citabria, Big Pistons:
Thanks for the data! I wish Piper would have released such detailed performance information.
Seems like my rough data points are not too far off from your data. The 1 kias / 2000 ft drop in Vy will do nicely for now.
By the way, I also found another article [1] on here which cites a Vy decrease of roughly 1 kias / 1000 ft.
[1]
http://www.pprune.org/tech-log/343259-vx-vy.html