Prop balancing

I know I'm getting off on tangents, but I figured I would write when I had a thought.  At my age, that thought might not come back!

Not too many people know the necessity of propeller balancing.  An imbalance can cause airframe vibrations that make it to the accelerometers.  That is not good!  All control programs have a filtering algorithm that they pass the accelerometer input through to reduce the effect of small vibrations, but it doesn't take too much shaking before your quad can't quite figure out which way is up.  

Many of the control programs have a parameter that allows you to choose the degree of filtering. If you have a lot of vibration, then increasing the  filtering will allow the craft to at least fly.  But that added filtering really hurts the handling. It prevents the craft from realizing it is not level until it is very 'un-level', and it will jump around and be a lot harder to fly.  For ease of flying and stability, you want to use as little filtering as possible - but for that to work, you can't have significant vibration.

As a real-world example:  I had a quad that seemed to fly OK, but when it hovered, it would jump up and down about 10' !  I couldn't figure out why until I had it hover very close to me.  I saw the spinner on prop #3 vibrating back and forth about 3/16 of an inch!  After I landed, I took the craft in to my basement and tied it down.  I then revved up the motors, one by one with a "servo tester" (highly recommended tool that costs less than $5).  At certain RPMs, but ONLY at certain RPMs, motor #3 vibrated a lot. A small imbalance was triggering a resonance in the airframe.

I didn't think this amount of vibration was possible, since I balanced my props with a magnetic balancer before I mounted them.  It simply shouldn't have been vibrating - but it was.  

I took some masking tape about the size of a large postage stamp and placed it on the top of one 'side' of the prop, then spun the motor with the servo tester.  The vibration was worse! So I put the masking tape on the other side and spun it.  This time, the vibration was much less.  So I used a single-edged razor blade and scraped some plastic off the side that did not have the masking tape, and spun it again.  This time, it vibrated even less. I scraped a little more and removed about half of the masking tape. It still vibrated a bit, but I was getting closer.  So I took the masking tape off entirely and spun it, removed a bit of material and spun it again.  Eventually I got to the point where it didn't vibrate at all.  My quad flew better immediately.  

So, my recommendation is to use a good magnetic balancer to get the prop statically balanced, then put it on the motor and spin it with a servo tester (make certain your craft is well tied down!) to do a dynamic balance.

An acquaintance said that they balanced the motors themselves.  An un-balanced motor may explain why the static balancer is not good enough. I'll figure that out and report my findings.