Jet engine modeling in X-Plane 11

OK I overhauled and upgraded the jet engine model as well.

Here is how it works: For SUBSONIC dynamics, I curve fit maximum engine thrust ratio to static max thrust as a function of density altitude, Mach number, and engine bypass ratio. This is pretty easy and boring and I have been doing this for years.

But here is where it starts to get good: 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.

No arbitrary losses above your critical Mach number, 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.

So that is for subsonic inlets being dragged above their critical Mach number. What about supersonic inlets?

OK this gets good: As we move through Mach 1, we transition from the subsonic curve fit for subsonic engines to the pressure-recovery of the total energy of the airstream. Here is where this gets interesting: The faster you go, the higher the Mach number of air incoming to the inlet, and the more energy is available from the airstream to turn into THRUST!
So, the faster you go, the more thrust you get! This is one reason that supersonic jet airplanes just keep speeding up, and up, and up, and up!

Planes like the F-4 Phantom, for example, take about FIVE MINUTES to get from Mach 1 to Mach 2 (a long time because the thrust only builds as the speed builds) but darn they hit Mach 2 and are still slowly accelerating!

Now, nothing this good lasts forever. At some point, the aircraft speed overwhelms the inlets’ ability to accept the shockwaves, and losses occur. We simulate this with a normal shock, and the inlet efficiency gradually moves from ideal (total pressure recovery) to the worst possible (normal shock) as the inlet moves to and then past it’s maximum allowable Mach number.

Here’s the equation for the losses across the normal shock, by the way:

	const xflt gamma   =1.4    ;
	const xflt gamma_m1=1.4-1.0;
	const xflt gamma_p1=1.4+1.0;

	xflt nrm_shock_press_rat= xpow((gamma_p1 * sqr(M_use) ) / (gamma_m1 *sqr(M_use) + 2.0			) , gamma	/gamma_m1)	//
							* xpow((gamma_p1			  ) / (2.0 * gamma * sqr(M_use) - gamma_m1	) , 1.0		/gamma_m1);	// normal shock total pressure ratio

So, if you open the F-4 Phantom in Plane-Maker, go to the engines window, and then the Jet Curves tab on the right, you will be able to SEE EVERYTHING that I just talked about.

On the left, at Mach 0, you see the static thrust for each altitude.

Then as you move right to Mach 0.5, the thrust falls as the turbine can’t deliver much ‘oomph’ due to the rapid inflow of air… like trying to climb a rope ladder while the rope is falling, trying to get thrust from an airstream always coming at you is simply an uphill battle that does not work too well. So the thrust FALLS as you speed up.

Then, above Mach 0.5 or so, something interesting happens: the energy in the oncoming airstream becomes significant, and the inlet starts decelerating that incoming airstream, using that deceleration to INCREASE the air pressure inside the inlet, which actually helps the inlet do the job FOR the engine! Now, that thrust starts BUILDING!

Now as we move to Mach-1, it’s crazy-time. The airstream pushing at the airplane is packing HUGE energy from all that speed, and nice, efficient, oblique shocks start capturing all that energy, slowing and pressurizing that air efficiently, and handing that high-pressure to the engine. A well-designed inlet at this point might develop MORE thrust than the engine itself… the job of the engine is simply to pressurize the inlet here. And, the faster we go, the farther to the right we move on those curves, and the greater the thrust becomes as we speed up. This is a recipe for an airplane that just never seems to stop accelerating. Enter the F-4. And the SR-71.

But, at some point, the shockwaves overpower the design of the inlet, and we start heading to the (terrible) efficiency of the normal shock. Here you see the curves dropping thrust hugely, on the fast-side of the max expected Mach number for the inlet.

So, you can see the thrust curves in Plane-Maker and now know what forms them. Set the reference Mach number on the lower left for you inlet on your plane to get the thrust peak right around the top speed for your airplane.

And then finally, MAXIMUM thrust is not the only thing here: We also need thrust variation with N1, and DRAG from the engine at idle at various speeds. Those things have been tuned and tested as well.

For testing:

I have a full Citation Mustang POH with aircraft speeds and power settings, to test and tune the low subsonic flight regime for jets, and a recently de-classified F-4 Phantom Pilots Operating Handbook with subsonic and supersonic deceleration times (to tune the DRAG) and acceleration times (to tune the THRUST) to test and tune the high subsonic and supersonic flight envelopes of jet engines. All of the math above checked out very well with the POH’s for these airplanes… much of the accel/decel timing on the F-4 Phantom to within 1 second to get to and from various subsonic and supersonic speeds at full and idle thrust.

And a quick little detail: Low/high jet engine bypass types: GONE! Now we ONLY go off the bypass RATIO that you entered! This lets cool things like exhaust smokiness and engine mass for mass distribution all be floating point with bypass ratio for infinite variation, which is nice.

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!

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40 Responses to Jet engine modeling in X-Plane 11

  1. Tom Knudsen says:

    Thank you for all your hard work. I would place you amongs those of Linus Torvalds, Steve Wozniak, Bill Gates, Paul Allen, Steve Jobs, Tim Berners-Lee, Jack Tramiel, Guido van Rossum, Dennis Ritchie, Robert E. Kahn and Vinton G. Cerf. I belive my choice to place you among my “heros” are justified as you have dedicated your life to bring us a hobby, a way of life, a community to learn from, and simulator to enjoy and participate with.

    I could not imagine a world without so thank you and extend my gratitude to the rest of the team for their also hard work.

    • Greg says:


      I fly X-plane as often as real life allows me, the ‘feeling’ is just incredible. The fact it gets better and better… just fantastic! A great thanks to Austin, Ben, Chris.. all the LR staff.. A Very Happy New Year to you all

    • Sven Melander says:

      Another thanks from Scandinavia, from Sweden. I’m really thankful for all the work Austin and team are putting in. A huge thanks and to all of you I’m wishing a prosperous and joyful 2017!

  2. Luigi Linguini says:

    Holy Moly! Wish I were 20 yrs younger to again work as hard as you do. Great news getting those jet engines programmed and fined tuned, especially for the ss ones! Looking forward to that! Tho I’m not a jet fighter kinda guy (it relates to war and not peace), I’ll def take a spin or two in one…without weapons loaded if possible lol!!!
    Thanks to you and the guys/gals making xplane the BEST Flight simulator on our little planet, perhaps the Universe 😉

  3. Elios says:

    so next beta when?

    and all the other new stuff you guys kept talking about thats now missing like the g1000, gns keyboard input, Citation X…. GROUND HANDLING that isn’t broken ….

    • carrotroot says:

      Patience … X-Plane 11 will eventually get those things, the lifecycle for an X-Plane run is usually around 4 years. Also, when did ground handling become a thing!? Is it because 1 or 2 guys on Avsim’s forums have made a stink about it? Not to lessen the importance of ground handling, but I would imagine higher priority would be given the flight model while in the air than on the ground; after all this is where you will likely be spending most of your time … in the air, that is unless you are one of those folks who spends a lifetime taxi-ing around X-Plane’s magnificent looking airports (ok I’m kinda guilt of this one lol).

      • Austin Meyer says:

        Oh ground handling is fun, and really matters for run-up, short-field take-offs as the nose comes down under power and braking and then pops up on brake-release, nose-dive under braking for short-field landings… all so fun and you REALLY feel it in a Cessna 172 or other light plane! When I first got it (finally working perfectly, I pretty much cannot describe the sense of joy I got from feeling like I was in a real airplane on the ground during this type of maneuvering)

    • Eric says:

      Calm down ! Quit asking when the next beta is going to be. This is not a fast food drive thru. You asked the same question in the last blog post. It will be ready when it’s ready !

    • Alexander says:

      I do not think the next Beta will come before January. But I am sure everyone works hard to get it out within the next 3-4 weeks, maybe earlier.

      • Ben Supnik says:

        Next beta will be early January – the original plan was to have it out this week (the time between xmas and NY) but pretty much everyone I know has some kind of cold this week, so I’m behind in my own bug fixes. We’ll be evaluating a beta 4 candidate early next week.

        • Bruno says:

          Awesome Ben, thanks for that.

          I was already really happy with improvements in pb3, especially fine tuning of the B738.

          By the way, there’s a guy here doing fixed and improving the default B738. He’s got VNAV already working and all. Personally, I’m always sorry to see double work. I’m a programmer by profession and every time I see I’ve just reinvented the wheel I feel that I just wasted my time immensely (unless if I learned something new).

          Maybe you could get this guy on board with you guys? 😀

          Anyway, I’ll have to close this reply by saying I would LOVE it if you would ever be as enthusiastic about working on the weather engine, as you are with these things you’ve been sharing. Getting multiple METARs around you and “interpolating” them to get seamless transition from one area to another, that would really make my day (and from many others). Smoothing out when new METARs are loaded would be awesome as well.

          Thanks a lot! I’m a new-comer to XP11, and asides from the weather engine, I’m really liking it.

        • Bruno says:

          PS: Speedy recovery for “almost everyone you know” 😀

        • Ben George says:

          Hey Ben, this question is unrelated, this is in regards to WED, I am updating my airports, and I get the message “Taxi route 1M is too close to runway 01 and now must be marked active for runway 01 departure.” when I run validate scenery, but that doesn’t make sense because the taxi route I am creating is at runway 19, what am I missing, why is it doing that, and I can’t update my scenery.

          • Ben Supnik says:

            Runway 1 is runway 19 from the other end. Is your runway labeled 1/19 or JUST runway 19? What’s the name of the runway in WED?

          • Ben George says:

            Hi Ben, it’s label as 1/19, I am adding arc traffic live at 19, 01 is on the other end.

          • Ben Supnik says:

            This is “as expected” then. If the runway is 1/19 and you don’t have flows that STRICTLY limit you to only ONE direction operations, then you need to have a hot zone labeled for BOTH directions, e.g. this area is “hot” for ops on runway 1 OR runway 19.

        • Ben George says:

          Ben, you are awesome, I didn’t think of that at all, thanks for the tip.. will make the necessary modifications..

    • We can finally update the CF-104… I had given up on it because of the poor supersonic performance.

      • Elios says:

        its still going to have poor supersonic do to the physic engine
        it has nothing to do with the engine modeling

        fix the super sonic flow model first …

        but i would rather they work on things like the missing G1000 and the Citation X first

      • Teryn D. says:

        awesome. Love your 104

      • Arthur says:

        OOhhh yes, waiting for a long time to buy the F-104. I hope it will happen……..

  4. TheSergey says:

    You x-plan 10.51 forgotten forever?

  5. Hendrik Maroske says:

    Jet engines finally, yay!

    This is a very promising area. I am tweaking my Learjet 35 model since V9.7 and just now have begun tuning it for V11. That is going to be tons of fun because I already have the whole collection of datasets in a plugin and can easily see how precise the new calculation is. To achive maximum fidelity, the model adjusts the fuselage drag until numbers match up, compared to the manuals. Small drag adjustments indicate a good correlation.

    Thanks a bunch for your good work,

  6. Will there be support now for multi-stage afterburners like on the F-4? Since it uses the same engine as the F-104, I believe there are 5 stages. Perhaps the throttle quadrant range can be mapped so that the first 75% of travel gets you to military thrust and the last 25% controls the afterburner stages. The lower afterburner settings are commonly used on takeoff. The mid-afterburner setting will be selected be the lead aircraft and the aircraft following can catch up on full afterburner if they fall behind slightly.

    Also, is there a new afterburner flame yet using particle effects? It would be nice if X-Plane supported a variable flame depending on the afterburner setting and it would be nice of course if we could build this into object models with other particle effects.

  7. xplanefan says:

    “we start heading to the (terrible) efficiency of the normal shock”

    What is used to simulate aircraft such as the SR71 which never reached this state because of it’s fancy movable inlet cones and bleed valves?

    Does there exist any engine inlet that would suffer from a single normal shock at its highest operating speeds? Wouldn’t they ALL have an oblique shock system followed by a final normal shock. This shock system would have less loss than a single normal shock for a given inlet Mach number… It sounds like Xplane approximates the latter for all aircraft.

    What is recommended for aircraft developers to overcome this artificially elevated loss accounting in planemaker?

    • Austin Meyer says:

      Oh yes all of your comments are true!
      The NEXT level is to actually design the inlet (complete with moving cones and ramps and overflow ducts) and run all the shocks accordingly.
      So, that is the NEXT level! What we have now runs from total energy to normal shock across a range of Mach numbers to simulate the inlet losing efficiency at some point… and you SET where that normal shock happens in Plane-Maker engine screen… and you can set it to be FASTER than the redline on the airplane, and you will run out of steam before you get to that point for sure as the inlet efficiency deteriorates.

  8. Chris Ettinger says:

    Where can I get the CF-104?? Can’t find it…

  9. Milktrout says:

    Great news! Thank you!

    The broken supersonic model has been a shame for years.

    How are you going to handle drag? Even a close approximation with good documentation would be a huge step up.

    How will you handle SFC?

  10. Jim Zane says:

    Allow me to go off-topic for moment and ask you guys a question regarding networking. A couple of online demonstration teams who currently fly in another program have Asked me to relay this question. Will X-plane 11 allow for some robust multiplayer Interaction?. With a centralized lobby system? The reason I am asking this for them is because we all fly Together and I do create aircraft for this program And the other and have been with you guys (X-plane) since the very very beginning So many years ago. Personally I think it would be a really good PR boost to get some online demonstration teams flying Close formation routines in this program the way they currently do now in the other. What Do you think guys,?

  11. Stephan says:

    Ben, the update just went online but it gives us an error message regarding a navdata fail and regarding a problem with processing a GLSL pixel shader.

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