Prop balancing

I had the opportunity to watch a professional balance an 0320 this week. He was using the honeywell vibrex which shows a full frequency of vibrations. It was interesting that it showed a strong vibration over 3 times the prop speed. He said it was probably the alternator which did measure over 3 times the speed of the prop. The owner then commented that he had replaced a broken alternator bracket last year.

I use a MicroVib II and do a spectrum analysis with each balance. I forget exactly but the alt rate is about 3.8 if I remember correctly.

Dug up some old plots I had hosted, here's an example of a 2 blade prop, pretty normal to have a 'spike' at the 2 blade rate, a 3 blade will typically have a 3 order rate spike. You can see the alt rate in the plot



Here's a 3 blade on an RV12 and you can see the 3 order spike, the Rotax also has some other accessory drive speeds you can see.



Here's a typical polar plot of the balance:

It's interesting that the out of balance alternator cracked the bracket but since the frequency is high, lt was not a annoying vibration.