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Finn,

The FFT processing required two channels recorded simultaneously, one for the position reference and one for the accelerometer.

The system I used allowed sampling of each channel at 5khz with no external amplification or filtering.

My guess as to the prop 3x(blade) signal is that it is an interaction with the ground. The prop is 76” dia resulting in about 8” of prop tip to ground clearance.

Also a guess on my part is that the engine 1x vibration may be coming from the flex plate shifting on the rubber dampers. My system has about 0.020” of clearance between the corners of the damper plate and the flex plate rim when centered. When attempting to balance the engine, a point was reached where no further improvement was made and the phase angle was not consistent between tests. I don’t know how much the splined shaft can restrict the possible shift off center of the damper plate, but the intensity of the vibration was consistent with the weight of the damper plate and 0.020” of eccentricity. This may also be affected by the load on the rubber dampers.

Steve Boese

On Mar 19, 2021, at 8:34 AM, Finn Lassen finn.lassen@verizon.net <flyrotary@lancaironline.net> wrote:

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Thanks Steve.

Seems that having the right hardware and software makes all the difference.

I my attempts the phase signal was all over the place. That's why I moved to adding a nut. Appears than one needs two channels being sampled simultaneously: trigger pulse and accelerometer, to get proper phase info?

Can you explain the prop "3x (blade)" ? Some kind of air interaction between prop, airframe and ground? In flight the air would quickly move past the prop, sitting still on the ground one could imagine some standing waves?

Also,ï¿½ why does so much engine vibration remain (relative to prop's)? I think you mentioned that you first balanced the engine and then the prop.

Finn

On 3/18/2021 9:45 PM, Steven W. Boese SBoese@uwyo.edu wrote:
This photo shows the Signalexpress FFT processed data from balancing the prop on the ground.ï¿½ The prop 3X (blade) signal disappeared in flight.

Steve Boese
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