new location and magnitude. The response of the rotating assembly to the trial weight provides the needed information for the Dynavibe to calculate the correction weight and location required. After applying the trial weights, I ran the engine again to get the resulting imbalance reading.
Run 3
After the second run, the Dynavibe calculated a correction vector and displayed the location and weight to apply for the final correction. The Dynavibe split the correction weights between 2 screw locations to achieve the correct weight and angle for the correction. When applying these weights, they must be in the exact location indicated and the precise weight specified. Here I used a combination of different washers and adjusted the weights using a bench grinder as needed. The weights were within 0.1 gram of the weight calculated by the Dynavibe. These weights were to be removed later so I did not need to make them pretty, just accurate and securely attached. After the weights were applied, I ran the engine again to get a final reading of the corrected balance. After this run, I ended the automatic balance program and the Dynavibe created a report on the SD card. I later printed this report, see image. This procedure balanced my propeller to an acceptable level. I could have run additional routines to achieve a finer balance.
Relocating the balance weights to the spinner backplate
After the final balance, the correction weights were on the spinner screws which is not acceptable for flying. The correction weights needed to be moved to the spinner backplate. Here is where I consulted the McCauley Propeller balancing procedure. The McCauley procedure describes the approved location and fastener requirements. My spinner backplate is 0.090" thick so I had no concerns about supporting the balance weights as directed by McCauley.
On my airplane, I already had one balancing hole in the backplate. I used the existing hole and added one more so I could split the correction weight between the two. Because my existing hole and the new hole do not line up with the spinner screws and they are at different radii, I used vector calculations to determine the location and weights to be applied on the backplate as shown in Figure 8. I ran the engine once more after moving the correction weights to the backplate for a final check.
Logbook Entry and Marking the Propeller as a Balanced Assembly
After completing the balancing procedure, I removed the reflective tape from the propeller, applied a placard indicating that the propeller was balanced to the engine as an assembly and I was ready for my A&P to inspect the work and make the logbook entry.
John Earl