Morten

XP 11 flight model [status]

59 posts in this topic

Just to clarify a bit more as this is a bit confusing to many.

The new downwash model and other new planemaker stuff inn XP v11.10 is OPTIONAL for the designer (atleast for the time being).  This is something new that I cannot remember Austin did before. The reason is that LR does not want to "break" any aircraft as always happened before in a new version. What this means is that many designers *probably* will not update their aircraft for a while, since the aircraft will need a recalibration, updated airfoils etc. so you will be flying with "old" model on those. But sooner or later those will have to be converted to take advantage of new stuff and Austin probably will soon get tired of maintaining "two" flight models.

However, we at IXEG have already converted so you will be flying the new model as of IXEG v1.21 (and old model on current v1.2),  :)

Hope that makes sense.

 

Edited by Morten
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So to see the new ground effect with 3rd party airplanes we need to wait for the developers to update them or save the planes our self's in the plane maker? 

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38 minutes ago, Denco said:

So to see the new ground effect with 3rd party airplanes we need to wait for the developers to update them or save the planes our self's in the plane maker? 

Well i guess you could save them yourself but since the flight model is not updated to match the new ground effect calculations the behavior will be  wrong/weird.

Edited by mfor
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On 1. november 2017 at 9:50 PM, mfor said:

Well i guess you could save them yourself but since the flight model is not updated to match the new ground effect calculations the behavior will be  wrong/weird.

Not recommended, you'll see behavior like in beta 4...

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We are starting to get questions about this so I thought I'd start a topic to explain and clarify how things work in XP and what you can expect ahead.

As you know all of us at IXEG are old timers and have been developing aircraft since XPv5 or so, (like 15-20 years ago) and have been through this process many times before.  Since it's been years since XP10 got released we assume a big portion of the current user base have not experienced a new XP version run change and what this actually MEANS to your favorite aircrafts.  Basically, in a new XP run, it is "tradition" for Austin to implement lots new flight model stuff.  Some new stuff, some big stuff, some minor tuning here and there of various things.  Typical ones are;

  • Thrust (engine)
  • Lift
  • Drag
  • Ground model
  • Controls
  • etc.
Also, this usually takes place throughout the entire beta phase (where we are now), and by experience will continue through to at least  XP 11.1 when things start to stabilize.  During this phase, XP11 is a moving target flight model wise, and attempts by the designers to chase austin's tail and "fix" flight model related stuff is a waist of time because chances are big things will change again in the next beta.  So designers tend to hold off for a while before we start looking at it.  THEN at some point we start looking at it, and usually we discover more errors/bugs/new stuff in the new FM, which again leads to debates with Austin and hunt for documentation. Also, it is tradition for Austin to tune stuff that he does not mention in the release notes, so there are usually a few surprises. Further, LR policy is that the designers have to prove they are right, and LR is wrong, not opposite...  So sending Austin emails like "Hey, the thrust has changed 20%, fix it!" is not going to change anything.  So basically you have 3 choices as a designer;

  • Prove to Austin you are right (takes a lot of time, knowledge, effort and documentation) 
  • Reverse engineer your own flight model (Also takes a lot of time and effort)
  • Pretend your aircraft is performing just fine, ignore user reports, and do nothing
So basically, as it looks today, all quality aircraft will have issues that needs to get fixed moving from 10 to 11.  For those aiming at the highest level of accuracy in the FM (like IXEG) designers *might* need to do a complete recalibration of the aircraft.  This is not a trivial task as you can imagine, XP's FM is far more complex than e.g. FSX

SOOOoooooo, what do we know at this point about XP11 PB8 that might/will affect us;

  • Drag has changed
  • Ground model has changed
  • Engine model has changed
As we digg deeper into XP11 we might find more things, but we'll see.

Like I said, this is perfectly normal   It is just a phase everyone has to go through and everyone needs to be patient. Kind of like moving from FSX to P3D from a designer point of view. There is no "quick fix", this is hardcore engineering and programming. The good news is we will do all this work for free, and in the mean time, we recommend using XP10 if flight model is important to you  

M

Very well stated,my friend.

Sent from my Swift 2 Plus using Tapatalk

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IMPORTANT

As I tried to explain above, as of IXEG v 1.21 the flight model is now updated to XP v 11.1.

THIS MEANS THAT YOU NEED XP 11.1 OR NEWER FOR THE IXEG v 1.21 AIRCRAFT TO PERFORM AS IN INTENDED!

So, if you use - say - XP 11.05 with IXEG v1.21, you will probably experience trim issues, aircraft nose wanting to lift off to early on takeoff etc. And possibly other strange issues. So if you for some reason wan't/need to stick with XP v 11.05 or older, stick with IXEG v 1.20.

M

Edited by Morten
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Hi Morten, Jan and all you other great developers of the B737,

I have followed the ground effect discussion for years and remember the extensive work Morten put into it years ago. I also noticed your recent comments on the ground effect and that you believe it is within the ballpark.

Not being a pilot I have no right to say what is right or wrong but within the last 50 feets, keeping a steady  descent rate and speed, the descent rate increases rapidly and there is very little elevator authority within the last 100 feet before touch down.

I have attached the data-file as a CSV (european separators). Also I have attached a chart with the last 300 feet plotting lift and drag over ft (agl). I have no knowledge of aerodynamics but a loss of lift in the range of 60.000 lb below 100 ft (agl) seems extreme, doesn't it? Had I cut the throttle I would have thought it natural but with a constant N1 of ~51 % I would have guessed it would drop a lot smoother before hitting the ground.

The gross weight at landing was 45 tons and I was aiming at a vref+5kts of 132 KIAS.

This is with IXEG B733 v1.21 and X-Plane 11.20b2.

 

Best regards,

Mikkel

flight_data.csv

ld_chart.PNG

vvI_chart.PNG

Edited by Mikkel
Extra info
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Something I noticed in XP11 (that wasn’t in XP10) is that the nose will want to pitch down when you get to around 30 ft AGL, even if the plane was in neutral trim before. If you don’t anticipate this it will lead to a hard landing.

The key I found for a smooth landing is to fly the airplane all the way to the ground. What I mean is don’t chop throttle at 50 ft as you pass the threshold, and rely on a last minute flare to arrest descent.

I flare with power around 25 ft AGL, and only after I’m established in my flare attitude do I slowly retard the throttle and modulate pitch to fly the plane to the ground. Usually I will still have power on as the wheels touch the ground, then quickly retard to idle and go to reverse thrust.

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Yes, with ?excessive? back-pressure on the yoke I can also make a good landing.

 

The above was done with N1 at ~51 % all the way to the ground (hence you see the hump in the charts), no thrust reduction, no elevator input to ease the landing etc. As such the pilot interaction is removed and what you see is the pure output of the equations from the simulator.

As an added info (will add it to the original post):

Gross weight: 45 tons.

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Yeah with that drop in VVI around 40 ft AGL I’m curious if the nose pitched down right there.

Nice job with the data btw. Do you also have the corresponding pitch (or even better AOA), and IAS data as well? As you probably know those are the determinants of lift.

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Yes the nose dropped quite significantly. The numbers in the data seems odd, though. The lift is on quite a high scale.

Attached is a new dataset where I try (with questionable success) to keep the pitch (not AOA) steady. It is recorded with more datapoints to give a more precise analysis. The clock is too many for me to look into it. Will do tomorrow. Oh what the heck: Attached a coulple of extracts from the data. I am curious about the required amount of elevator input to keep the pitch.

flight_data_w_AOA.csv

wAOA_elev_pitch.PNG

wAOA_lift_alpha_pitch.PNG

Edited by Mikkel
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Hi Mikkel,

Getting late here as well, but as you mention yourself, you need to do a constant AoA (not pitch) approach do tests in ground effect. Also you need to separate lift, drag and pitching moments as they are all are different effects.  Jan did a lot of tests and was OK with what we ended up with in XP 11.1.  AoA will decrease as you get close to ground, so if you do it right you will need to trim up all the way down to maintain the AoA, and in theory you will make a very smooth landing in the end :)

We did a lot of testing, and so did others, so I don't expect you to find anything... . The pitching moment is a tad on the high side, but austin disagreed.

M

Edited by Morten
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Almost all my landings are hard. Even greasing it is hard to achieve. There's definitely something wrong with the FM during flare.

All the heavies in XP 11.10 seems to be affected by this "suck down" during flare, so I don't know who's the fault it is, but it's very unrealistic.

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Can Jan explain how to flare properly then? Pilots in youtube videos recommend to initiate flare at 30 ft AGL, but with latest update plane starts to madly pitch down its nose at 50 ft AGL. I have to pull my yoke (Saitek Pro Flight) all the way to compensate it but still am landing hard. Full movement of yoke is not sufficient to compensate ground effect. Can it be real?

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I am really not sure what the problem is for you folks experiencing lack of elevator control or "sucking down" effects - make sure that you have upgraded to the latest version of the IXEG 737 (1.21) and the latest version of XP11.

I have made a little video to show that things work perfectly fine on my end - with plenty of elevator authority remaining during landing.

 

Cheers, Jan

 

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Just my humble .02 cents worth and I will say this, I had a real and current 733 pilot from the UK in the sim and he noticed the exact same thing.  Bottom falls out at 50ft and we have to bury the yoke in our gut to save it.  You will never notice it using a joystick, guarantee it. ;)     I have my yoke set to boeing specs on travel and it is impossible to land realistically with real control movements.   I have fooled it temporarily by limiting amount of aft control movement in the calibration setup within x-plane and it is much closer to normal now but still not "right"   All of us need to remember that control set-up, plane set-up and x-plane set-up, all have an affect on our perception of feel.  But when I saw the description of the other poster's I had to chime in to confirm the presumed issue.  It is there but why and how I have no clue. I just know it is different from XP10 version and Pre-11.11 i believe.   

Rob

Edited by 737NUT
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3 hours ago, Shanwick said:

No problems with touch downs here either. I can grease every landing if runway length and exits allow.

Landing in New York:

https://youtu.be/hEtzHV3Upyo?t=20m50s

Here are some touch and go:

https://youtu.be/cN0zAk66GbQ?t=6m50s

 

 

Watched the NY vid, you dove it down the last few hundred feet,  What was your GW?  You didn't have much fuel so I will guess around 104K lbs,  if so your airspeed was way high at what looked to be 145, Blury so hard to tell exact numbers.   If I fly it in that manner it is easier to flare and hold since it is coming in Hot.   I also bet you are using a joystick ;)   It's ok, just an observation    

Rob

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Rob, what you are experiencing is an intentional nose pitch down starting about 60' - which will also happen in real. This is a result of of XP's new downwash model. The effect is a *tad* overdone in our opinion, but we could not convince austin about it at the time. We might adjust this ourself in a later update if austin sticks with it.

 

Cm3.jpg

Edited by Morten
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Thanks Morten and I agree it started with the update Austin did in XP11.  I put a stupid amount of hours on my sim weekly so i probably notice more then most and can tell when something has changed.  I think my setup allows me to feel or sense things in a different way as i don't use much automation on my flights.  :)   /W and OEM for the win.

Rob

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1 hour ago, 737NUT said:

Watched the NY vid, you dove it down the last few hundred feet,  What was your GW?  You didn't have much fuel so I will guess around 104K lbs,  if so your airspeed was way high at what looked to be 145, Blury so hard to tell exact numbers.   If I fly it in that manner it is easier to flare and hold since it is coming in Hot.   I also bet you are using a joystick ;)   It's ok, just an observation    

Rob

Yes there was some diving involved plus a bit too fast (6 knots too fast), but not a problem since it was visual. Speed was just right before touch down. ZFW was 104.9k lbs + 8.2K lbs of fuel, which gave me an approach speed of 139 knots (+5 knots included). I see all kinds of airliners diving a bit on approach almost on a daily basis when they approach visually. Not a good idea on an precision approach of course.

Edited by Shanwick
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Just so you know i wasn't knocking your approach as Lord knows most of mine are not any better. LOL  But your approach helps mask the issue of the XP11 issues near the ground.

 

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3 hours ago, 737NUT said:

Just so you know i wasn't knocking your approach as Lord knows most of mine are not any better. LOL  But your approach helps mask the issue of the XP11 issues near the ground.

 

No worries. I just don't think that there is something to mask.

Check out the landing below. 140 knots approach speed calculated by the FMC with plenty of elevator throw/authority left. Latest x plane and IXEG 737 release. Video was recorded a little over two weeks ago.

https://youtu.be/cN0zAk66GbQ?t=22m24s

 

Edited by Shanwick
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Also note that if you have XP's pitch control sensitivity setting set to a HIGH % (more non-linear), you will need to input MORE elevator to counter the pitch down tendency in ground effect!

Our recommended setting is 25%.

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Hi Morten, do you care to show your sensitivity options in X-plane? I have a pretty good setup now (using a heavy joystick), which works great doing smooth turns at higher altitude, but I haven't been able to find a good setup that also works during the landing phase. The reason I ask is that trying out different sensitivities will take time to learn by "muscle memory", but if you already have a recommendation, that would help tremendously :)

 

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