Holy Crap, Rusty, I'm really impressed!

 

 

I don't think it is just one force involved. When the blade is accelerated violently, the blade hub moves forward and the outer 2/3 or so lags behind a bit. The blade then is hinged in effect at a point about 1/3 of diameter, and since that hinge line is not parallel with the propeller shaft centerline,  the tip of the prop moves off its preferred track. It is then amplifying the expected  bending of the blade in opposition to the thrust being generated.

 

Try reducing the propeller pitch to nil, so that there is no thrust at all and each blade will be at its stiffest against the acceleration, and deceleration forces. (sell the gyro)

 

Try a different prop.

 

Even thick fixed pitch wood props move fore and aft at the tips in response to thrust.

 

A wood prop would not be acting up like the very light weight and flexible piece you have there.

 

You might try over balancing, or adding weight to the counter weights, and then add just a bit to the very edge of the starter ring. (sell the gyro). 

 

I suspect that the problem gets less impressive at higher revs.

 

Lynn E. Hanover