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Old 12-15-2015, 01:42 PM
Tom Delaney Tom Delaney is online now
Join Date: Dec 2007
Location: St. Louis, MO
Posts: 156
Default Prop balancing questions

I have an ELSA RV12 with the standard Sensinich prop. I have balanced the prop once before and achieved an .1 result however I think some of it was luck!

I have two questions:

1. While recently checking the balance at 2000 rpm (4850 rpm on the skyview tach) I am getting a lot of bouncing of the plane while in chocks. I thought I read somewhere that this has a negative effect on the readings. I guess our lighter plane will see this but I was wondering if a lower rpm could give satisfactory end results

2. Very basic question...when one superimposes the polar chart over the prop to visualize the correction is this done in front of the plane or from in the cockpit?
I always thought I stood in front however someone said the opposite.

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Old 12-15-2015, 05:29 PM
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AX-O AX-O is offline
Join Date: Jun 2005
Location: SoCal
Posts: 2,513

1) With a fixed prop, you want to get as close as possible to your cruising rpms to get the best result. You balancing is optimized for that RPM and conditions. So a lower RPM may of may not work for cruise depending on what your unbalanced forces are at a lower RPM.

2) The main thing is the direction of prop rotation regardless of if you are inside the cockpit or outside looking at the prop. Make your chart match the rotation. In most cases you would view the chart and the prop from the front. But you can do whatever you want. Just switch your 1 o'clock with 11 o'clock. 1 o'clock being in the direction of prop rotation regardless of point of viewing.
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Old 12-16-2015, 07:00 AM
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WA85 WA85 is offline
Join Date: Apr 2010
Location: Huntsville, AL
Posts: 151

Depending on what equipment you are using, prop balancing is typically done using synchronous measurements, meaning you synchronize the vibration to a specific frequency. This filters out all other vibration frequencies and allows you to balance just the prop and not worry about the affects of other rotating components on the acft. Typical prop balance equipment uses a accelerometer and a frequency measuring device, such as an optical tach (photo electrical cell and reflective tape, or a magnetic pick up (steel tab passes by a mag field sensor). It is common for acft jump around quite a bit during static engine runs and it should not have any affect on the prop balance. Try holding max aft stick and chocking the main wheels or using the tail tie down to aid in holding the acft static. The polar chart orientation does matter. I typically orientate my polar charts to have the target blade at 12:00, standing behind the prop, that turns clockwise, based upon the accelerometer pointing down (6:00) and the tach mounted at 12:00). The worst thing that can happen if you incorrectly orientate your polar chart is that your move lines will be 90 / 180 out of phase orientation. Many times I install the equipment and take a reading, then add weight to target blade, then plot the move, that will tell you right away if your polar chart is somewhat correct. Two bladed props are simple. Not every prop will react exactly alike. Generally it is best to balance the prop at the highest practical RPM you can run on the ground, however if you can only get 75% of the max RPM, that is generally OK, especially with fixed pitch props. Generally, if you balance to well below limits (goal) at 75% RPM, the prop should still be under limits at max RPM. Most props have a 0.20 IPS or less goal. I try for under 0.1 IPS if at all possible. Anything over 0.75 IPS might be out of limits based upon how much weight will be needed to be added. In this case, re-static balance the prop.
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