ORIGINAL: perreback
I doubt if the different readings you get actually depend on where in the prop arc you point the tach. Tachs can give you wildly erratic readings sometimes. Try sitting in the shade with the front of the plane pointed towards a sunny area of your field. Sit behind your plane when reading your tach --- safer than in front.
perreback has the best analysis here,
Another point to consider is that these tachs have a swicth to change from 2-3 bladed propellers - where is your switch positioned? Is it working properly? Are you changing it accidentily? [
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The math would indicate that this is a possibility...
