Beta 4 of X-Plane 12 Early Access is available on Steam & through the Laminar Research installer. You should be prompted to auto update the next time you launch the sim, if you haven’t already. Here is a link to Early Access release notes.
This build contains multiple crash fixes, as well as our first go at fixing the magenta (aka pink or purple) sadness. X-Plane renders magenta when it hits a numeric error (a NaN value) inside the rendering engine. There may be multiple causes of NaNs – it’s not all one bug because magenta is a symptom, not a cause, and each bug has to be squashed one by one.
Don’t be surprised if some users see less magenta in the this beta and others do not. If you still see it, please file (or re-file) a bug that includes a screenshot or two of where you see it, and any steps needed to reproduce it.
We’ll start with the wings, which I’ve found to be very important:
The entire flight model is based on a new type of memory-access that is fast for the product you get, but slower and constantly self-checking for the internal builds we run for testing. We now have vectors of props, wings, and bodies… and access those vectors by accessors that hop right through to the memory for speed in the delivered sim, but do bounds-checking first to make sure no illegal accesses are even REQUESTED in our internal self-check builds. This entire new architecture, coded by me in the first month or so of the virus – when I could not even leave my house – sets us up with a platform that is flexible, fast, and bullet-proof to use. This new architecture allows for up to 16 engines and props, which is useful for the new generation of eVTOLs, many of which have more than 8 motors and props!
Now, for these wings and prop blades, we now allow THREE airfoil files per wing, not two like we used to, so you can have root, middle, and tip airfoils, which is especially useful for propellers! Well-modeled props have thick airfoil files at the root, mid airfoil files at the mid-span, and then go to a very thin foil right out at the tip to delay shock-wave formation at high speed. We now allow all the Reynolds numbers you like for each foil, so the way to get variation with Reynolds numbers is to put them in the airfoil files in Airfoil-Maker.. we don’t have multiple slots for different Reynolds numbers in Plane-Maker any more: that was always limited and awkward: It’s much better to save all your data for different Reynolds numbers for your airfoils in Airfoil-Maker, so that’s what we do now.
So everything you are about to read about is based on a new, high-speed, self-checking memory-layout that allows 16 engines and props, and 3 airfoils per flying surface.
First off: Wing sweep improvements. As air approaches a wing, it has to SPEED UP to get out of the way of the metal. It has to speed up to go around the wing! As a wing approaches the speed of sound, therefore, the air near the wing (still speeding up!) must EXCEED the speed of sound to get out of the way of the wing fast enough! In other words, the air around the wing goes supersonic even when the wing itself is still below the speed of sound. This causes shock waves, huge drag, and even loss of lift. The thicker the wing, the more the air has to accelerate to get around it, and the greater this effect. Nobody wants all this extra drag as they fly at airliner speeds (which DO approach the speed of sound) so in World War 2 the Germans came up with a way to cheat: Sweep the wing!
When the wing is swept back, the airfoil SEEMS THINNER to the air. That means that the air is in less of a rush to get out of the way: That localized supersonic flow, and shock waves and drag that result, are delayed! You can go faster before you run into these shock waves. But here’s the thing: You can only cheat so much! No matter how swept the wing is, once your airplane is going Mach 1.00, you have fully supersonic flow over every bit of that wing: You can’t escape supersonic flow with wing sweep! Wing sweep only makes the wing seem THINNER, letting the air accelerate LESS get around it, letting you get CLOSER to Mach 1.0 before you see supersonic flow over the wing! The thinnest, most highly-swept wing in the world will not AVOID supersonic flow, it will just let you get much CLOSER to Mach 1.0 before you get supersonic flow, and drag. X-Plane 12 now understands all of this, and invokes transonic drag at the right time based on wing-sweep, and transitions to fully supersonic flow by the time the aircraft Mach Number hits 1.0. It’s a nice interpolation from the subsonic to supersonic flow models as the flow goes from transonic to fully sonic on the wing! Back in X-Plane 11, the simulator delayed supersonic flow based on wing sweep until some value PAST Mach 1.0, which was erroneous! Forget that! So here is what version 11 did wrong: X-Plane 11 simply imagined that the effective airflow over the wing was multiplied by the cosine of the wing sweep. In other words, X-Plane 11 though that if you had wing sweep, that meant that you got out of challenging the air head-on, and it reduced the effective airspeed over the wing since the wing was getting out of a direct confrontation with the air by sliding through it sideways. This was so close, but not as good as it could have been.
To REALLY get it right, we need to understand that as you approach Mach 1, we do NOT imply REDUCE THE SPEED over the wing by the cosine of the wing sweep: Instead, we are making the AIRFOIL THINNER by the cosine of the wing sweep. Mach-1 flow hitting the wing is still Mach-1 flow over the wing! Wing sweep only makes the wing appear thinner to the arriving airflow.. it does not actually reduce the speed over the wing. This is really subtle, but matters a lot for transonic drag (drag from the formation of shock waves as we APPROACH Mach 1), so we will see more accurate formation of shock waves and resulting rise in transonic drag in X-Plane 12 than we did in version 11: Make sure your wing sweep is entered correctly, make sure you airfoil thickness is entered correctly in Plane-Maker, and expect drag rise as you approach Mach 1. Thinner, more-swept airfoils will be called for, the closer you get to Mach 1. The real Citation X (and the simulated Citation X in X-Plane, of course) are perfect examples of how important these two factors are in approaching Mach 1: The wing of that airplane is very very thin and very highly swept.
New tire rolling coefficient of friction data:
Observe these charts:
As you see above, rolling coeffs go from about 0.006 to 0.01 for high-pressure tires, and 0.01 to 0.02 for more medium pressure tires.
So your planes should probably use 0.008 for the airliners and fighters, And about 0.016 for the general aviation airplanes, And about 0.040 for the piper.
Everyone that enters airplanes themselves should go to Plane-Maker and set tire coefficient of fric to about 0.008 for airliners to 0.01 for lower pressure airliners to 0.02 for light planes to 0.04 for very low pressure tires like the tundra tires on the Piper Cub.
Hydroplane speed more accurate, and on-wet friction a bit higher.
Water: Improved floatplane dynamics!
I’ve been working with Bridger Aerospace to get their fire-fighting seaplanes simulated, and the result is really incredible. It took about FIFTY different alpha builds, each one with further-refined water dynamics, but we finally have the parasite drag, wave drag, plowing, step-taxiing, running on the step, wave interaction, and even drag from the scoops on the firefighters that grab water all simulated properly for Bridger to train their next generation of pilots! So the float-plane dynamics in X-Plane 12 are now professional-grade, for people that fly float-planes to put out fires for a living!
In addition to the dynamics of the floats themselves, we have better rendered wake, wind and wave correlation, and even docking with hard docks!
One subtlety is the new wave height paradigm:
Enter the wind and stuff, and look at the wave height slider: You see the range of heights as a little tip at the bottom, so you can drag the wave height slider accordingly for the conditions you want to fly… Then, at right you see the little tip on the wavelength and speed that results from the height entered We are doing first-principles with wave length and speed, and they match up with your chart pretty close, so I think we are good there.
For real-weather, the wave height is auto-set to the SHELTERED case, based on the lowest-alt winds So I think that has us where we want to be, with plenty of little notes there in the UI to show us what is going on… and give instructors the ability to set conditions as desired.. with the notes on the defaults available to them of course
OK I like this paradigm a lot: Total flexibility, with super-easy normal-case references available at-a-glance.
Corrected spoiler dynamics, very noticeable on airliners and the F-14!
This is big: The spoiler effects were TOO WEAK in version 11. This meant that in the sim, all the planes that used roll spoilers, which certainly includes airliners, rolled too slowly!
As a result, many people over-sized their spoilers to get the desired roll-rate… and got too much DRAG from those over-sized spoilers as a result!
This has now been fixed for X-Plane 12!
NOTE: Aircraft designers should now go into their aircraft and make sure that they have entered the correct sizes for their spoilers.. they should now get more-accurate results!
Beta 4 is currently in the works, going through test so we can release it next week. We expect to have our first go at fixing the magenta (aka pink or purple) sadness in that release, so please hold off on filing any further bug reports on this until you can try beta 4.
And this just in: X-Plane 12 for Steam is now available!
Well, this has been a crazy couple of weeks. X-Plane 12.00 is now available for Early Access – in other words, everyone can get X-Plane 12. Over the next few weeks we will post more about ongoing development and get into some of the new features in depth – there’s a ton to talk about in X-Plane 12. For today, here are just a few notes on some issues that have come up over the last few days.
X-Plane 12 has been in a private alpha test program with third parties since December (!) – almost nine months. During that time we built 38 (!) official alpha builds, recut the global scenery five times, and committed over 4000 checkins to X-Plane’s source code (plus more to the aircraft, scenery tools and art libraries). The alpha program included completion of major features, lots of debugging, and changing the product in response to early alpha feedback.
So why Early Access now? Not because X-Plane 12.0 is done – we still have over two hundred open bugs and a lot of things we want to do. X-Plane 12 is in Early Access so that the entire X-Plane community can be involved in X-Plane 12’s growth, not just a limited number of testers.
With X-Plane 12.0 in early access, we don’t have to say ‘no’ to users and devs who want to get started with 12, and third parties can get their entire teams using the new sim and run their own test programs.
(We can also finally open up our developer relations program to a wider audience.)
Major Areas of Work
Here are some of the major areas of work ahead of us:
Clouds – we are working on the shaping and quality of clouds, improving resolution, fixing artifacts, and improving performance. Clouds are probably the single most expensive part of the renderer, so they are a constant tug-of-war between quality and speed.
Lighting – there are quite a few lighting and atmospheric scattering bugs that affect the sim, as well as work to do improving auto-exposure and tone mapping.
Philipp is working on an airbus MCDU, which we expect to ship during early access.
Third party interfaces – we have a few new SDK and authoring features that are mostly completed that will ship during early access. The elephant in the room is third party access to the weather system.
That’s One Blurry Airbus
X-Plane 12 moves some work that used to be on the CPU to the GPU (looking at you, ocean waves!), and virtually all new computing work in X-Plane 12 is on the GPU. When we discussed this before Early Access, there was a lot of teeth gnashing. “You’re gonna use more GPU power? I can’t buy a 3080, I’d have to sell my kidney!”
We still have a lot of GPU optimization left to do, but we also spent some time before beta 1 working on performance, particularly at intermediate settings. User with high end hardware have been pleasantly surprised to see production-level FPS in beta 1, and a common request is “I have 60 fps and blurry clouds, can I get a higher max setting.”
(I do suspect there is a huge gulf between the haves and have-nots for GPU power – because there’s a huge range of hardware performance amongst our users. We will keep optimizing.)
What we didn’t optimize was VRAM use, and this is why blurry textures is a common problem with the first beta. X-Plane 12 uses Vulkan/Metal as its renderer, always, so it uses our Vulkan/Metal memory management strategy: we dynamically bring the resolution of textures down to fit within your available VRAM, with some guessing as to which textures are most important.
The texture slider in the UI sets the maximum texture resolution X-Plane will attempt – if you have a card without a lot of VRAM, setting this lower can help avoid “thrash” as X-Plane tries to fit 4 GB of textures into 2 GB of VRAM. But X-Plane will further lower res until it fits – X-Plane will not use system memory as backup texture memory, nor will it slow the framerate and stutter by shuffling textures between vRAM and system memory on the fly.
I’m afraid I don’t have any useful information about how much VRAM will get you a better experience – we’re going to do an optimization pass and see what we can tighten up.
I suspect the big driver of VRAM is memory used for effects – X-Plane 12 has HDR always on, but also has extra VRAM reserved for screen space reflections, 3-d water, dynamic weather effects, clouds, etc.
One thing that can help (and I know no one wants to hear this) is to run at a lower resolution. The sim has to internally use VRAM proportional to the size of the winddow or monitor res you fly at. Jumping from 1080p to 4K doubles the resolution in each dimension (making each pixel half as big) but uses 4x the VRAM for surfaces. Full screen anti-aliasing increases VRAM by its factor (4x MSAA = 4x VRAM) for some of those textures, so it’s more efficient than higher res.
What’s All This Magenta
X-Plane renders magenta when it hits a numeric error (a NaN value) inside the rendering engine. Right now there are multiple causes of NaNs – it’s not all one bug because magenta is a symptom, not a cause. A few we know about:
We believe there’s some kind of problem specific to the GeForce 900 series. Sidney bought one on eBay so we can debug this.
I’ve seen NaNs caused by the traffic debugging lines for ATC – I suspect that particular shader has a bug.
We can sometimes get NaNs from the past frame – they get “reflected” by SSR and propagate from one part of the frame to the other.
There’s no easy answer here – each bug has to be squashed one by one. These are high priority bugs and we’re working on them now – hopefully each fix will make things a bit better, but don’t be surprised if some users see less magenta in the next beta and others do not.
Fuzzy Scuzzy Rendering
FSR stands for FidelityFX™ Super Resolution. FSR is AMD’s free open source up-scaling technology. The idea of up-scalers is:
Lots of people have 4K monitors.
Not as many people have GPUs that can run games and simulators at 4K – they’re expensive.
Upscaling a 2K image with a little bit of smarts uses a little bit of GPU and looks a lot better than just running the monitor at low resolution.
When you move the FSR slider to the left, X-Plane renders its 3-d image at a lower resolution and then upscales it to the monitor. This saves GPU time and VRAM at a cost of image quality. The image should look better than running at low resolution but not as good as running at high resolution.
Should you use FSR? I would only recommend using FSR if you want to/need to run at 4K and your GPU is struggling. Support has had a number of complaints about blurry rendering from users with FSR on – FSR is resulting in a less detailed image on purpose just like reducing resolution does. If you are going to use FSR, use full screen anti-aliasing – it helps.
We are still undecided about the future of FSR in the simulator. We added the option of up-scaling based on user requests, and if we didn’t ask for it, we’d probably be asked for it. But we’ve also had lots of “I set this slider low and now everything looks terrible.”
(Why don’t we use FSR2 or DLSS? Both of these upscalers require motion vectors as inputs from the rendering engine, something X-Plane does not provide. We may support them in the future, but adding motion vector generation is not trivial.)
Beta 3 Coming Soon
Over the past weekend part of the team met in person to do planning and roadmapping; beta 3 should be available shortly, with some of the bug fixes we’ve already coded. X-Plane 12 for Steam is in review — hopefully it will be available Real Soon Now™.
Our next milestone is the Early Access release of X-Plane 12. As we get closer to this public stage, we wanted to share a quick look into where we are in the development process, what’s being worked on currently, and what else needs to be completed before we release.
During Early Access:
Everyone will be able to purchase X-Plane 12.
X-Plane 12 will be available for purchase as a digital key for $59.99, just like always.
During Early Access, X-Plane 12 digital keys will include access to X-Plane 11 as well.
DVD copies of X-Plane 12 will be available later on, once X-Plane 12 is out of Early Access. These will be available from X-Plane.com in North America and by Aerosoft in Europe.
Early Access will also be available on Steam.
In Final Testing
Austin is working on a blog post now to provide an overview of the changes that have been made for X-Plane 12.
World Features/Global Scenery
Rain/Snow shaders for airports and global scenery
Complete data import/finish scenery tile generation.
We import piles and piles of data and then use it to create our global scenery. Due to the sheer size of this data, the import process alone takes several days .
Improve performance of 3D trees.
Tune coastlines/beaches embedded new scenery tiles.
Boat/Seaplane 3D wake drawing.
Complete ortho photo compatibility.
We know that users have all sorts of ortho photo packs that they are using currently in X-Plane 11. Our goal is to maintain compatibility as much as possible.
Finish implementing geographically-aware water colors.
Fully animated jetways.
Modular tower building kit for Gateway artists.
New ground vehicles with updated driving logic.
FMOD sounds for ground vehicles.
Dozens of new static aircraft + new airline/airport based logic for livery selection.
New (all electric ⚡️) passenger buses
R22 (skids/mariner versions)
While this aircraft is ready for an initial release, there are some additional features that we are still working on – specifically a custom Airbus FMS.
To-Do: Finish liveries and finalize autopilot behavior/cockpit details.
Largely complete – including fresh data sources for real weather, new logic, new rain/snow accumulation/windshield effects, new volumetric 3D clouds, etc.
Fix a handful of bugs in the volumetric cloud system.
Ensure weather continuity between networked simulators.
Here’s a quick update on X-Plane 12 to give you an idea of where we are, what’s being worked on right now and what the next steps are. At the end of last year, we started sharing early, rough builds of X-Plane 12 with a private alpha group, mainly 3rd party developers and subject matter experts. Everyone in the alpha program has signed a non-disclosure agreement, so please do not ask them to share X-Plane 12 or technical details. (The alpha program is also ful, please don’t email us for access… sorry.)
Here’s a snapshot of some of the development tasks we’re working on right now.
Default Fleet Updates
Rain/Snow on Runways
Prop Physics Updates
Anti-Aliasing and FSR
Recut Default DSFs
Third Party APIs
There’s a lot more that is already completed, too – this is just a window into our current work. Once we complete all of the open development tasks, there are still plenty of bugs to fix.
At some point during this bug fixing process, we will transition from the closed private alpha to an open “Early Access” beta program. Once we make this transition, anyone who wishes to participate in Early Access will be able to purchase a license of X-Plane 12 and use the new sim right away.
Here are some details on our recent development work.
Light Up the Night
X-Plane 12’s lighting engine is completely photometric and runs in true HDR at all times. This includes updates to how we do night lighting and artificial light sources. We are finishing up a very careful pass over a wide variety of light sources – urban lights, street lights, and most importantly light sources that affect pilots (e.g. approach lights, runway edge lights, PAPIs, etc.). The intensities of these lights are calibrated using spec sheets from the FAA.
The new lighting engine also requires us to take a new approach to low visibility flying conditions. Low visibility daytime conditions in X-Plane 12 are naturally darker than sunny days, but also still lighter than night time flying. Low visibility isn’t just about making X-Plane look nice – X-Plane is used as a training simulator, so we need to make sure that visibility is limited by just the right amount to train for instrument approaches.
To solve this problem, our art director Alex Unruh built…a monolith.
The monolith is a calibration target for tuning the simulator – based on a certain position on the runway and the right visibility settings, the monolith will be just barely visible. The monolith is surrounded by runway lights so we can make sure the approach and edge lights respond to fog appropriately as well.
Water, Water Everywhere!
X-Plane 12’s water is 3D. Not only does this make ocean waves more realistic, but this 3D water interacts with the flight model. Austin has worked closely with seaplane pilots during alpha testing to dial in seaplane behavior.
A new feature in a simulator can create new bugs that need to be fixed – this is why we invited our third party developers to try out the simulator early in the process. For example: When we made the water waves 3D, they started sticking up through orthophoto scenery. (In X-Plane 11, orthophotos just “paint over” the 2D water.) Last week we implemented water masking, so that orthophoto scenery can cut out the 3D water to avoid these bugs.
X-Plane has featured a deferred renderer for almost a decade; with X-Plane 12, deferred rendering is the exclusive mode our graphics engine runs in – this is important because it makes the new weather effects and lighting possible.
With X-Plane 12 we now support real multisample anti-aliasing (MSAA) with our deferred renderer. In some versions of X-Plane, MSAA wasn’t compatible with deferred rendering, and the AA options were FXAA and SSAA. This was frustrating to users because SSAA severely hurts framerate. If your GPU is maxed out, 4x SSAA will typically cut your framerate by …. 4x.
The new MSAA code path should be much more efficient. We are also implementing AMD’s FidelityFX Super Resolution (FSR). FSR lets us render the world faster and then scale the result up to 4K – it’s a great option for users who want to fly in 4K but keep their framerates up. You can read more about FSR here.
Supporting Third Parties
With the new major version we are making some changes to the plugin environment:
The X-Plane SDK supports the new ARM M1 Macs.
We are removing OpenAL from X-Plane. X-Plane itself hasn’t used OpenAL in years, and we are in no position to support it. Plugins that use OpenAL in X-Plane 12 will need to package OpenAL themselves.
We are making the FMOD API available to plugins (in a few different ways) so that plugins that generate sound can interact with the full 3-d sound environment.
In X-Plane 12 we run Chromium Embedded Framework (CEF) at startup. In X-Plane 11, the first plugin to run CEF would ‘own’ it; this new setup allows X-Plane and all plugins to share CEF and should make it easier for plugins that need to access web pages to do so.
We are building our own test plugins and working with our third party developers to make sure these new pathways have been tested experimentally.
A major focus of the private alpha is to make sure that these changes will work with third party add-ons the way we expect, so we’ve prioritized getting the changes into the alphas early so our third party developers can try them.
The next phase after private alpha will be a completely unrestricted Early Access beta program – everyone will be able to run the X-Plane 12 beta (either as a demo or with the purchase of an X-Plane 12 license).
We have some feature work to wind down with some of these third party cases and new features, and once that is done, it’s going to be bug fix, performance tune, repeat, repeat, repeat to get to Early Access.
All aircraft in the X-Plane 12 world cast a wake turbulence – a wing cutting through the air in X-Plane 12 leaves a vortex in the air that swirls inward over the wingtip, and sinks slowly as it dissipates energy over time. The strength of the vortex and its lifetime depends on the lift force generated by the wing (i.e. a wing that has to lift a 172 does not create a strong vortex, whereas a wing that supports a 747 surely does). Over the course of its life, the vortex sinks slowly and is displaced by the prevailing wind.
Flying through such a vortex can be dangerous! If you cross the vortex left by an airliner while flying a 172 yourself, be prepared to be tossed around or even flipped upside down. If you do the same with roles reversed, you might see a slight bump just enough to ripple the surface of your coffee (be sure not to do this in an A350 as spilled coffee can cause in-flight engine shutdowns).
Wakes left by AI aircraft
AI aircraft in X-Plane run the full flight model. That is, each wing is calculated using the same methods and with the same accuracy as for the user aircraft. Thus the amount of energy left in the wake vortex is clearly known, it just comes from the flight model. Therefore, if ATC clears that 747 to take off before you, be sure to stay above their flight path until you can turn away from it. For landing, stay above the preceding planes path and touch down slightly further down the runway than they did to stay safe.
Wakes left by online traffic, live traffic, and other plugins
For aircraft that are not run by the X-Plane flight model, such as other players’ aircraft from an online network, or real-world traffic injected from a plugin like Live Traffic using data from an ADS-B exchange, X-Plane makes a best effort guess based on the data provided by the plugin. The plugin can tell X-Plane how heavy the aircraft is, and its wing area and wingspan. In the absence of this data, X-Plane will fall back on a fairly conservative light aircraft estimate, assuming a Learjet-sized aircraft weighing 10 tons with a 12m wingspan. This means you are not going to get flipped upside down in your 737 if you end up flying through a wake left by an old plugin. This is to minimize user frustration with existing online flying plugins. Since the wakes are technically an extension of the TCAS override API used by plugins since X-Plane 11.50, all plugins that show traffic in X-Plane 11.50 are compatible with wake turbulence generation and will gain that base functionality automatically when used in X-Plane 12.
Wake turbulence data for plugin authors
Plugins can use new datarefs starting with X-Plane 12 to inform X-Plane of physical properties of the non-player aircraft that are then used for a more accurate strength and duration of the wake. By writing to the new datarefs, a plugin providing traffic data can upgrade from the “generic Learjet wake” to an accurate wake representative of the aircraft they are actually drawing.
Learn about wake turbulence avoidance
In X-Plane, you can cheat and make the wake left by an aircraft visible by having it drawn in the sky in a color scheme showing its danger (from red over orange and yellow down to green) so you can avoid it (or fly through it on purpose to experience the effect). Wake visualization is just one of the many graphical flight model outputs available. Press Ctrl+M to toggle graphical flight model output in X-Plane. By repeatedly pressing Ctrl-M you can cycle through all the visualizations available, while a small white label tells you what you are looking at. Keep toggling until you see “Wake Turbulence” displayed and marvel at the air disturbance waiting to make your day interesting.
You can also use X-Avion on your iPad to have wake turbulence danger zones visualized – this works in real airplanes using ADS-B data, and it works in X-Plane when driving X-Avion over network.
One of my favorite planes the F-4 Phantom. And one of our alpha testers recently casually mentioned that he was an Israeli Air Force F-4 Phantom flight instructor. I’ve been exchanging emails with him about every 30 minutes or so, on average, for 12-hour-long days, for the past 5 days or so. We have been doing DOZENS of changes of EXEs and ACFs to get the F-4 in X-Plane flying PERFECTLY.
And the success we’ve realized is beyond my wildest expectations.
So here we go: First off, thanks to this intensive, minimum-sleep, once-in-a-lifetime week, we now have an F-4 flight model that is accurate enough to be used for actual F-4 flight instruction: It flies JUST like the real plane through its’ huge flight envelope, from dirty, low-speed approaches, to high-G maneuvering, to Mach 2.2 travel at FL500, to the ceiling of FL600, with acceleration, deceleration, gliding, all representing the real plane as close as an instructor pilot can tell by flying it.
So here is the thing about the F-4: It has a HUGE speed envelope, going from 150 knots to… 1,500 knots.And a HUGE angle of attack envelope, going from zero to… 25 or 30 degrees AOA without losing control, as that huge delta wing rolls up a giant vortex to maintain low pressure above it, without stalling, up to absurdly-high angles of attack.
And it does this with almost no flight computer to speak of at all… just a rate-damper, nothing else! So, with these huge ranges of speed and angle of attack, hugely-detailed requirements for performance, maneuvering, and handling, this airplane makes the ULTIMATE airplane to test the X-Plane flight model. If it’s wrong in the X-Plane flight model, the pilot of the sim will NOTICE it! There are no flight control computers to hide the errors, and across this huge envelope, any shortcoming in the flight model will stick out like a sore thumb at SOME portion of the operating envelope!
So the F-4 Phantom has always been my dream airplane, and it exposes any error in the X-Plane code. It’s perfect.
And now, with an Israeli Air Force F-4 flight instructor putting in 12 hours days for five days or so with CONSTANT emails and iterations, we have this airplane DIALED IN TO FLIGHT PERFECTION.
And here is where it gets REALLY FUN: We did this not just by perfecting the ACF file (which we surely did) but ALSO by perfecting the flight model code in X-Plane, rolling in refinements that will subtly benefit ALL airplanes. There are NO hacks in the aircraft file. NO plugins. EVERY VARIABLE is entered as accurately as could be in Plane-Maker, and the flight model is now improved and refined to represent this model perfectly.In the past, people would sometimes have to put fake values in the airplane file in Plane-Maker to get realistic performance, but that is NOT the right solution. I wanted to enter this bird ACCURATELY, and then get perfect results.
So here’s where we start: The FIRST major issue we noticed was that the simulated F-4 was just not flying dangerous enough: It was limiting the angle of attack to safe values, not pitching that nose quickly and aggressively into the target, not going to huge angles of attack and lift and drag to CLAW at its’ target. The reason? The OLD X-Plane flight model was treating the wings of the F-4 like any normal airplane wings, limited to 15 or 20 degrees to avoid the stall, and with a flight control computer limiting the airplane to nice, safe, low angles of attack to prevent that stall.
That’s not what the real airplane does! The REAL F-4 has a huge DELTA wing and DELTA horizontal stabilizer. These triangular wing plan-forms roll a VORTEX up off their leading edge, rolling the air up from underneath the wing, up around the leading edge, causing a HUGE VORTEX, or HURRICANE, literally to form over the ENTIRE TIP OF THE WING! This vortex does NOT allow the wing to exceed critical angle of attack and stall! No! The MORE the angle of attack, the STRONGER the HURRICANE over the wing, as the angle of attack comes up and the air ROLLS UP OVER THE LEADING EDGE AND INTO A VORTEX OVER THE ENTIRE WING.
Here is what is so amazing about this: For traditional wings like X-Plane has always simulated, air never comes up over the leading edge! Instead, at the stalling angle of attack, the air separates from the top, loses suction, and the wing stalls! And all of this is carefully orchestrated based on the AIRFOIL, or CROSS-SECTION SHAPE, of the wing!So X-Plane has always used the airfoil cross-section, corrected for the plan-form, or top-down shape of the wing, as classical wing theory calls for.
But now, with a delta wing, the cross-section of the wing hardly matters! Now what matters is the plan-form, or top-down shape, of the wing! That’s what lets the air roll up over that highly-swept leading edge to form the huge hurricane over the wing that SUUUUUUUCKS the wing up… and never stalls!
And X-Plane now used this classical wing theory based on the AIRFOIL CROSS SECTION for non-delta wings, vortex-generation based on the PLAN-FORM of the wing for delta wings, and even interpolate smoothly between them for partially-delta shapes, consider both the airfoil cross-section, and the delta plan-form, for a real three-dimensional understanding of the wing.
And that is what lets us simulate the delta wing of the F-4, with that huge over-the-wing hurricane that lifts the wing up without fear of stalling even at crazy-high angles of attack.
Of course, this gives HUGE LIFT, bit also with a resulting HUGE drag! So, to get this airplane right in X-Plane, job number one was to get this vortex formation working in the simulator, so we did! Now, as the wing as entered in Plane-Maker gets closer to the proper shape for vortex lift, X-Plane smoothly departs that oh-so-boring land of smooth flow up to the stall that we have gotten used to, and into the land of vortex-generation as a function of angle of attack! Now, when X-Plane detects the SHAPE of the DELTA wing, higher AOA means more lift (and boatloads more drag!) without any stall! And what is beautiful is that X-Plane PHASES SMOOTHLY INTO the delta-wing dynamics based on how CLOSE to a delta-wing the planform is. The F-4 wing is NOT QUITE a perfect delta-wing shape, so SOME classical wing dynamics remain in place: X-Plane is blending largely into the vortex dynamics, but still using some of the classical dynamics at the same time… and if you look at the slats on the leading edge of the real airplane, you will see that the designers telegraphed this to us, equipping the wing with slats to resist stall in the classical flight dynamics regime as well.
And, once that stall-proof delta-wing was done… I RIPPED THE ARTIFICIAL STABILITY COMPUTER RIGHT OUT OF THE AIRPLANE! NOTHING LEFT BUT A DAMPER, LIKE THE REAL PLANE! NOW, starting with THAT, we were able to feel the REAL F-4 Phantom: Hurricane lift, no computer in the way. Boom. NOW we feel the air, and can CLAW into the turn at crazy-high angle of attack and G-load. The the NEXT thing we noticed is that the simulated F-4 did NOT tuck the nose down coming through Mach-1, as it should!
The reason was quickly found: IN REALITY, for SUBSONIC airplanes, the lift acts at about 25% of the way back along the wing. The OLD flight model had this just fine.IN REALITY, for SUPERSONIC airplanes, the lift acts at about 50% of the way back along the wing. The OLD flight model had this just fine.IN REALITY, for SUBSONIC DELTA-WINGS airplanes, the lift acts at about halfway between these two values… at 37.5%.
And, and that LAST item is what the OLD flight model did not quite understand. NOW, I have coded that center of pressure for SUBSONIC DELTA WINGS to be right in the middle between 25% and 50% of the wing chord, which results in just the right stability for the real airplane. But here is where it gets interesting: Now, as you accelerate though Mach 1 in the simulated F-4, the center of pressure scoots BACK from 37.5% (delta-wing) to 50% (supersonic) suddenly pushing the lift BACK, and causing the airplane to suddenly become a lawn dart, wings at the back and nose in the front, causing the nose to suddenly drop. This is called Mach Tuck, and it’s exactly what the real airplane does. This is interesting enough, but here’s where it gets scary: Let’s say you are supersonic, turning hard. The center of pressure is back at 50% (because supersonic). Compression shocks blanket the front half of the wing, expansion fans blanket the back half. All of this is what happens in the real plane, and is simulated by X-Plane. Your stick is WAY BACK IN YOUR LAP as you try to get that nose-heavy lawn-dart to pull the nose UP though the turn. At some point, you drop below sonic. The moment the speed comes below Mach 1, what happens??? The center of pressure suddenly moves FORWARD from 50% (supersonic) to 37.5% (delta-wing)! SUDDENLY, the nose LURCHES upwards! The lift has just JUMPED forwards, now well in FRONT OF the heavy engines, and the nose suddenly lurches up from that lift coming forwards. The G-load suddenly jumps up, the angle of attack skyrockets, and the shock waves have vanished to be replaced by the hurricane vortex forming over the wing! The lift is huge, the drag is huge, the g-load is crazy by maxing out angle of attack at just barely below the speed of sound, and the nose claws around the turn at crazy angle of attack and g-load. All of this is a well-known characteristic of the real F-4, and all of this is simulated perfectly by X-Plane as EMERGENT behavior, because the center of pressure shifts on the wing, and all the physical dynamics follow it without any additional code at all. Lift from shock-waves to hurricanes: Welcome to the F-4 Phantom.
Boom. NOW we feel the transition from subsonic vortex lift to supersonic compression-shocks and expansion-fans and back to hurricane lift if we pull the nose hard in a supersonic turn to drag us back down to subsonic. But it wasn’t happening at quite the right SPEED. The transition to supersonic flow and back, with the resultant severe pitch-change, was happening at about Mach 1.25, when it SHOULD have been happening at Mach 1!
Why? Let’s look at transonic flight for a moment.
Imagine you are in a GLIDER. Nice long straight wings sticking out there. You are in thin air at 100,000 feet and diving down at a 45-degree angle. There’s probably RedBull logos on your airplane, and you’re in a pressure suit. As you dive, air speeds up to MORE than the speed of the airplane to get AROUND that thick, cambered wing. The THICKER the wing, the MORE the air speeds up to get around it. The GREATER THE CAMBER, or LIFT the wing, the MORE the air speeds up to get around the wing. (Remember, air speeds up to generate lift!) With your straight, thick, high-lift wing, as you get to Mach 0.60 or so, the airflow over the TOP of the wing has accelerate to the speed of SOUND to race up OVER that thick, curved airfoil! So, even at Mach 0.60, you have SUPERSONIC FLOW over the TOP OF THE WING!You have supersonic drag at just Mach 0.6! Bummer! Your speed is limited as you have hit an aerodynamic brick wall of shock waves.
But you are high on RedBull and have to pee so bad for obvious reasons that you want to get back down to Earth FASTER than Mach 0.60.You want more speed.How do you get it? OK, let’s operate with LESS LIFT. Let’s use a foil that is NOT SO CAMBERED on top. Without this airfoil camber and lift, the air does NOT speed up so much over the top of the wing! Of course, you have less lift now so the plane has a higher stall speed and HAS to fly faster, but who cares? You’re in a hurry. Reducing that camber and local airflow acceleration lets you get up to mach 0.65 before the flow goes to supersonic over the top of the wing. More speed!
But you still want to go faster. Nobody wants to pee inside a pressure suit.Ok, let’s make the wing THINNER! This means the air does NOT have to speed up as much to get around the wing, so now you can get to maybe Mach 0.70 before those thrice-damned shock waves form over the wing and block your acceleration. But you still want to go faster. As you get older, you can’t hold it as long. So now we SWEEP THE WINGS BACK. If we sweep the wing to a 45 degree angle, then the flow acting at a right angle to the chord is only 71% (cosine of 45 degrees) of the actual local speed.
Or, put another way, the wing is SLIDING out of a direct conflict with the air by slicing through it SIDEWAYS. Or, put another way, the wing appears THINNER to the air, because the air is sliding sideways ALONG the wing from root to tip, not suddenly being jerked around the airfoil in a direct confrontation. No matter how you look at it, you can go a LOT faster before the local flow over the top of the wing goes supersonic, invoking those shock waves that impede further acceleration.
So that wing sweep EFFECTIVELY lowers the speed over the wing: Less airspeed acting at a right angle to it! And X-Plane was applying this speed reduction to the MACH NUMBER on the wing as well! Ooops! BAD MOVE.NOT RIGHT. While wing sweep does indeed lower the effective airspeed at a right angle to the wing, localized flow still starts to go supersonic as we approach Mach 1, and by the time we are AT Mach 1, ALL of the flow is supersonic. No wing sweep in the world can avoid this. So wing-sweep lets you get CLOSER to Mach 1 before localized flow starts going supersonic, but that’s all. It doesn’t let you get PAST Mach 1 and still have subsonic flow.
So the subtlety is that: Wing sweep lowers effective wing speed, by reducing the airspeed acting at a right angle to the chord and lowering the resultant lift… Wing sweep lowers effective wing camber and thickness, reducing localized flow acceleration to supersonic speeds… But once you are going Mach 1, the shock waves form on a wing of ANY sweep… the sweep just DELAYED that localized shock-wave build-up until CLOSER to Mach 1. Wing sweep doesn’t let you exceed Mach 1 with subsonic flow.
So based on this, we would expect an airplane with a long, thin, lightly-cambered, highly-swept wing to get very very CLOSE to Mach 1, but never quite HIT it. Because that thin, un-cambered, swept wing sees almost no localized sonic flow until almost the very moment the plane hits Mach 1. There’s very little localized flow acceleration. So look at the wing of the Citation X. It’s all right there.Hidden in plain sight.Going Mach 0.925. Of course, as soon as I corrected the flight model to understand this subtlety, the transition from subsonic (tornado) to supersonic (compression-shock and expansion fan) happened smoothly between mach 0.9 and Mach 1.0, and the airplane Mach-Tucked or smacked you in the, um, everything, with G-load at just the right speed.
The the NEXT thing we noticed is that the simulated F-4 did NOT want to raise the nose quick enough in take-off!The real airplane could raise that nose at 155 knots in the take-off roll… so why was the simulated airplane building up to 185 knots on the runway before we could get the nose up? The first thing we found is that the simulated airplane was carrying all its’ fuel in the wings. OOPS! The REAL F-4 carries almost all of it’s fuel inside the body… ABOVE the engines!In the real airplane, under full thrust, the weight of all that fuel ABOVE the engine thrust was pulling the nose UP from the push of the engine UNDERNEATH it! Look below: All those black boxes are fuel… and all ABOVE the engine! The engine pushing forwards under all that weight will help RAISE the nose!
So the next thing I did was get all the fuel placed properly in Plane-Maker. Doing this, the nose came up at CLOSER to the right speed (175 knots?) under full acceleration, but still not he 155 knots we know the real airplane could do.
In the REAL airplane, downwash from the flaps certainly smacked DOWN on the horizontal stabs, giving more nose-up moment, but in X-Plane, we were just not seeing that downwash hitting the stabilizer during the take-off roll (but it was just fine in flight!!!) WHY?? As you may have guessed, it was GROUND EFFECT. IN REALITY, ground effect is BOUND to flatten out the downwash from the wing onto the horizontal stabilizer, and the old X-Plane flight model was OVER-DOING that effect, flattening out the ground effect a bit TOO MUCH. I went back to the ground effect reports I used, and found a better way to curve-fit the experimental data. As well, I looked at the induced drag reduction with ground effect, and since induced drag is tied to downwash (one is the cause of the other) blending the downwash in with induced drag as well mode the model more accurate yet. Doing these things, we shaved another few knots off the rotation speed, but only a few knots… the ground effect impact on downwash was not huge.
What ELSE were we missing?
We were still raising the nose at 170 knots… 15 knots too fast. We needed to get that nose up at 155 knots, 15 knots sooner.We need 15 knots more ‘oomph’.Somehow. 15 knots. Myself and the F-4 instructor wracked our brains wondering WHY we could not raise the nose until going 15 knots too fast… ALL elements of the aircraft definition in Plane-Maker were checked and checked agin. Vortex lift, which also applies to the stabilizer, was checked and checked again.
What were we missing?
Suddenly it hit me: There is a thing called ENTRAINED FLOW. ENTRAINED FLOW is wherever you find a fast-moving JET of air, and the air nearby is GRABBED AND DRAGGED ALONG WITH IT to some extent, speeding up the air all AROUND the jet. Look at video of any of Space-X’s or anyone else’s rocket engine tests on the test-stand: You plainly see air RUSHING through the test stand, entrained in the hypersonic rocket flow, and being dragged along with the current. Do you know of any jets of air located near the horizontal stabilizers of the F-4 Phantom?
If not, then here’s a small, subtle hint:
Now this is subtle, but if you look real careful and squint just right, you will see these HUGE FREAKING AFTERBURNERS PUSHING AIR OUT AT ONE FREAKING THOUSAND FIVE HUNDRED MILES PER HOUR. This entrains flow, without question. In other words, the entrained flow around the jet exhaust is getting dragged along by the supersonic core, speeding airflow over the horizontal stabilizer! But how MUCH? How MUCH entrained flow is near this jet blast???
I asked the F-4 instructor, really assuming I was wasting my time asking a pilot such a weird technical question that nobody ever even thinks about.
Then, in his endless series of mic-drop moments, he knew. He literally KNEW how much entrained flow there would be. How? Because the people making the airplane wanted to make sure that nobody working on the ramp ever got BLOWN AWAY BY THE JET BLAST! They MEASURED the entrained flow (and temperature!) to a careful degree, ALL AROUND THE JET EXHAUST, so they could tell the ground crew where to never go!
And here it is:
Boom. My jaw dropped when I got this. The answer is right there. NOW YOU KNOW how the jet-blast, complete with entrained flow, for both subsonic and supersonic jets. A careful look here shows that the plume expands out at a ratio of about 4 or 5 feet back to 1 foot out. As well, look carefully and see that as you move aft, the flow slows down as the volume increases (no surprise there, if you follow Bernoulli on Twitter).
Another fascinating thing you see is that as you move out to the SIDE from the CENTER, the flow cuts its’ speed almost exactly in HALF each bit of CONSTANT DISTANCE you move.
In other words, at a certain distance aft (call it 100 feet) the flow speed exactly cuts in HALF for each ten feet you move to the side! The EDGE of the drawn gray plume is NOT where the speed is ZERO… it is where the speed is fifteen knots… 1% of the core emitter velocity.And, as you can easily extrapolate from this, in another 10 feet it will be half of 15 knots! And, as you can easily extrapolate from this, in another 10 feet it will be one quarter of 15 knots!And, as you can easily extrapolate from this, in another 10 feet it will be one eighth of 15 knots! And, as you can easily extrapolate from this, in another 10 feet it will be one sixteenth of 15 knots!
I think you get the picture: The entrained flow goes out very very far (do you REALLY thing it WON’T be windy standing behind an F-4 Phantom at full afterburner?), but the flow speed simply gets very very small compared to the core speed as you move farther away. Carefully transcribing this flow into X-Plane, white lines showing flow boost speed on each element:
Boom. Average 15 knots of help on the stabilizer from the entrained flow. The horizontal stab of an F-4 gets help from flow entrained in the jetwash, increasing the speed over the stabilizer. By an average of 15 knots. There was our missing 15 knots. With the entrained flow jet-blast model right from the ground-personnel warning, we suddenly had an extra 15 knots on the tail, and the nose coming up right at 155 knots, JUST LIKE THE REAL PLANE.
The performance was all dialed in, but something was still not quite right: The plane seemed to be too sluggish in roll. Sort of… wallowing. Even more than the real plane! Especially at low speeds. At high speeds, it was basically perfect.
Why? How do you have the roll being too sluggish at low speed, but just fine at high speed? Suddenly it was obvious: The AERO effects were just right. The plane had too much MASS INERTIAL in roll. That mass inertia (the ‘radius of gyration in roll’) was too high, and at low speeds, under low aero forces, you really noticed that it just took too long to get that big mass rolling! At higher speeds, with the higher aero forces at play, you really did not notice it much: The large aero forces over-ruled even the too-high inertia enough that no human could detect the problem.
So, it was time to go into the way X-Plane distributes the mass across the airplane to DETERMINE the the inertias in pitch, roll, and yaw.Sure enough, X-Plane could be a bit better there! First, I updated X-Plane’s mass estimate for the ENGINES based on all the latest engine data I could find based on number of engine spools and bypass ratio. Then, I added consideration of the EMPTY fuel tanks to the mass-distribution estimate. Fuel itself was already added, but why not take it to the next level of accuracy and also add the weight estimate for the empty tanks themselves? They are mostly just above the centerline of the airplane, tightening that roll inertia right up. As well, why not consider the fact that THINNER wings probably weight less, since any airplane with really thin wings is not storing too much stuff IN those wings! Fighter jets store most of their stuff in their bodies, not in their wings! This is needed to keep the wings thin to keep the drag down! So, with fuel tank empty masses and locations considered, and thin wings pushing the innards of the airplane out of the wings and into the bodies a bit, X-Plane now had a more accurate moment of inertia estimates (on all axis) and that tightened up the roll inertia, resulting in proper roll even at low speeds, where the aero forces were low, and the body inertias very very apparent.
But now we got to a really odd subtlety: (this one also explained in a video found by Scott Manley on YouTube)
At really high angle of attack, the speedbrakes become blanketed in turbulent, separated or cyclonic flow, and frankly don’t do anything. Think about it: They’re in the middle of the wing, far from the leading or trailing edge, in the middle of a chaotic and turbulent zone of air. Why WOULD they do anything in that scenario? So, at a very high angle of attack, the roll spoilers become use lawn-ornaments on the wing, and lowering an aileron just increases the DRAG on the wing you want to raise, pulling it AFT and dropping it, while of course advancing and lifting the other wing, causing the pane to wallow in the OPPOSITE direction you commanded! As my F-4 instructor put it: “If you try to pitch and roll at the same time, the Phantom will not obey”. So now, with blanked flow diminishing and then finally at max lift eliminating speedbrake effects, sure enough in X-Plane: Near max angle of attack, full left stick causes the nose to yaw right, and the plane to gradually wander off to the right, and vice-versa. All of this is emergent behavior in X-Plane: None of it was tacked on.
But now we got to a really odd subtlety: (this one also explained in a video found by Scott Manley on YouTube… youtube.com/watch?v=iZiduQboyow, t = 9:33) At really high angle of attack, the speedbrakes become blanketed in turbulent, separated or cyclonic flow, and frankly don’t do anything. Think about it: They’re in the middle of the wing, far from the leading or trailing edge, in the middle of a chaotic and turbulent zone of air. Why WOULD they do anything in that scenario? So, at a very high angle of attack, the roll spoilers become use lawn-ornaments on the wing, and lowering an aileron just increases the DRAG on the wing you want to raise, pulling it AFT and dropping it, while of course advancing and lifting the other wing, causing the pane to wallow in the OPPOSITE direction you commanded! As my F-4 instructor put it: “If you try to pitch and roll at the same time, the Phantom will not obey”. So now, with blanked flow diminishing and then finally at max lift eliminating speedbrake effects, sure enough in X-Plane: Near max angle of attack, full left stick causes the nose to yaw right, and the plane to gradually wander off to the right, and vice-versa. All of this is emergent behavior in X-Plane: None of it was tacked on. Again, the huge performance envelope and nearly-absent computers, coupled with high mass and small wings made every single shortcoming in the flight model stand out like a sore thumb… so I could quickly address it!
So, we now have the F-4 flying at a level needs to give flight instruction. Think of the thousands of tons of fuel that would have been saved if we had had this in the 60’s, or even 10 years ago, when the plane was still in service!
But is there any OTHER use for these improvements in X-Plane?
Well, flying the new F-14 Tomcat, we suddenly do NOT need a fake, stall-proof airfoil for the horizontal stabilizer. Why? Because the horizontal stabilizer of the F-14 is a delta-wing. It doesn’t stall: It vortex-lifts itself to do any job. Now, the hurricane forms over the F-14 stabilizers, preventing any stall of the tail surface. Now, as well, the downwash modification due to ground effect is more accurate, thus giving more accurate downwash onto the horizontal stabilizers of the airliners, helping them get their nose up just a little earlier, as the A-330 has been needing. As well, this downwash tuning with ground effect will perfect that nose-down that airliners encounter in reality as the wash onto the tail reduces in the final bit of the approach and flare. Also, the transonic effects that I tuned carefully with Mach number are JUST STARTING to become apparent as airliners like the A-330 move into the their MAX-MACH cruise, and the Citation-X tops out at precisely Mach 0.925, just like the real airplane. As well, the new more-accurate transonic drag rise we developed for the F-4 exposed a teeny error int he A-330 aircraft file: As the A-330 approached its’ max cruise of Mach 0.85, the drag was just skyrocketing. Why? ONE TINY BIT OF THE WING did not have quite the right sweep: And it was tossing shock waves and slowing the whole airplane. Thank the F-4 tuning for finding this.
As well (and this is the big one) if you look at videos like this, you will see the horizontal stabilizers of large airliners shaking during their take-offs. And no, it is NOT from a bumpy runway: The main wings are as still as a rock in a field. Check it out:
So why is the horizontal stabilizer jiggling along as if something is smacking into it? Well, as you now know: THERE IS! It’s the entrained flow from the engines! X-Plane now applies the entrained flow model from the F-4 ground-crew warning to ALL jets of air (physics are physics!) and sure enough, about 15 knots or so is making it to the stab of the A-330! Again, helping raise that nose at a lower speed, as our model needed. Hit command-M a few times to see the various flow fields.
So, this tuning and upgrading of the flight model to NAIL the F-4 has been great, fleet-wide!
And, of course: Wow, what an airplane.
And the final note from the F-4 instructor:
Hi Austin, Tested this thoroughly….it is *ON THE SPOT* now (!) Perfect takeoff (!!!), landings, flameout landing path, aerobatics….X-Plane has a very fine F-4 simulation now 🙂 …. The F-4 nuances are very accurate now…amazing in fact. Well done! “Tuning” an aircraft is a familiar process to us developers, but Austin did not “tune” it. He simply increased the model’s design resolution, or “density”, accurately, using actual data and design aspects…and on the other hand, extended his flight model’s “blanket” to cover more of the edges of flight dynamics envelopes. …so the LR F-4 is not “tuned” at all. It is a 100 percent natural, “raw” aircraft in X-Plane…Impressive!
This is David Byrns’ show, where he does our favorite songs from the Talking Heads with a really incredible way of presenting both the band and the music… A free and liberating way I’ve never seen before. The theater is a small venue where you are really right there with David enjoying his creativity. It’s the best show I’ve ever been to. It’s shutting down in 30 days so if you want to see this incredible experience you’re just about out of time. New York is opening back up again so there’s just a narrow window left to hang out with David Byrne and see this. Seize the day while you can!
Simply put, a photometric renderer is one that tries to create realism by using actual real world light levels (specified in real physical units) in its internal calculations. In other words, we render the world as it is.
A decade ago, the image of the world you saw through your simulator was essentially built out of pre-made images drawn in Photoshop by artists. These images were drawn as realistically as possible, but they were low dynamic range (LDR) because that’s all the monitor could handle. The sky was as blue as the art director decided, and then created with Photoshop. This worked great in its time, but with today’s modern graphics cards we can do much better.
First We Have to Go Higher
A traditional low dynamic range (LDR) renderer has colors in a range from 0 to 255, but if we want to model the real world, we’re going to need some much bigger numbers. We measure luminance in candela per meter squared (cd/m^2) or “nits” (nt). Here’s a Wikipedia chart listing the luminance of a wide variety of stuff. A few examples:
Flood lights on buildings at night – 2 nts.
An old crappy LDR monitor – 80 nts.
A nice newer LCD monitor – 500 nts.
The clear sky – 7000 nts.
Clouds – 10,000 nts.
The sun at sunset – 600,000 nts
The sun at noon – 1,600,000,000 nts.
(That last one is why your mother told you not to stare at the sun.)
Note how wide the range of numbers are: daytime images are made up of things in the “thousands” of nts, but with a wide range of variation, while night ones might be single digits.
So to be more realistic, the sim needed to render using bigger numbers. X-Plane 12’s rendering pipeline is entirely HDR, from start to finish, using 16-bit floating point encoding to hold a much wider dynamic range of luminance.
You Can Stare at the Sun in X-Plane
Obviously you can look at the sun in X-Plane on your LCD monitor and not suffer direct eye damage – the peak brightness of your monitor might be 100-500 nts. How do we show a scene with 10x the brightness of a monitor, or more? To solve this, we needed to model a real camera to serve as your “eyes” in the simulator. This camera in X-Plane sets an exposure value that maps our HDR scene to your monitor.
The dynamic range of computer monitors isn’t very large, though. To address that, we applied a tone mapper to the exposed image. The tone mapper is a tool that “squishes” some of the bright areas of the image so that we can fit a wider range of bright colors onto the screen at the same time. The tone mapper can give the simulated scene a look that’s more like an image from a film camera, rather than a cheap shoddy digital camera. Using the tone mapper, our art directors tuned the parameters of X-Plane’s camera to make our scene look brilliant and realistic, given the constraints of computer monitors.
The exposure levels in X-Plane 12 are set by our art team according to time and weather conditions. They are also modulated by auto-exposure, so that as you look around the scene the camera becomes more sensitive in dark areas (to help read panels) and less sensitive in bright areas (to avoid being blinded).
A Physical Sky
Now that the sim had a rendering that accurately modeled the real world, a sky that was painted by our art team using Photoshop just wouldn’t work. It needed a new sky that would match the light values (nits) of the real sky.
To do this, we calculated the light levels of the sky by considering the composition of the atmosphere, the viewing angle, and the brightness of the sun. The sky is blue for a reason (oxygen molecules) and we get a blue sky by simulating that scattering effect.
This math for the sky works when looking in any direction from any location, so we not only get a blue sky, but we get the correct “blue-ish” tint when looking at the ground from the air, and this matches the sky without the artists trying to hand-paint two effects to match.
Lighting It Up
To create a photometric world in X-Plane, we needed light sources in the sim to be specified in real-world units. X-Plane comes pre-programmed with the brightness of the sun, but how bright is that LCD screen in your glass cockpit? In X-Plane 12, aircraft designers specify these values in real world units.
One of the advantages of this real world approach is that the “right” value for setting up an aircraft can come from the real specifications of the aircraft, rather than tuning some numbers in a 3D editor until it looks right.
One of the big advantages of this approach is that all of the elements that make up the sim play well together because they are all calibrated to the same standard – the real world. How bright are landing lights compared to the airport lights? How visible are the taxi lights when the sun comes out? With a photometric rendering engine, the answers are determined mathematically and by measurements that can be checked against real life, so the entire scene fits together.
With photometric rendering, we’ve taken another step closer to real life – the new X-Plane 12 renderer simultaneously produces realistic images and is more straightforward to work with, all thanks to its use of real-world values as inputs. Check out the video below for an A/B comparison between the lighting in X-Plane 11 and X-Plane 12.
First, I appreciate everyone’s cooperation with the RFC on scenery; we’ve had an ongoing discussion in our developer Slack as well as the comment section, and I don’t think I had to nuke any off-topic comments. The feedback was wide-ranging and there’s no one clear answer but it does give us a really good picture of how the scenery system is working (and isn’t working).
It’s Friday, so let’s do something completely different – her’s some show and tell from a few things people have been working on things week.
Light It Up
Alex has been recalibrating the runway and airport lights for the new photometric lighting engine. This spurred an internal discussion about how best to calibrate artificial light sources. Does the author specify the luminance of the bulb before a tinted plastic housing goes on top (this way is good if you have the bulb specs from the internet) or based on what you’d measure when the finished light is tested? (This way matches FAA specs for airport lights.)
After going back and forth a few times, our answer is “well, both”, and we have a system that now allows this, which should solve use cases for both aircraft (where often the bulb properties are known because you can look up replacement parts) and for airports (where the FAA has standards for the light’s final results).
Something to keep in mind: urban airports are quite dark compared to their surroundings. Ther are very few light sources near the runway that aren’t tightly controlled for brightness and direction. I used to fly over KLAX on a regular basis at cruise altitudes (commuting from San Diego to San Francisco for work) and KLAX was always an inky black void in the sea of lights that is the LA basin; at 34,000 feet no runway lights are pointed up at us.
Petr and Sidney have been working on the weather surface shader, which applies water and other weather effects to surfaces. This is how we dynamically make the pavement wet when it rains.
The shader is tricky because the effect of a surface being wet changes a lot once the water forms a real puddle. When I took my kids to their swim lesson, I couldn’t help but notice the useful reference material all over the place.
Stop Writing on the Windows
I must be a dad, because I get annoyed when my kids get finger prints all over the windows when they “write” things in the frost on a cold day.
Turns out Sidney does the same thing.
What you’re seeing there is programmer art. Programmer art is when the programmers make their own texture files to test code. In this case, Sidney is testing the defrosting system for windscreens, which use a special texture to specify the pattern of defrosting. This lets artists control the defrosting effect and get faster defrosting near vents.
Another “behind the scenes” thing you can see here: that popup window is a set of internal controls for testing, debugging and developing the windscreen effects. The parts of these internal controls that are generally useful will become third party developer tools (like the texture browser and particle system editor in X-Plane 11).
Cessna In Spaaaaaaaace
Daniel rewrote the planet shader. In X-Plane 12, water is treated separately from land (so that it can be 3-d). The new planet shader shows a far view of water and a far view of land at the same time and correctly shows atmospheric scattering, which is normally pre-calculated in a special “froxel cache” for regular scenery.
If you haven’t noticed the pattern, it’s that the art team’s screenshots all tend to look good enough to ship, and the programmer’s screen shots tend to be very, very silly. In this case, the Cessna in space is pretty silly, but what we were looking for was the smooth atmospheric effects all the way out to the horizon.
Here’s one more goofy programmer screenshot:
I was calibrating the runway lights according to Alex’s spec, and typed an extra 0 into one of the internal art controls by accident. The result was this fantastic screen shot.
What you’re seeing is: the billboards for the runway environment are accidentally huge and are filling up the entire reflection cube map. The reflective underside of the Cessna wing picks up this blue lights and it looks like a rave.