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CRJ owners, can you please repeat this flight test (static and dynamic longitudinal stability)

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I am a new user of X-Plane and an aerospace engineer in real life. I was initially disappointed in X-Plane but was convinced that payware aircraft fly much better than the horrible default models. I recently bought the CRJ-200 and although the plane flies much better than the default aircraft, it's pitch stability is very wrong/unrealistic on my computer. I was hoping some of you can perform this flight test just so that i can eliminate hardware issues, or issues with my operating system/install. The CRJ flight model on my system shows very low positive/neutral static stability and a highly underdamped phugoid mode which gets easily excited and and is very difficult to arrest. All of these features are highly unrealistic and not what i was expecting in a payware aircraft.

 

Flight test

I deleted/reset my preferences, set number of flight models to 4 and adjusted weather (low visibility) so that i can get about 40 FPS. Set data input and output to output of flight control deflections to make sure there are no ghost control inputs. Get a stopwatch.

 

Do not use the autopilot at any point in the flight. Get the plane up to about 12,000 feet and a speed of 240kts. Trim for straight and level flight. It took me about 4 minutes just to trim the plane as it was in a constant phugoid that would not stop oscillating. This shows very poor static stability bordering on neutral (low positive). Assuming you can get it staight and level (vsi=0) perform a small quick elevator control pertrubation: simply pull back on the elevator until data I/O reads 0.500 (50% elevator up) and release the controls within one second. Make sure the elevator centers back to neutral (0.000 control deflection). After this action start the timer and record the following:

Time until straight and level flight (VSI=0)

Maximum and minimum altitude

Maximum and minimum vertical speed

Number of oscillations until straight and level flight

 

What should happen in real life? Jets are subject to strict FAA requirements regarding stability (FAA part 25.181). A pitch up like this should excite the short term period oscillation (not the phugoid) and should dissipate within 2 cycles and 4 seconds.

 

In regulatory practice, “heavily damped” means in no more than two cycles. Test pilots for the Raytheon Premier I took the aircraft to 35,000 feet to evaluate short period behavior in gusts. After pitching up and releasing the controls (stick-free), they found that the aircraft took approximately 2.5 cycles and 5 seconds to return to level flight. The FAA agreed that this presented no safety issues, but refused to wave their criteria (2 cycles and 4 seconds). The designers fixed things by adding wedges to the trailing edge of the elevator to change the hinge moments, making the elevator’s response to vertical gusts more neutral and bringing the aircraft into line with FAA requirements. (from Flight Emergency & Advanced Maneuvers Training, Inc. dba Flightlab, 2009. All rights reserved. For Training Purposes Only)

In real life the phugoid takes some effort to get excited, usually a long duration elevator deflection from trimmed flight that allows the speed to drop 20 kts and then a release of the elevator. In X-Plane it gets excited instantly, even with minor elevator deflections.

 

What happened to the CRj on my system? After i pitched up (50% elevator) and released the elevator quickly the plane entered a large amplitude phugoid with a VSI of +1.9K FPS. The plane climed to 12,200 ft after the peak of the phugoid the VSI read -2.2K FPM and it started a descent. It settled down at 11,550 ft after which it started to climb again at about 2.0K FPM the first cycle took about 37 seconds. The plane was still oscillating after 6.00 minutes!!! with about half amplitude and half the vertical speed (1.1K) not even close to being damped out. I shut down X-Plane before it could ever return to straight and level flight - waste of time to wait more...

 

I really want to know if this is an issue with my system. I can't comprehend that a model this wrong would be sold as a payware aircraft. It is impossible to fly accurate IFR with such poor stability. This really makes me question if this model was flight tested by real CRJ 200 pilots and how involved real world pilots were in the making of this aircraft.

 

There are plenty of things i like about X-Plane, but the flight model is a major deal breaker for me. As soon as i feel the phugoid and see the VSI start to dance out of control i lose all sense of realism.

 

I currently fly Rob Young's models for Fly 2K! and they show accurate static and dynamic stability.

 

(I assume users are familiar with the concepts of static and dynamic aircraft stability. They are a little tricky to understand especially the difference between static stability and the two dynamic modes: short period and long period)

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What should happen in real life? Jets are subject to strict FAA requirements regarding stability (FAA part 25.181). A pitch up like this should excite the short term period oscillation (not the phugoid) and should dissipate within 2 cycles and 4 seconds.

 

In regulatory practice, “heavily damped” means in no more than two cycles. Test pilots for the Raytheon Premier I took the aircraft to 35,000 feet to evaluate short period behavior in gusts. After pitching up and releasing the controls (stick-free), they found that the aircraft took approximately 2.5 cycles and 5 seconds to return to level flight. The FAA agreed that this presented no safety issues, but refused to wave their criteria (2 cycles and 4 seconds). The designers fixed things by adding wedges to the trailing edge of the elevator to change the hinge moments, making the elevator’s response to vertical gusts more neutral and bringing the aircraft into line with FAA requirements. (from Flight Emergency & Advanced Maneuvers Training, Inc. dba Flightlab, 2009. All rights reserved. For Training Purposes Only)

 

 

Good analysis. I don't have the payware CRJ200 but I don't have difficulty believing your results, and I don't think it's a control issue.

 

I (and not only me) already noticed for a long time that X-Plane FM tends to predict a longitudinal stability (both static and dynamic) much lower than it should be. Probably it's the main culprit of the infamous "twitchiness" many complain about.

 

Obviously there's not a "stick free" mode in a PC flight simulator (elevator deflection is fixed when stick is free, and it does not respond to gusts), but this make the results of analysis actually worse, since with a fixed elevator, static and dynamic stability should improve.

 

The suspects I always had on the poor static/dynamic stability predictions are my biggest issue with X-Plane FM, but I didn't make a quantitative analysis until now.

 

As you probably know, by writing a plug-in, one could analyze in real time the output of X-Plane flight model. I always wanted to write such a plug-in, in order to compare the X-Plane FM predictions with the linearized control and stability coefficients data yuo can find on engineering texts (I guess you know Roskam, etc.).

 

If you decide to do such an analysis, count me in to help you (even if I don't have a lot of time right now). This way, you/we could present a hard case, with hard data, to Austin and try to convince him to take a look at X-Plane's flight model.

 

Marco

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I can't comprehend that a model this wrong would be sold as a payware aircraft.

 

.....and this is why engineers usually work for business types :). The CRJ is fine for it's intended purpose.....satisfaction and entertainment to at least 2 standard deviations of it's customer base. If you are concerned about x-plane's computational abilities, then I recommend you should prepare your own models and do your own testing and not rely on an "off the shelf" implementation not targeted for you.

 

Open up plane-maker and also investigate the data out abilities from x-plane and evaluate the solver. Make a copy of the CRJ perhaps and play with the design parameters....Do some free-bodies, run some calcs...compare those with X-Plane's data out, etc....but just don't buy a model and guess it was assembled by an aero engineer intereted in design elements that you are interested in. Everybody's goals with x-plane is a bit different, but it is a much more accessible software than anything else you will find.

 

Tom Kyler

Laminar / IXEG

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Good analysis. I don't have the payware CRJ200 but I don't have difficulty believing your results, and I don't think it's a control issue.

 

I (and not only me) already noticed for a long time that X-Plane FM tends to predict a longitudinal stability (both static and dynamic) much lower than it should be. Probably it's the main culprit of the infamous "twitchiness" many complain about.

 

Obviously there's not a "stick free" mode in a PC flight simulator (elevator deflection is fixed when stick is free, and it does not respond to gusts), but this make the results of analysis actually worse, since with a fixed elevator, static and dynamic stability should improve.

 

The suspects I always had on the poor static/dynamic stability predictions are my biggest issue with X-Plane FM, but I didn't make a quantitative analysis until now.

 

As you probably know, by writing a plug-in, one could analyze in real time the output of X-Plane flight model. I always wanted to write such a plug-in, in order to compare the X-Plane FM predictions with the linearized control and stability coefficients data yuo can find on engineering texts (I guess you know Roskam, etc.).

 

If you decide to do such an analysis, count me in to help you (even if I don't have a lot of time right now). This way, you/we could present a hard case, with hard data, to Austin and try to convince him to take a look at X-Plane's flight model.

 

Marco

 

Thanks for your offer, i am glad that i'm not the only one seeing this large discrepancy with stability. I would first like to eliminate all hardware/install issues by seeing if others are experiencing this kind of stability error.

 

I understand X-Plane has a large base of very intelligent users including pilots and engineers and even some who are both. Surely they would have noticed that something is wrong if the error is this large. (Something that should take 4 seconds happens in over 12 minutes.. >18000% error.. little bit more than two standard deviations) I am not a pilot in real life and ideally this case is best presented by someone who is both a pilot and an engineer. I am also a junior engineer, again it would be great if a senior pilot/engineer can present something to Austin.

 

My obesrvations with the CRJ: static stability is very low, bordering on neutral, control response/authority is very high, probably 300% more than what is should be. Short-period dynamic stability is not modelled (understandable, related to flutter), long period (phugoid) is inacurate and easily excited due to a lack of static stability and overly high control authority.

 

I first have to confirm that nothing is wrong with my system/hardware/install.

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.....and this is why engineers usually work for business types. The CRJ is fine for it's intended purpose.....satisfaction and entertainment to at least 2 standard deviations of it's customer base. If you are concerned about x-plane's computational abilities, then you should prepare your own models and do your own testing and not rely on an "off the shelf" implementation not targeted for you.

 

I would recommend you open up plane-maker and investigate the data out abilities from x-plane and evaluate the solver. Make a copy of the CRJ perhaps and play with the design parameters....Do some free-bodies, run some calcs...compare those with X-Plane's data out, etc....but just don't buy a model and guess it was assembled by an aero engineer intereted in design elements that you are interested in.....that too is difficult to comprehend.

 

Tom Kyler

Laminar / IXEG

 

So what is X-Plane's intended purpose? Provide 2 standard deviations of error? I think you need to examine your competition and at least provide something that is in the same ball park in terms of stability and control feel. Getting the steady state portion of a flight model is not good enough. It's 2012 and i think the user expects more, especially since great flight models have been available ever since Fly! and maybe even before that.

 

There are other products out there that are available to the flightsim consumer base that simulate static and dynamic stability to much higher degree of accuracy. I am not much different than the rest of of X-Plane's user base. I'm just like any other user looking for a realistic experience. I really like X-Plane's lack of stutter, smooth gauges, lighting, replay, night textures, failure and analysis options. I also like the user interface, but i think the core of this simulator (flight model) needs some work.

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So what is X-Plane's intended purpose? Provide 2 standard deviations of error?

 

Gives us something to do and pays the bills while we do it.

 

Tom Kyler

Laminar/IXEG

www.ixeg.net

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Gives us something to do and reduces our ability to pay other bills :-)

 

Brilliant inspiring analysis, but Tom's right, it's not meant for testing multi million dollar jet stability tolerances.

 

Im no engineer but I definitely notice some weird handling traits in many xplanes, usually as I rarely fly within their intended performance envelopes! But only at extremes or if I put ny pedantic pants on.

 

If you widen your tolerance levels, you'll find it pretty good. What do u think of Fsx's fm? Flight? Or a better plane's performance, like the NGX by pmdg? I'd be really interested to hear your opinion.

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So what is X-Plane's intended purpose? Provide 2 standard deviations of error? I think you need to examine your competition and at least provide something that is in the same ball park in terms of stability and control feel. Getting the steady state portion of a flight model is not good enough. It's 2012 and i think the user expects more, especially since great flight models have been available ever since Fly! and maybe even before that.

 

We've often heard.............that X-Plane must be more "real" because it's more of a challange to fly. I suppose most, or all of these statements are from armchair pilots. Truth is, most real airplanes are very easy to fly in cruise mode, or small deviations in altitude. And this also includes airplanes such as a Pitt's aerobatic variety. If an X-Plane model needs "artifical stability" turned on, then fine. It's still a computer program, any way you look at it. It's not the real thing, in which an added stability mechanism might be employed.

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If you widen your tolerance levels, you'll find it pretty good. What do u think of Fsx's fm? Flight? Or a better plane's performance, like the NGX by pmdg? I'd be really interested to hear your opinion

 

I took a long break from this hobby and i only recently got back to it. i don't own FSX or earlier versions of X-Plane. I have a Mac and wanted to buy the latest flight simulator. I bought X-Plane without enough research. I have to admit the FAA certification was what swayed the bar. The only other sim i can speak about is the now outdated Fly 2K! and the v88 aircraft versions made by Rob Young. I believe these models were even more refined in Fly! II, but i never bought Fly! II so again i have no opinion about it. I would not hesitate to recommend the Fly 2k! (v88 freeware patch) aircraft if you are looking for the sensation of good positive static stability. I believe Rob now works for a company called Real Air and I am pretty sure his flight modelling work there is just as outstanding as in Fly!, probably even more accurate. I have also heard that ELITE has a good, stable flight model.

 

I have to remind you again that i am not a real pilot (not for at least a year) and i believe that the best way to evaluate a flight simulator is to get feedback from an unbiased real world pilot of that same aircraft, not an engineer. Although we understand the math and all those things I am sure that there are intricacies that i don't have a feel for yet. I am not one of those engineers that think they know everything.

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There is a discussion on the similar problem in their support forum. This problem happens with autopilot on, but probably can be somehow related...

 

http://forums.x-pilot.com/index.php/topic/3106-progressive-pitch-oscillations-in-cruise-with-ap-on/

 

 

 

 

I am a new user of X-Plane and an aerospace engineer in real life. I was initially disappointed in X-Plane but was convinced that payware aircraft fly much better than the horrible default models. I recently bought the CRJ-200 and although the plane flies much better than the default aircraft, it's pitch stability is very wrong/unrealistic on my computer. I was hoping some of you can perform this flight test just so that i can eliminate hardware issues, or issues with my operating system/install. The CRJ flight model on my system shows very low positive/neutral static stability and a highly underdamped phugoid mode which gets easily excited and and is very difficult to arrest. All of these features are highly unrealistic and not what i was expecting in a payware aircraft.

 

Flight test

I deleted/reset my preferences, set number of flight models to 4 and adjusted weather (low visibility) so that i can get about 40 FPS. Set data input and output to output of flight control deflections to make sure there are no ghost control inputs. Get a stopwatch.

 

Do not use the autopilot at any point in the flight. Get the plane up to about 12,000 feet and a speed of 240kts. Trim for straight and level flight. It took me about 4 minutes just to trim the plane as it was in a constant phugoid that would not stop oscillating. This shows very poor static stability bordering on neutral (low positive). Assuming you can get it staight and level (vsi=0) perform a small quick elevator control pertrubation: simply pull back on the elevator until data I/O reads 0.500 (50% elevator up) and release the controls within one second. Make sure the elevator centers back to neutral (0.000 control deflection). After this action start the timer and record the following:

Time until straight and level flight (VSI=0)

Maximum and minimum altitude

Maximum and minimum vertical speed

Number of oscillations until straight and level flight

 

What should happen in real life? Jets are subject to strict FAA requirements regarding stability (FAA part 25.181). A pitch up like this should excite the short term period oscillation (not the phugoid) and should dissipate within 2 cycles and 4 seconds.

 

In regulatory practice, “heavily damped” means in no more than two cycles. Test pilots for the Raytheon Premier I took the aircraft to 35,000 feet to evaluate short period behavior in gusts. After pitching up and releasing the controls (stick-free), they found that the aircraft took approximately 2.5 cycles and 5 seconds to return to level flight. The FAA agreed that this presented no safety issues, but refused to wave their criteria (2 cycles and 4 seconds). The designers fixed things by adding wedges to the trailing edge of the elevator to change the hinge moments, making the elevator’s response to vertical gusts more neutral and bringing the aircraft into line with FAA requirements. (from Flight Emergency & Advanced Maneuvers Training, Inc. dba Flightlab, 2009. All rights reserved. For Training Purposes Only)

In real life the phugoid takes some effort to get excited, usually a long duration elevator deflection from trimmed flight that allows the speed to drop 20 kts and then a release of the elevator. In X-Plane it gets excited instantly, even with minor elevator deflections.

 

What happened to the CRj on my system? After i pitched up (50% elevator) and released the elevator quickly the plane entered a large amplitude phugoid with a VSI of +1.9K FPS. The plane climed to 12,200 ft after the peak of the phugoid the VSI read -2.2K FPM and it started a descent. It settled down at 11,550 ft after which it started to climb again at about 2.0K FPM the first cycle took about 37 seconds. The plane was still oscillating after 6.00 minutes!!! with about half amplitude and half the vertical speed (1.1K) not even close to being damped out. I shut down X-Plane before it could ever return to straight and level flight - waste of time to wait more...

 

I really want to know if this is an issue with my system. I can't comprehend that a model this wrong would be sold as a payware aircraft. It is impossible to fly accurate IFR with such poor stability. This really makes me question if this model was flight tested by real CRJ 200 pilots and how involved real world pilots were in the making of this aircraft.

 

There are plenty of things i like about X-Plane, but the flight model is a major deal breaker for me. As soon as i feel the phugoid and see the VSI start to dance out of control i lose all sense of realism.

 

I currently fly Rob Young's models for Fly 2K! and they show accurate static and dynamic stability.

 

(I assume users are familiar with the concepts of static and dynamic aircraft stability. They are a little tricky to understand especially the difference between static stability and the two dynamic modes: short period and long period)

 

 

Sent from my iPad using Tapatalk HD

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There is a discussion on the similar problem in their support forum. This problem happens with autopilot on, but probably can be somehow related...

 

http://forums.x-pilo...ise-with-ap-on/

 

I have the same problem with this aircraft. It doesn't look like the developer is willing to take a look at this issue as the thread has been dead since March. I'm willing to bet 800xp has part of the answer to this problem. It may not be a simple fix though.

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Yes, I also have this problem and the plane is not usable for me because of that. However, I did not try the suggested xplane parameters changes by that user...

Still not sure if the OP here have the same issue...

 

 

 

 

 

I have the same problem with this aircraft. It doesn't look like the developer is willing to take a look at this issue as the thread has been dead since March. I'm willing to bet 800xp has part of the answer to this problem. It may not be a simple fix though.

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I have a Mac and wanted to buy the latest flight simulator.

 

Allow me...

 

http://www.flightgear.org/download/

 

Version 2.7 with the latest version of JSBSim is de out in a month or so, with it's new helicopter flightmodel, wich happens to be the same Outerra is using :-)

 

Pronto!

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As a recent poster loving the new XP10, I do realize the CRJ by Rollon is the BEST flight model of any airplane to date, at least for transport jets. Flying the Beechjet in real life, and having thousands of hours in that airplane, as well as many more as a simmer, the CRJ has made me into the XP10 advocate I am now today.

 

Too twitchy, and stability issues are XP problems... I miss FSX for the more stable feel. However, with the massive lists of plusses I see in XP10, I can live with the twitchiness and touchiness for now, hoping it will get fixed soon. Yet, the realisim of the CRJ, has me forgetting I'm in XP ... more like the FSX add-ons I have... and the smoothness, and great graphics still amazes me!

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how can the flight model be bad?

 

from the x-plane web site fairy tales:

 

"The most advanced flight simulator in the world."

 

and ...

 

"I’m speechless! There are absolutely no words that can describe the beauty in X-Plane 10. I’m an avid flight simmer and current student pilot. X-Plane 10 is the closest to reality that I’ve experienced."

 

why quote aerodynamics engineers or FAA examiners when you can quote inexperienced student pilots instead :Party:

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how can the flight model be bad?

 

from the x-plane web site fairy tales:

 

"The most advanced flight simulator in the world."

 

and ...

 

"I’m speechless! There are absolutely no words that can describe the beauty in X-Plane 10. I’m an avid flight simmer and current student pilot. X-Plane 10 is the closest to reality that I’ve experienced."

 

why quote aerodynamics engineers or FAA examiners when you can quote inexperienced student pilots instead :Party:

 

Yet again, your intelligence and expertise shines through.

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What happened to the CRj on my system? After i pitched up (50% elevator) and released the elevator quickly the plane entered a large amplitude phugoid with a VSI of +1.9K FPS. The plane climed to 12,200 ft after the peak of the phugoid the VSI read -2.2K FPM and it started a descent. It settled down at 11,550 ft after which it started to climb again at about 2.0K FPM the first cycle took about 37 seconds. The plane was still oscillating after 6.00 minutes!!! with about half amplitude and half the vertical speed (1.1K) not even close to being damped out. I shut down X-Plane before it could ever return to straight and level flight - waste of time to wait more...

 

From what I can tell (IXEG 737) the short period damping in XP is is right on the money. about 2-3 oscillations in a couple of sec.

(No use of artificial stability).

 

M

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Getting the steady state portion of a flight model is not good enough. It's 2012 and i think the user expects more

 

Honestly I think *most* users actually just want a pretty looking plane that has a lot of buttons that they can push and nice scenery.

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I know this is an old topic, but one that I find to be very important to showcase the obvious flaws still present in X-Plane FM. Not in a bad way, but to let us go to LR and ask for changes for the better. I wonder if today's X-Plane Beta still demonstrate this kind of behavior, I tried with the default aircraft, and they did, but obviously I can't test with addons since I don't own them.

 

MortenM has shown his addon doesn't fail in this test, so that's a proof that it's possible to have it modeled accurate in X-Plane. My PMDG 737NGX was very stable, I don't know if the aircraft is supposed to return to level flight after this, it didn't, but it didn't start going like a roller coaster either.

 

Again, not trying to bash X-Plane, just want to make it visible for the newer X-Plane users so that we can have it fixed in the near future!

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Alec

With regards to this...there is nothing to fix. I promise you. All the tools are there in Planemaker. The autopilot is extremely customizable (I just spent 2 weeks making sure the autopilot on my latest project works correctly without the bobbing up and down or chasing the heading marker), so to tell Laminar Research to fix it, would be like telling them to add/fix what is already there.

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Actually you're right, there's nothing to do with LR here, since MortenM has already proved he successfully designed a FM that doesn't fail the stability test. It's another of those "default aircraft that aren't made for serious flying" examples.

 

And about the autopilot, it's very important that hand flying the aircraft also demonstrate a stable and predictable flight, or we end up chasing the needles like crazy! I Wish I knew more about this, maybe I will start reading some serious material about how these factors have to be taken into consideration by the airplane manufacturers and how they do their testing. Now that I'm flying in real life I can clearly see why it's so important to be flying a stable airplane, you as a pilot just isn't able to focus all the time on the flight, there's so much to do up there, the aircraft must do some work on it's own!

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Alex,

 

I can only reinforce Goran's reply... X-Plane is becoming a huge positive surprise to me each time I try to explore it's multiple features, Plane-Maker being one of them...

 

It's a shame, but certainly not unique to X-Plane, that many nice looking aircraft have nothing to do with being nice-looking when it comes to replicating as plausibly as possible in a PC-based simulation platform, their RL counterparts. Sometimes we download "excellent" models, in any sim, but unfortunately most fall short from even having been designed with the use of RL data extracted from a simple POH...

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Let's hope more developers acknowledge these issues and start doing these kind of tests before releasing an addon. It's so important! If it were a car sim people would complain that they're starting a turn and the car starts to oversteer every time unless they turn the wheel in the other direction. Actually some cars may be setup for this, but it's a very specific condition, not one you would want to have on your daily basis driving around town.

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