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