Rapid pitch excursions on LPV VNAV

[carlmill]

Started training today to get back to IFR currency for the first time in my new to me RV-9A with full Dynon hardware +

Found that when tracking the glide slope in VNAV with speed 90-95kts, the pitch servo would command quick sharp up/down commands, the screen would show slipping clutch and commands for trim up (it has working electric trim). The glide slope deviations were modest and on average it stayed on the GS.
The doesn't happen on a non coupled descent.

Thoughts/ suggestions?

Carl

 

[airguy]

Started training today to get back to IFR currency for the first time in my new to me RV-9A with full Dynon hardware +

Found that when tracking the glide slope in VNAV with speed 90-95kts, the pitch servo would command quick sharp up/down commands, the screen would show slipping clutch and commands for trim up (it has working electric trim).  The glide slope deviations were modest and on average it stayed on the GS.
The doesn't happen on a non coupled descent.

Thoughts/ suggestions?

Carl

I'm doing this exact thing right now, pursuing my IFR in a 9A with Skyview. I found that my coupled LPV approaches did the same thing if I flew them 90-100 knots with zero flaps. I dropped 10 degrees of flaps and flew it a 90 knots, and the performance is much better. On a whim I dropped 20 degrees flaps and flew it at 80 knots, and it followed the glideslope like it was on rails. You might want to try that.