On Monday, September 19, 2016 at 9:05:26 AM UTC-4, BG wrote:
There are a total of no less than 6 static ports on that vintage of Grob. The 4 on the tail boom are connected together and there 2 on each side of the cockpit area as well. By design if any of these ports are clogged, a yaw will give a problem as you describe. Use a wire and probe the ports. You can blow carefully on each one and watch for a response on ASI.

By design a yaw will give some ram pressure to one side with a compensating negative on the opposite side , together should be close to true static..

Design must not have been great as it was replaced with a triple prob on the tail in the later models.

Buzz

Thanks - this is exactly what I was looking for/figuring. I have a lot of time in tracing pneumatic systems issues on other gliders, so I'm comfortable with the process. Interestingly, the fuselage on this glider was refinished before we bought it. Makes me wonder if they got a lot of sanding dust, filler, and paint in the system.

On the good news side, the new USB Boroscope I bought will come in handy for inspection the tubing from the inside of the fuselage to see if there are any obvious problems (like tubes dangling or open T-fittings).