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.

Mustang

Just remembered that I read this thread some weeks ago when I had no time to reply. I've nothing scientific to add as I didn't run any tests, but I was not surprised to see this video because I find that getting the Duke to stay trimmed by hand is generally difficult to the point of being impossible. It's always wandering in pitch in a merry little phugoid.

It's the one single aspect of the aircraft which I don't enjoy (which shows how great the rest of this aircraft is), meaning I resort to the AP more often than I normally would - and then watch the trim wheel jostling around constantly, instead.

I hope Nick is able to take a look at it - when he can find time! - for a future improvement, especially as he has a hunch about what may cause it. I don't know how the real aircraft flies, but I do expect that it should have positive stability in pitch at most (if not all) power settings.

With MSFS, the weather is always a factor as the atmosphere is not as static as older simulators, but other aircraft don't exhibit the same behaviour so it cannot be the sole reason.

Buzz

I watched your video and if you were hands off with a set power setting, then what I saw in your video was positive static stability and negative dynamic stability. No aircraft would get certified for normal category if it exhibited negative dynamic stability. And the BE60 is certified for normal category. I have the duke combo and never noticed this.. I'll give it a try and report back.

Buzz

So gave the B60 a cruise stability test and the model exhibited both positive static and positive dynamic stability. Test parameters... ISA, no winds, 4500' msl, half tanks, centered CG, 30" @ 2600rpm. Set a stable cruise after speed settled for the power setting. PIO by pushing the nose down 5 degrees and let go with a fingertip on the yoke just to maintain wings level. The pitch oscillations reduced in magnitude until about 5 minutes later the aircraft was in level flight with no oscillations.