Dynamic instability in descent

C-FJDH

Picked of the Duke package during the recent sale, and have put the first few hours on them. Lovely airplanes, really well done. Small twins are my happy place on the sim, and these really deliver.

I've run into a issue on the various models of the Duke, and I'm surprised I have not seen it mentioned here in the forum. The aircraft exhibit dynamic instability, which is most evident during descent.

I'll start a descent, and set power and trim for -1000fpm. It's quite a challenge to keep a constant descent rate, and if the aircraft is disturbed by turbulence or control input, it will begin to oscillate up and down. The oscillations increase in magnitude until the VSI is pinned in each direction. Without hard correction, the aircraft would depart controlled flight or break-up.

In a dynamically stable flight model, the oscillations should decrease on each cycle, until the aircraft reestablishes equilibrium.

All that said, since no one else seems to experience this, is there something I should be doing differently? Something important I'm missing?

Cheers, and thanks for any helpful comments.

Black Square

Any chance I could convince you to upload a video of this behavior for me? I've never seen such behavior in all my testing, and I would like to think that I've put the aircraft through its various extremes. Is this while hand-flying, only on autopilot, or both? I have a hunch, but I don't want to assume anything before I have a chance to see it. Thanks for sharing.

C-FJDH

I'll have to figure out how to record a video first, but I'll see what I can do.
Hand-flying only, which is how I do most of my flying except for longer cruise segments. I've seen it happen on the Grand Duke and the turbine. CG was just slightly behind center at takeoff.
About to do another turbine flight, so I'll see if I can capture it.

C-FJDH

Figured out video, but had a 80kt quartering tailwind on the flight and descent that made things a bit too lively for testing. Will try again tomorrow.
But what a fun and totally immersive airplane. One of my best flights yet on a Duke.

Rolo Bach

I've flown more than 250 hours on the piston and grand duke. Never the plane had this sort of instability on manual or autopilot fly, in any weather condition.

C-FJDH

Here's the video: https://youtu.be/tKcCebRK_Wg
Grand Duke. No pitch inputs made once the oscillations begin, just a few nudges of aileron to keep the wings level. Previous flight on the Turbine Duke did not show this characteristic, even though conditions were pretty rough.

Here's a short video on the observed effect of negative dynamic stability: https://youtu.be/Q2DOus05Qso?si=v8QeLNWaT-Q414y8

Rolo Bach

Seems the CG is some way rear.

C-FJDH

No, CG is one tick mark behind center.

Rolo Bach

OK, try to put the CG halfway forward (as I usually set). Trim neutral. If this don't work, I don't know what matter.