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