Possible work around for GA aircarft "Dynamic Pitch Stability" - (10/10/2020 not an Issue)

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UPDATE: 10/10/2020

The Asobo C172’s Dynamic pitch Stability matches almost Exacrly, that of the RL C172.

There is NO issue… It just seems bad, because currently, the c172’s AP, does not initialize to the planes VS, when the AP is turned on, causing a violent pitch change, and the resulting Oscillations.

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Possible (temporary) work around for GA aircraft “Dynamic Pitch Stability”

This has NOTHING to do with AP, so Mods, please do not combine this with other such AP “porpoising” threads

The current GA planes, ( I am here going to specifically talk about the C172), have unrealistically high “Dynamic Pitch Instability”

Nothing to do with AP. When you hand fly them, the are verging on being pitch Unstable, one effect of which, is that they are are unrealistically difficult to correctly trim.

One factor that can make a Plane Dynamically Pitch Unstable, is the COG being to far towards the rear. (even if set within manufacturers limits)

So one way to make the plane MORE Pitch stable is to move the CoG Forward.

You cannot put bags of helium in the back (ie negative Baggage weight), but since the Pilot & CO-Pilot are also behind of the Cog Point, reducing their weight, will effectively move the COG forward, and this make the plane more dynamically pitch stable.

Excuse me if I am not 100% precise here, I am NOT an Aeronautical Design Professional.

Bottom Line, Throw out the Co-Pilot and go on a very severe diet (ie set both Pilot & Co-Pilot weight to ZERO)

This will both move the COG forward (to nearer the forward limit) as well as, with this less weight, improve climb performance – a double PLUS

Try it – you have nothing to loose, (except some weight !!)
See if you notice a difference. On paper, plotting the Dynamic Pitch Response to an Impulse disturbance to the yoke, there is a very significant improvement.

— which also greatly HELPS the AP in controlling the plane.
It helps you when flying with the stick , so it helps the AP – another Double PLUS.

and Unlike “Updrafts and Downdrafts” this is something that is NOT
“To difficult for you to understand” (sigh)

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I can’t say I’ve noticed any obvious pitch instability in most of the GA aircraft.

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Fair enough - so this post does not apply to you … but nether the less, it is an issue that some can notice, and might like to address…

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UPDATE !!! patch #5 ???

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It should be noted that removing the weight of the copilot in some of the GA aircraft can affect the lateral balance - probably more than it should. If you want to experiment with moving the CG, and you find doing so causes the aircraft to turn, try changing the pilot and copilot’s weights together.

You also need to check that the CG doesn’t go out of range as the fuel tank empties. And note that having the CG too far forward can lead to problems - you may run out of elevator authority as you try to flare for landing.

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All very true …

At the moment, personally, I am more interested in determining the Bugs, than doing a series of 100% realistic flights. (the Engineer in me !!)
To that end, I have set Fixed 50% Tanks, and Dev Mode on, so I do not fill my logbook with “Test Flights”
With all the GOC item set as RL, there is a Stability issue, that is cause elsewhere, probably somewhere where I have no control. Asobo will need to eventually fix it.

In the meanwhile, I want to be able to be able to trim the plane, and test the AP, the GPS etc, without the instability affecting it… so hence my “temporary” solution.

Its more to check is everything else works as expected …

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That’ why i suggest, they BOTH need to loose weight !!!

Which also demonstrates that the Dihedral aerodynamics does not seem to be working, set the Dihedral to any value, even ridiculously high angles, and the wing leveling effect does NOT change.

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That’ why i suggest, they BOTH need to lose weight !!!

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The default C208 has this problem bad. Or at least some problem, I cannot quite make sense of it. Short term it behaves like negative static pitch stability since a pitch disturbance initially grows.

Here is the scenario: Level flight, in trim (well, to the extent possible). Push yoke forward into a 500 fpm descent and let go. Watch the aircraft go into an even steeper descent, beyond 1500 fpm. Thus far it looks like negative pitch stability. But then it eventually starts pitching back up into some sort of phugoid, so it does display some level of static stability. I have not let the phugoid progress far enough to determine if dynamic stability is positive or negative.

Same thing happens in the other direction when pulling into a climb.

This makes hand flying the aircraft very tedious, it needs careful watching all the time. Especially challenging when doing pattern work, easy to develop high rates of climb after takeoff or high rates of descent during downwind to base or base to final turn.

A real aircraft with these flight characteristics would never be certified.

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EXACTLY : Finallly someone is looking at the Pitch Stability Impulse response, and not just blaming the AP for everything.

You momentarily disturb trimmed stable flight, with a momentary control deflection, and the plane oscillates. The Oscillation does die away,

In Th C172, it dies away at about 50% per cycle, provided the oscillation is low enough in amplitude, tat it does not have too much effect on the aircraft speed,

Turning on the AP, just trims the plane faster and more precisely than you can, which helps reduce the Oscillations.

What does not help, is that when you turn on the KAP140 AP, it does not set its VS to the current VS of the plane.

Instead it sets it to the last VS entered, when then AP was turned off.

BTW: It GROWS initially, because of the extra STEP of the AP not matching the Current VS, when it is engaged,

I’m speechless as to how it ever passed any form of testing …( just as well I am speechless)

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Yeah, if you have not tried this in the C208 yet, give it a try, you will find it highly entertaining! In the C172 I could live with it, didn’t feel too bad hand flying it. But in the C208… Yikes! Definitely takes some getting used to.

What do you make of the impulse response initially growing but then reversing, is that negative or positive stability in the field of aerodynamics? If it works like stability analysis in math it would be the initial response to a small disturbance that matters, and then the C208 is actually unstable since it initially diverges. This is all with no AP involved.

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The dynamic stability’s damping is a function of the oscillation amplitude.
Small Oscillation are under damped.
Big Oscillations increase in amplitude.

The point is, you don’t LIVE with it … you DIE with it !!!

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Yeah agree. I’m thinking in the C208B case it is actually statically unstable, since the initial disturbance grows. But once the disturbance is big enough there is a force countering the disturbance, and then dynamic stability comes into play. Anyway, just semantics or an aerodynamical technicality - but interesting on an intellectual level.

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Still nobody in the Community ( of 1,000,000 + ) able to tackle this issue.
All it needs is 1 !!

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Yup. This is definitely an issue. And one that probably only IRL pilots and simulation enthusiasts will notice.

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Eh…to be honest, it may vary from aircraft to aircraft. I don’t notice any particular issue with the G1000 Skyhawk, it handles like I remember a Skyhawk handles (then again I’ve only flown RGs and Seminoles for the past few years).

It really depends on what you’re doing with it too; of course if you push the aircraft to its limits by yanking the yoke all the way back out of nowhere, you’ll see some odd behaviour because a plane is never even meant to be there to begin with (how does a developer simulate abnormal flight conditions accurately if they’re dynamic and not very well understood?)

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No need for any of that to see the issue.

From level flight, in trim (to the extent possible), gently push the yoke forward to about a 500 fpm descent and let go. Watch what happens.

Agree it is different in different airplanes; odd but relatively benign in the C172, bizarrely unstable in the C208B.

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Exactly – and the way the C172 behaves in MSFS is NOT EVEN CLOSE to RL.

The Carenado MSFS Cessna is a lot closer to RL, and returns to its former stable trimmed state with a very good, critically damped repsonse, and is back in the same trimmed state it was within about 5 seconds.-- the key being, getting back to that trimmed stable state, before the airspeed has had time to significantly change, and require a different trim
— that changes the airspeed - that require a different trim
— that changes the airspeed - that require a different trim
— that changes the airspeed - that require a different trim
— that changes the airspeed - etc

slowing the response has the opposite effect to what is wanted, the slower the response, the more time the plane has to change speed, that requires more time to return to original pitch-- that change the speed etc etc etc

Each cycle takes about 10 seconds, during which time, the plane’s speed is changing significantly

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I think you are absolutely right, and I figured out what is going on with the C208B as well: At least in the conditions I tried this in (light, forward CoG) it has negative static pitch stability, but trim changes in the correct sense with airspeed changes. So initially it increases its rate of descent, but as airspeed picks up it starts pitching up again, and from then on it is “statically stable” (by means of speed changes, not aerodynamic static stability caused by things like decalage) and dynamically unstable.

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That’s seems right … but to be honest, I am not an expert, or even a very knowledgeable in the details.
I know how to fly, and I know what a C172 should behave like, and the Asobo planes are not acting correctly.
It’s quite clear what is wrong, but how to correct it totally eludes me, and just messing about blindly with .cfg setting is not a way I want to spend the next 6 months.
I have looked at some of the Modded planes, bit most of the mods are to do with Auto Pilots, and although a few of the flight parameters have been changed a little, and are different to those in Patch 35, that may well be because, when the Mod was first made, it used parameters from an earlier patch version, and it really changes by Asobo I am seeing.

So I am back to the conclusion, that until some with the right technical knowledge looks at this issue, it is never going to be correct in the right way, and any changes that are guesses as to what to change, while maybe having an effect on one thing, are probably completely messing up other flight characteristics.

Ironically, the planes appear to have a long period oscillation, which, if faster, might dies away faster – ie the Plane need to be made MORE “Twitchies” !!!