flight dynamics help understand

[rwy12 16]

Hi

I have both msfs2020 and recently x plane 11.

when I fly in the msfs at any at level altitude, the plane always needs constant control input to stay on same heading and altitude.

In x plane 11,  the plane keeps flying at the same altitude and heading for long long time without any control input. The longest about 20 minutes.

weather is  set to real world in both cases.

same planes-airbus toliss.

Can somebody please advise if this is how x plane behaves, wind seems to have zero impact on flight dynamics in x plane 11.

I have a feeling my controls need fine tuning.

I have managed to perfect the control sensitivity in msfs2020.

I have managed to set the curves in x plane, but they need further fine tuning.

I have the fulcrum yoke-if any other simmer has this can you please share your control settings.

Thank you

Qas

 

 

[jcomm 5,939]

Hi,

a possibility, since you mention you're on XP11 and not XP12, is that you have picked the default settings for the axis modulation through "Artificial Stability"...

When you go to the Settings / Input to define the pitch, roll and yaw axis, in XP11 you will have two separate types of settings - sensitivity and artificial stability.

Make sure you move the slider full LEFT on the three axis under the "Artificial Stability" GUI.

 

Edited May 19, 2023 by jcomm

[blingthinger 649]

Is real weather the best test of this? Maybe you're flying in a stable pocket of the same weather conditions.

The Airbus flight control system is meant to lock to a selected pitch/roll attitude and hold it, right? Does the IRL system drift in a situation like this? 

Manually setting a weather condition and then letting it get better or worse on its own would be a good test. I know XP has this capability.

[Bob Scott 7,721]

57 minutes ago, jcomm said:

Hi,

a possibility, since you mention you're on XP11 and not XP12, is that you have picked the default settings for the axis modulation through "Artificial Stability"...

When you go to the Settings / Input to define the pitch, roll and yaw axis, in XP11 you will have two separate types of settings - sensitivity and artificial stability.

Make sure you move the slider full LEFT on the three axis under the "Artificial Stability" GUI.

 

Well, he's flying a Scarebus, which is the very definition of "artificial stability"...if anything but a pilot or AFCS input disturbs the attitude, the computer-controlled flight control system will return the acft to the set attitude.

[rwy12 16]

many thanks everyone-I will check the settings and try the suggestions.

Qas

[blingthinger 649]

8 hours ago, Bob Scott said:

the computer-controlled flight control system will return the acft to the set attitude

So is it safe to say then (based on IRL behavior) that if an Airbus flight model is moving through perfectly still air and is requiring pilot input to stay on attitude, the model isn't quite right? 

[C2615 319]

3 hours ago, blingthinger said:

So is it safe to say then (based on IRL behavior) that if an Airbus flight model is moving through perfectly still air and is requiring pilot input to stay on attitude, the model isn't quite right? 

Airbus normal law is to stay on given flight path, rather than attitude, if speed \ config or wind(up/down draft) changed, attitude will change for that then.

[mSparks 2,086]

7 hours ago, blingthinger said:

requiring pilot input to stay on attitude, the model isn't quite right? 

disclaimer: Ive never piloted an aircraft above 10,000 feet. I cant talk for msfs.

that out of the way, I have spent an insane amount of time studying the 744 systems, getting feedback from people who have, and learnt a lot from the XP flight model doing so.

There are some really interesting/none obvious things happening when you start flying fast and/or high.

For fast, the faster you go the more impact the flight surfaces have, so much so that above certain speeds the 744 disables the outer ailerons (aileron lockout) and reduces the rudder movement via a module called the Rudder Ratio Changer, there are whole chunks of proceedures for dealing with these failing and misbehaving. 

For high, the higher you go the more sensitive climb/descend is to pitch.

These both combine to mean when you are high and fast control inputs need to be absolutely minimal to hold her stable, even in sketchy weather. whereas coming into approach particularly with sketchy weather it can be a real fight to keep a stable approach.

All of this is done well by the xplane flight model as far as I can tell, and needed a lot of customisation in the 744 over what came as default in XP11 to get the correct behaviour.

So it will depend a lot on the aircraft even within xplane, and I doubt anyone really knows for msfs.

There are also a lot of people in all the sims pretending they are real pilots for clicks - getting even the most basic things wrong, which has been incredibly frustrating.

[blingthinger 649]

9 hours ago, C2615 said:

Airbus normal law is to stay on given flight path, rather than attitude, if speed \ config or wind(up/down draft) changed, attitude will change for that then.

Gotcha. So assuming perfectly still air with A/T engaged, the only thing that would lead to an attitude correction by the computer (to maintain flight path) is weight/balance changing as fuel burns? There's no fun quirk IRL Airbus systems (e.g. surprisingly bad gyro drift) that would lead the pilot to have to correct every so often when not on A/P?

OP doesn't specify what they are correcting for (attitude vs path) and is comparing different real weather models and different weight/balance models. Lots of variables at play here.

 

6 hours ago, mSparks said:

the faster you go the more impact the flight surfaces have

I would imagine this would have implications on A/P tuning as well?

 

5 hours ago, mSparks said:

the higher you go the more sensitive climb/descend is to pitch.

I always figured Maverick almost died because of this. Or flutter. But pausing at exactly this frame:

https://youtu.be/ngxlg8zzQos?t=214

we can see it was little more than a battery issue. These E/V hybrids are dangerous!

[mSparks 2,086]

1 hour ago, blingthinger said:

I would imagine this would have implications on A/P tuning as well?

 

yep, when you see XP releases with complaints about porpoising, thats why, they dialed it in low and it drives everything to much when high.

Actually, we saw that in MS, so they did model that at one point- as long as they didnt "fix" it by removing the physics.

default XP AP doesn't have any options to correct for this, so the best you can do with the default AP is being very lazy when low and slow.

took a good chunk of the last 12 months getting it to behave well throughout the ranges - all stuff you only notice when it doesnt work right.

2 hours ago, blingthinger said:

we can see it was little more than a battery issue

I was impressed with the all the effects they demo'd in that scene, all round enjoyed that movie.

proper plasma interference, decent data out displays, nicely captured what I know of the skunkworks way of doing things.

I kinda wish someone would bring the XP space shuttle back to life, wrap it in some decent particles and failures.

 

[Bob Scott 7,721]

On 5/19/2023 at 4:38 PM, blingthinger said:

So is it safe to say then (based on IRL behavior) that if an Airbus flight model is moving through perfectly still air and is requiring pilot input to stay on attitude, the model isn't quite right? 

In that case it's safe to say that the model's behavior is not realistic. 

Generally when we refer to "flight dynamics" in the sim world, we're referring to the simulation of an aircraft's flight physics, e.g. how the plane behaves physically as the result of gravity, aerodynamic forces, weather etc...things like reaction to control surface inputs, acceleration, inertia etc.  That's a function of the platform simulation (XPlane is very good), and the parameterizations given to it (the .acf file).  With a Scarebus, the model's behavior is also a function of the constantly-active automation in the FBW system, so behavior can still be boogered even with great flight dynamics if you have a lousy implementation of the FBW and/or AFCS automation.

On 5/19/2023 at 8:01 PM, C2615 said:

Airbus normal law is to stay on given flight path, rather than attitude, if speed \ config or wind(up/down draft) changed, attitude will change for that then.

OK, I should have caveated that as being in stable, constant-speed flight.  But...as I understand it, the normal law maintains a load factor, which in most cases is congruent with maintaining a flight path.  However the jet moves within an air mass, so if the air mass moves up/down in a draft or fwd/aft in a wind shear, I'm not certain the flight path (relative to the ground) is unaffected.

[blingthinger 649]

7 hours ago, Bob Scott said:

In that case it's safe to say that the model's behavior is not realistic. 

That's what I figured. I was going to be very surprised if Airbus had a quirk like that.

Yes, as for "model" that's why I left out "flight". Though FBW does enter a fuzz band of where that dividing line can be drawn.

Likewise, a 3rd party FBW system could theoretically be made to compensate for shortcomings of the physics model. Thing with FBW systems is that they generally seek to eliminate the dynamic behaviors of the underlying physics. You can load up all the real-world-accurate wing forces XP can handle with wind tunnel-acquired coefficient polars, etc and the computer will just smooth it out with gust alleviation and flightpath tracking.

Seems as though you'd almost have to have a 6DOF motion platform to really feel a difference between 2 well-written FBW models. The FBW equations are ironing out what you would normally notice on the screen. Have to start feeling the bulk movement in the seat of the pants. And that's a whole different ball of wax.

[jcomm 5,939]

Yet another factor is that some stuff is provided directly by the ASOBO core FM and quite incomplete and somehow also inconsistent... They provide a "fbw airbus-like" control mode, but honestly I don't believe any developer should revert to it f modelling a FBW aircraft, being it an airliner or a military airplane because I seriously doubt it can be trusted...

A simple test of how inconsistent the built in FBW implementation in MFS is can be to try an engine failure and see how the system deals with it... 

I guess Fenix, probably also FBW, use their own code to model Airbus C* law and other nuances of the FBW. I also believe PMDG when they bring the 777 to MFS will hand-code their own C*u law as well as other specificities of the Boeing-style FBW.

In the real Airbus, under "calm weather", a pilot does not have to input any pitch rate demand for turns using bank up to 33º because the FBW system will do it for her/him (better her... tired of ugly pilots in the cockpits...).

From the Airbus FCTM:

"At bank angles less than 33° with no input being made on the sidestick, the computers maintain a zero roll rate and, consequently, the aircraft will maintain a constant bank angle. Within this range, there is no need to make a correction in pitch, as this will be compensated for by the computers. Beyond 33° angle of bank, pitch compensation is no longer available. On releasing the sidestick to neutral, the aircraft rolls back to 33° angle of bank."

A good source of information on the Airbus (and other airliners) can be found here:

Airbus Control Laws (code7700.com)

Edited May 21, 2023 by jcomm