Thanks
to all who responded on and off the list. The problem did indeed turn out be
worn flap hinge pins. I replaced them with nuts and bolts and flew the plane
for two hours today with no buzz.

For
those who are not familiar, the Legacy flap hinge pin is a machined part with a
large button head that fits flush into a recess in the stationary part of the
flap hinge. The pin’s head is safety-wired to the stationary hinge half
so it can’t rotate, and the small end of the pin is secured with a snap
ring. The moving hinge half (which is attached to the flap) has a pressed-in spherical
bearing.

The
problem is that the snap-ring does not provide any clamping force against the
ears of the spherical bearing. In my case this resulted in the bearing turning
on the pin instead of in its race. The pin eventually wore until there was enough
free play to cause serious buzz above 160kts.

I
believe the root cause was the designers’ attempt to achieve a nice,
streamline pivot joint at the expense of good aircraft practices. For the time
being I am using AN3 bolts and stop nuts. I’ll eventually switch to AN526
machine screws with low profile AN324 stop nuts. That hardware is available in
stainless and should improve the cosmetics a bit.

Can
anyone explain how the Evolution flap hinge pin is secured? I’d be
curious to see if there was any change from the Legacy design.

Thanks,

Adam
Molny

Legacy
N181AM