Yosimite Sam wrote:
I have an acquaintance with a 582 powered kitfox that had this same
problem...It turned out that the prop was very slightly changing pitch....I
remember it took a lot of time before this was found out. There was wear in
the hub area. It was fixed by adding a shim. It seems like (I'm trying to
remember here) the prop hub was slightly distorted where it clamped
together. What type of prop are on the planes? Are they using GSC props?


Hmmm... Very interesting. The props on both planes (actually Pegasus
powered parachutes) are 3-blade Powerfins. They are similar to the
WarpDrive props in that they have a CNC machined hub which clamps down
on the end shank of the blade.

I'll try to round up a WarpDrive prop and try that on them and see what
happens. The Powerfins have a lot of twist in the blades so I guess it's
possible that they are flexing and changing the pitch.

Regarding the prop shims, I know that the early (and possibly later)
versions of the ground adjustable Ivoprop had a problem with the tip of
torque rods that rides on the swash plate that varied the pitch would
wear and allow the pitch to wander.