AUSTIN, who else ? 🙂

I can't avoid to share here, without digging into details, how much I respect this guy's personality and dedicaton to his causes, X-Plane being surely one among the many.

I have interacted with Austin along the years, since X-plane was first revealed to the World in the nineties of last century and he started participating on forums, then sometimes through private messages, and I really have to acknowledge how much this interaction, even if not always ending up giving the results in terms of X-Plane development I wanted to, always happens in a exemplar way, showing his will to accepts critics, suggestions, but also to defend his points.

Recently I and Alexis started a deeper investigation into one theme that we thought might have some glitch in X-Plane 11 FDM, and, Murmur as usual and since he is by far a much more knowledgeable reference in terms of aerodynamics and  very professional / structured / scientific approach to investigation of any problems we give him to think about ( just like the torque bug .... ), was involved in the cycle 🙂

Murmur's contributions are saved in my e-mail, text files, etc... since I started interacting with him, or simply reading some of his more technical posts. He takes the problem and examines it in a rather methodic way ( methodic is kind of a direct translation from the Portuguese "metódico", so it might not sound right ? ) pragmatic in his investigations and conclusions, and exaustive in looking for supporting documentation. Alexis complements with his passion for aviation and his hands-on experience, and capacity to run exaustive flight testing, recording precious data and looking for any potential clues...

Austin, OTOH, took the challenge but also showed us, without any secrecy as he always does, how he calculates that "stuff" in XP's FDM, and heck, at the end of the day I simply have to ADMIRE the way he works, and the dedication to accuracy he uses when developing stuff for X-Plane, now fully dedicated to XP12 and coding it afresh !

So, if it wasn't for the simple fact that X-Plane is always a reference in terms of flight dynamics modelling, I can only be enthusiastic about XP12, and looking forward to get my hand on it.

The subject of this exchange of oppinions was one Austin, me and Alexis, and certainly others here at the forum who fly IRL too can assess based on hands-on experience, Murmur OTOH is able to evaluate these problems based on his very easy relationship with aerodynamics / phisycs.

The original "problem"  still has a "mistery" at it's root, but, the aftermath of all this exchenge on "Ground Effect Modeling" in XP11 was really enlightining, not only on the theme itself, but also and specially in terms of how a FLIGHT simulator like XP does it !

Edited October 3, 20214 yr by jcomm
typos

@jcomm

I really enjoy listening to Austin in how he explains things and the passion he has for this sim. Not because he wants it to sale because he thinks better, but he really believes in it as it tries to solve whatever problems he faces and works through them were you can actually see it by using the sim. He puts it right out there, no secret or magic to his method. He will throw a problem on the whiteboard  and if necessary, go out there, test to prove his theory; what any good engineer would do.  Because of his passion for the sim, we all benefit for it and it shows in his dedication to what has in store for X-plane 12.   

late comer to this party, but you didnt give details on what you were actually discussing.

It was only yesterday I was considering maybe asking Austin if fuselage angular momentum is modelled - when a fuselage starts spinning the momentum that needs to be cancelled, either by drag or control inputs.

"something" does feel a little off, and I wondered if it is related to that (very subtle issues)

Then I remembered there are a ton of fixes coming from the work with BETA that isnt in yet, and whatever it is its probably better to see how they affect things, that raw data will have been much more useful than anything I can provide.

1 hour ago, mSparks said:

It was only yesterday I was considering maybe asking Austin if fuselage angular momentum is modelled - when a fuselage starts spinning the momentum that needs to be cancelled, either by drag or control inputs.

XP calculates default radii of gyration for the 3 axes, from the geometry of the aircraft in planemaker, so I'm pretty sure it takes into considerations the dimensions of the fuselage (e.g., a longer fuselage means higher calculated inertia in pitch and yaw).

But the aircraft designer can override those default values ("custom radii of gyration" in one of the weight & balance screen).

With regard to the "rotational drag" of spinning bodies instead, I don't know if XP calculates it or not. For a conventional aircraft though, it should not be very important, compared to the moments caused by wings and tail surfaces (although it's there).

Edited October 3, 20214 yr by Murmur

Sparks, you're right that I was far from explicit regarding what I was referring to in my OP, and btw, I should already have found the time to fly your 744 mod !!!

Regarding your question, the subject under analysis was pitching moment due to ground effect, and how XP11 calculates it, which might differ from the fresh code Austin is writing for XP12.

Some, me and Alexis included, have noticed that when using the experimental FM most aircraft compatible with it exhibit a tendency to feel like "being sucked" by the runway on the last stages of their landings, alreading during the flare... (*)

This can be related to a pitching down moment, which is described on many technical texts covering ground effect and it's aerodynamics...

As a pilot I confess I can't say I notice that pitching down moment when landing the gliders I fly, but that might also be related to the way I trim for landing since more than 40 yrs ago when I was 16 and a student glider pilot and got scared to death after fighting the force required to deploy the flaps on my trainning glider, a Blanik L13 ( good old CS-PBY, lost in a CB downdraft accident... with no one on board... ), which could have ended on a stall/spin on the base leg 🙄.

Well, there are many nuances associated with different aircraft. As Yo-Yo from DCS World told me, the Mig-29 is well known to it's pilot's for the exacerbated pitching down moment due to ground effect...

https://forums.eagle.ru/topic/283273-how-is-ground-effect-modelled-in-dcs-world-pfm/?do=findComment&comment=4787983

We are still trying to find a reason for the way it sometimes feels in XP11, but one thing is for sure - Austin has modelled it with high detail and we can't actually find any glitch in the way it's theoretical base is used in that version of X-Plane with experimental FM enabled.

 

(*) there might also be some effect from the high wind gradient now programmed in XP11 to reduce wind intensity on ground along the last 30 feet before touchdown, so any tests should preferably be run with calm / null winds...

Edited October 3, 20214 yr by jcomm
many typos... as usual ...

1 hour ago, jcomm said:

This can be related to a pitching down moment

so.. maybe Im not wrong.

1 hour ago, jcomm said:

exhibit a tendency to feel like "being sucked" by the runway

It feels like VRS in the helo - but isnt.

There is also a missing / too weak effect of tail stabiliser effectiveness in the helo while accelerating.

Both of these "feel" to me like an angular momentum issue, like rotating the airframe was too easy, but as you say

1 hour ago, jcomm said:

there are many nuances associated with different aircraft

And its only sometimes.

1 hour ago, jcomm said:

Austin has modelled it with high detail and we can't actually find any glitch in the way it's theoretical base is used in that version of X-Plane with experimental FM enabled.

That much Ive seen, I think its more about how they interact with each other and how it changes with different airspeed over the flight/control surfaces, - low airspeed means controls are much less effective at putting rotational momentum into an airframe, once you start getting over 40kts it starts to become negligible, 60kts and its basically gone.

Theres a similar thing still to do on the 744, where the real one has a rudder ratio module so that rudder inputs behave the same accross the full airspeed range - slow gives large rudder deflection for a small input, fast and the rudder hardly moves at all.

It currently almost behaves like it has that even tho the rudder deflection is always the same

17 minutes ago, mSparks said:

Theres a similar thing still to do on the 744, where the real one has a rudder ratio module so that rudder inputs behave the same accross the full airspeed range - slow gives large rudder deflection for a small input, fast and the rudder hardly moves at all.

It currently almost behaves like it has that even tho the rudder deflection is always the same

If I understood correctly what you mean, the default XP 747 has rudder phased out between 280 and 400 KIAS (100% @280 KIAS -> 20% @400 KIAS).

If in sim the rudder appears to deflect the same at low vs airspeed, it's an error on part of the aircraft designer, who probably tied the visual rudder deflection to rudder input dataref, instead od the actual physical rudder deflection dataref.

EDIT: I checked and it behaves as it should, so I probably misunderstood your post! 🙂

Edited October 3, 20214 yr by Murmur

3 minutes ago, Murmur said:

the default XP 747 has rudder phased out between 280 and 400 KIAS (100% @280 KIAS -> 20% @400 KIAS).

And the real one is between 135kts and 250kts

1 minute ago, mSparks said:

And the real one is between 135kts and 250kts

Excellent reference, that should be an easy fix to do in Planemaker.

Just now, Murmur said:

Excellent reference, that should be an easy fix to do in Planemaker.

Not planemaker, because in the real one it fails with the hydraulics, we'll simulate the module in xtlua and tie it to the respective hydraulic system Stab Trim in: (N.B. Xplane probably does this just fine, the current one "hacks" it by having a lower defection)

(and hmm, someone deleted the black backgrounds from the OFFs... grrr)

3 hours ago, Murmur said:

XP calculates default radii of gyration for the 3 axes, from the geometry of the aircraft in planemaker, so I'm pretty sure it takes into considerations the dimensions of the fuselage (e.g., a longer fuselage means higher calculated inertia in pitch and yaw).

But the aircraft designer can override those default values ("custom radii of gyration" in one of the weight & balance screen).

With regard to the "rotational drag" of spinning bodies instead, I don't know if XP calculates it or not. For a conventional aircraft though, it should not be very important, compared to the moments caused by wings and tail surfaces (although it's there).

What I'm referring to here tho, is how that falls off at low airspeed - such as landing in ground effect, for example, the "pitch down" could be coming from it being to easy to rotate the aircraft pitch as elevator effectiveness disappears.

I think it will end up being a moot point though, because XP12 has a load of changes from RW testing baked in, so if it is just refining parameters we'll have to wait for the new shiny to pass judgement.

The main problem with XP flight model is he thinks it can be perfected into mathematical equations. Years and years of tweaking and adding equations. That's fine but the randomness and intricacies of mother nature, wind shear, wind currents, and how it flows over and up mountain cliffs, through valleys and across plains and water bodies. Mother nature effects has been completely overlooked because the whole thinking is just inside that cube around the aircraft. So to sum up, XP flight model can apply to a wind tunnel testing but for real flying it is dumbed down to basics absent of real world weather effects.

Thats another thing I love about Him, he does what is important and isnt twisted and turned in every direction by every user and complaint. I had a boss like that, if he is wrong Prove it.

2 hours ago, Greazer said:

XP flight model can apply to a wind tunnel testing but for real flying it is dumbed down to basics absent of real world weather effects.

Sigh... speaking of dumbed down... 😅

11 minutes ago, Janov said:

Sigh... speaking of dumbed down... 😅

Is there any point in starting a "discussion" here at all??

Edited October 4, 20214 yr by mjrhealth

4 hours ago, Greazer said:

The main problem with XP flight model is he thinks it can be perfected into mathematical equations. Years and years of tweaking and adding equations. That's fine but the randomness and intricacies of mother nature, wind shear, wind currents, and how it flows over and up mountain cliffs, through valleys and across plains and water bodies. Mother nature effects has been completely overlooked because the whole thinking is just inside that cube around the aircraft. So to sum up, XP flight model can apply to a wind tunnel testing but for real flying it is dumbed down to basics absent of real world weather effects.

Greazer, I have to disagree ....

X-Plane is actually one of the flight simulators that led the modelling of weather effects with much more detail than any of it's competitors up to date. This was true when each new version was released.

The option is no longer there in the "weather menu", but I recall the time when we had "downburst" / "microburst" options. Meanwhile these effects were wrapped with the overall weather modelling and happen "naturally" if the necessary conditions are present.

Wind flow affected by topography is yet another feature that X-Plane introduced I believe starting X-Plane 10, and that's around 12 yrs ago !

Could it be fine tunned ? Surely ! like pretty much everything in Live ...

Turbulence modelling is limited in all of the mass flightsims I've use, with or without weather add-ons. FSX and P3D, MFS and XP could do a better work in modelling turbulence. I have tested it in MFS ( there's a section of the weather menu available to manually set it... ) and it's as ridiculous in it's effects or even worst as what we presently have in XP. But "presently" is the word ! As we all know Austin is working on XP12 weather, so I look forward for enhancements.

Well, at least in X-Plane I can have consistent visibility simulation according to RW data, a problem that appears complex, as the developers ave even stated, to solve in MFS.

OFC I had to bring comparisons to the table ... and, BTW, there's a reference in terms of weather modelling, just as with many other aspects of the flight simulation - Aerowinx PSX ! There, and in it's 744, I have found what I consider to be the most true to real modelling of effects like shear ad local turbulence.

Horses for the courses, but there are objective details that we should take into consideration when making comparisons...

Edited October 4, 20214 yr by jcomm