Morten

XP 11 flight model [status]

30 posts in this topic

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

Edited by Morten
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I agree Tom,

Keep up the good work indeed

It's nice to be kept in the loop (up to date) as to what's going on

I personally think posts like this help products in the long run and the short term for that matter, set your stall out to speak for everyone to see

No doubt it'll be worth the wait

All The Best

Good Luck

Have Fun

Tony

 

 

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indeed, thanks a lot for taking the time to explain. I haven't deleted good ol'10 yet...

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Well said..it certainly opened my eyes to what is entailed..in which I never really knew..I have been simming since Microsoft way back when..and this time around I really wanted
a sim that actually mimicks true aircraft dynamics and challenges. Your aircraft is just simply amazing and so well thought out ..and the artwork is just amazing. Functionality will always be moving forward and getting better all the time..so to sum it up..My hats off to all of you..for such a great aircraft and to LR for giving us a serious sim platform to work from.

 

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Let's keep everything in perspective.

There are a lot more important things in this world that we need to be concerned about.

Thank you Morten and company for XP 10/11 and your continuing efforts.

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It's good to read people who think and do their job responsibly.
Many thanks "Morten".

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Thanks Morten for wiping out any false expectations on easy porting to XP11. This is professional. Keep up the good work.

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I'm on Beta 9..Flying 737 without any complains. Feels real enough for me, but of course I'm looking forward for tweaks and adoption of XP11 at some point of time.

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Also, in the wake of many developers that seem content to charge full price for XP11 updates, having one that is reasonably priced (let alone free as with this) is a breath of fresh air. Thank you for being transparent about what is necessary and I look forward to 1.1.

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Thanks for the notification, i'll be enjoying the fslabs on p3D in the meanwhile :)

Envoyé de mon iPhone en utilisant Tapatalk

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Just an example of what I was talking about "moving target". As some might have noticed in the beta 10 release notes;

  • More flight model and AI ground truck improvements.

 

Not alot of info to work on, but we happen to know this time its is about fuselage/body drag.

(A bug we discovered 6 years ago that is fixed on our v10 aircraft ;) )

 

M

 

Edited by Morten
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I noticed that that now trimming even just a tap makes huge pitch correction cant remember if it was the same in XP10

Also it seems that the pitch down moment during ground effect is way too overdone, felt like i needed to hold full backpressure to try keeing the pitch attitude , slamming the runway very hard each time now lol.

Also, im surprised how sensitive this aircraft is, only a 10deg defkection ob the yoke makes a very fast roll rate ( feel like an a320 or even more sensible, i can ecall that thos was the same in XP10

Envoyé de mon iPhone en utilisant Tapatalk

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Yea, Austin usually breaks a few things while implementing new flight model stuff...nothing new there -_-

Report all your FM findings to austin(at)x-plane.com. Remember, use the default aircraft as a reference when reporting flight model issues.

M

 

 

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Yea, Austin usually breaks a few things while implementing new flight model stuff...nothing new there 

Report all your FM findings to austin(at)x-plane.com. Remember, use the default aircraft as a reference when reporting flight model issues.

M

 

 

I was referring to the IXEG lol, that's the only aircraft worth flying on xplane, haven't flown anything else so far

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1 minute ago, cmbaviator said:

I was referring to the IXEG lol, that's the only aircraft worth flying on xplane, haven't flown anything else so far

hehe.. well, he'll just blame the aircraft designer if you refer to something else than the default aircraft :P

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hehe.. well, he'll just blame the aircraft designer if you refer to something else than the default aircraft 

I'll wait until you make the b733 officially compatible with xp11

Envoyé de mon iPad en utilisant Tapatalk

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Looks like Austin is still very busy updating the FM.  Massive update to the subsonic jet engine model in XP11 just announced!  As you know, we already have our own custom very accurate CFM-56-3 jet engine model.  Lets cross fingers that he hasn't messed it up for us...

 

Quote
Now, for X-Plane 11, I’ve overhauled the SUBSONIC engine performance as well, based on physical laws instead of curve-fitting, and the result is rather interesting.
 
SOOO, if you would like to learn about the SUBSONIC jet engine model in X-Plane 11, then read on!
 
( and, to get the latest on ALL my X-Plane 11 flight-model blogging, go here:
 
So here is a thing about jets: They get thrust based on the amount that the ACCELERATE the air coming out the back of the engine.
Here is the problem, though: All non-supersonic jet engines can only accelerate the air up to near the speed of sound… a little bit under Mach-1.
So, as the aircraft accelerates, the thrust of the engine deteriorates because as the INCOMING speed gets closer to the OUTLET speed, the thrust goes approaches zero!
So jets LOSE thrust as they accelerate, since the speed differential between input and output deteriorates… the jet can’t air accelerate the air as much since it already moving!
Now, if this were all there was to it, then jets would become pretty useless around 300 knots, like propellers are.
BUT, jets have a trick up their sleeve: Inlets. The INLET to the jet engine can SLOW the air and PRESSURIZE it, carefully using ram-air effect, to deliver slow-moving, high-density air to the engine!
So it’s all about the inlet slowing and pressurizing the air.
And now X-Plane does that starting with version 11, and here is how you control all that in Plane-Maker as you design YOUR jet airplane!
Fire up Plane-Maker 11 and go to the ENGINES window, and then the JET CURVES tab.
Though JET CURVES sounds like a really really bad James-Bond spinoff movie, what it ACTUALLY is is the thrust that the jet engine will put out in X-Plane, and when you understand the text above
you will see why the curves are shaped the way they are in Plane-Maker… go to that window right now so you see the curves as you continue reading.
First you see the many curves for many altitudes, and of course the thrust at higher altitudes is lower since the air is thinner.
Now, look at the left side of the curve to see the thrust at it’s maximum.
Then as you speed up, the thrust deteriorates. OOPS! That is the incoming speed building up, so the difference between entry and exit speed (the thrust!) falling apart.
BUT WAIT, THERE’S MORE! As you speed up towards Mach-1, you see the falloff start to ease, and then the thrust actually INCREASES! YAH! That is the INLET doing its’ part to capture the
RAM-air effect and deliver some nice high-density air to the engine compressor.
Look at the lower left where you enter the bypass ratio. Higher bypass ratios are more affected by the speed-based falloff since they have slower exhaust.
Then look at the inlet compression efficiency: This is how much of the energy is kept as the air is slowed and compressed by the RAM-air effect. Usually, most of it, for a properly-designed inlet.
Then see the normal-shock Mach number. This is the speed at which normal shocks start to form across the inlet, and efficiency is lost. An inlet designed for subsonic flight is likely to have a value of a bit less than 1.0 here,
since it is not designed to accept air near or above the speed of sound where those shock-waves start forming.
Only a supersonic airplane is likely to have a Mach number greater than 1 here!
As the inlet is dragged by an over-speeding airplane above it’s critical Mach number, normal shocks will now form across the inlet, DECIMATING the efficiency of the engine and robbing you of thrust.
The normal shock, only a few atoms thick, slows all air that hits it across the space of a few atoms, dumping a huge amount of the incoming streams valuable kinetic energy and turning it instead into HEAT.. the last thing you want coming into the front of your engine.
 
 
Also we don’t JUST want the engine to work properly at idle and full thrust… we want all the N1 settings in between!
SOOO I got a local charter company to run their LearJet from minimum-loiter speed all the way up to redline speed in steps, carefully measuring the N1 and speed at each step.
Then, in case they made any mistakes, do the whole thing all over again SLOWING DOWN in steps, from redline down to loiter.
By carefully looking at the speed associated with each N1 value, I mapped the thrust as a function of N1… this is the thrust CURVE, indicating what FRACTION of your max thrust you get at each N1 setting.
The result?
In flight, a jet engine like the ones found on the Lear-35, you get about 50% of your thrust at about 82% of your N1.
At an N1 of 50%, you only get about 9% of your max thrust!
So armed with that information, as well as thrust-versus-N1 curves for a number of engines, I found the following:
At very LOW speeds (like for idle and taxi) the thrust from the engine goes with the SQUARE of N1. This is not at all surprising, since DYNAMIC PRESSURE goes with the SQUARE of the speed.
And of course the thrust is scaling with the dynamic pressure on the system. So if you run the fan twice as fast, you get four times the thrust. 
But, at high speed, something DIFFERENT starts happening: The thrust starts running with the CUBE of the N1!
This must have something to do with the way the engine is only operating in its’ performance envelope as it approaches 100% N1, though I welcome a more technical evaluation on this from any reader.
Now, while I found from experimentation that the power curve of thrust versus N1 is about SQUARED for low N1, and CUBED for high N1, there will surely be SOME variation across engine type.
So, in Plane-Maker, ENGINE window, DESCRIPTION tab, right side of the screen, we have “thrust power curves with N1”, which you can enter.
We DEFAULT to 2 at low N1 and 3 at high N1 (with a smooth interpolation in between of course) but you can tweak these numbers as desired if you want to really dial in the thrust across a wide range of power settings!
 
So, jet simulation has been improved now for V11, especially in the supersonic regime… because getting that F-4 PERFECT is just going to be soooooooo cool!
 

 

Edited by Morten
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Status report.

The initial inquiries into XP11 are GOOD news :) 

  • Our CFM56-3 engine model has survived Austins major engine rewrite in XP11
  • The new drag model has not messed up a lot and we are now back in the ballpark
  • We have a fix in for the ground model

So, basically, what we need to do now on the FM side is more extensive testing and fine-tuning and check other non-announced areas for changes.  Unless Austin makes more changes that will set us back, things are looking good in terms of progress :)

M

 

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Really a great news....can't wait to fly IXEG 737 in XP-11 in all its glory

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OK. time for a heads up.  I'll start by quoting myself from almost a year ago;

Quote

Also, this usually takes place throughout the entire beta phase (where we are now), and by experience will continue through to at least  XP 11.10 when things start to stabilize.

So, things are developing as we expected :)

Those of you that are following the 11.10 beta know that Austin now has implemented DOWNWASH.  Downwash is simply put air that is deflected downwards behind the wing and is one of the factors that produce lift.  So behind the wing, the air flows at a downwards angle. HOWEVER, the downwash area is not  just limited to the area just behind and above the wing, and I bet most of you do not know the magnitude and forces that are in play. Have a look at this photo (Below) I took a while back.  You can see (on the very dirty A321) how the air moves around the wing, puts things in perspective. Upwash (in front) and downwash (behind).

So if you thought the air just rushes by your passenger window horizontally, you were wrong. As you can see, the direction of the air even far above and away from the wing is not horizontal.  This downwash area also extends all the way back to the tail and (can) hit the horizontal stabilizer.

And this is where things get complicated.  The downwash hitting the tail will cause an UP pitching moment.  How much depends on the aircraft geometry.  An aircraft with a high tail like an MD80 will hardly notice this, while a 737 will notice some even though the tail is 2-3 meters above the wing. So this affects normal flight.

ALSO, when an aircraft is landing, it enters what we call ground effect (GE). Many believe GE is one effect, but there are atleast 3 that we need to take into account.  One of them is affected by downwash and change in pitching moment.   When an aircraft approaches ground (less than 1 span) altitude, the downwash angle will get more shallow.  This causes less downforce on the tail and you will get a nose DOWN tendency which will affect your landing. Also, it will affect your takeoff where the opposite happens. This is very complex stuff to put it mildly.

So what is going on right now? We are working with Austin providing documentation and tests to get the magnitude of these effects right.  It also means that we will need to recalibrate the 737 to get it back to where it was.

The good news is that Austin now has implemented the designers need to save the aircraft for the new effects to take effect.  In other words the 737 will perform as expected until we release an update with the recalibration.  So nothing for you guy's to worry about. We do NOT recommend you open the aircraft and save it yourself. Most likely you will be out of trim in cruise and overpitch on takeoff etc.

M

 

upwash.jpg

 

Edited by Morten
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Thanks Morten for the interesting topic and for providing updates and feedback on this subject

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