Author: Austin Meyer

X-Plane 12 Flight Model Update – Supersonic Transition, Delta Wings and Mass Properties

Oh my GOSH this is getting good.

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.

BOOOO!

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.

Bingo.

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.

What’s next?

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)

Check out this video: //youtu.be/iZiduQboyow?t=573

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!

🙂

P.S…. I just saw American Utopia on Broadway in New York City, and I have to say: It was incredible.

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!

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Experimental Flight Model Changes in X-Plane 11.40

LR customers can now get the first public beta of X-Plane 11.40 by running the installer, checking the box to get betas, and completing the update. Steam will most likely be available tomorrow, barring any unforeseen crises with this build.

Release notes here.


This youtube video has a thorough explanation of the new flight modeling stuff.

PLUS new stuff for “official” beta 1: Read More

Posted in Aircraft, Aircraft & Modeling by | 31 Comments

Better Fuselage Dynamics Through Science

It turns out that my teachers in college, a ton of engineering textbooks, and the internet in general all seem to understand what wings do. Also my airplane has wings, and those wings are designed to interact with the air as much as possible, so I can flight-test my airplane at any time (and I constantly do) to collect information about what the wings in said airplane do. And then I use the information from the several sources listed above to really dial in the flight dynamics in X-Plane. Without question, on my death-bed I will look back on my many flights flown while frantically scribbling down notes and flying the airplane at the same time fondly. This is a challenge that not enough people get to enjoy, and then turning that knowledge into a simulator that then turns into money for me… well, let’s just say I have very little to complain about. Read More

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X-Plane 11 Propeller Modeling

OK the new engine modeling for X-Plane 11 is great, but what good is an engine to us pilots without a propeller?

X-Plane has historically done an excellent job of estimating the THRUST of propellers, typically to within just a few percent… but what about the SPIRALING SLIPSTREAM? This has been an area where X-Plane has been much weaker… I just don’t see any good solid references for determining the spiraling slipstream angles for propellers…
and it’s a real shame because the spiraling slip-stream hitting the vertical stab is so responsible for the left-turning tendency in single-engine props.

BUT, can we do better? How would we estimate the slipstream angle, exactly?

Well, as it turns out, there is a pretty darn cool way to do it, which is going into X-Plane 11 Beta-4: A spinning prop is just a spinning pair or trio or quartet of wings (as X-Plane has long understood) and those wings have LIFT and DRAG.

The LIFT from the propeller blade is referred to as THRUST. The DRAG on the propeller blade is what opposes rotation and makes them so darn hard to TURN. Read More

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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)	// //www.grc.nasa.gov/www/k-12/airplane/normal.html
							* 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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Flight Model Improvements Done for X-Plane 11.00

Thanks to a few hundred hours of flight experience in my Lancair Evolution so far, I am really improving the flight model in X-Plane in the area of PT-6 engines, electrical, and pressurization systems! And, while in the systems code, I’ve improved a lot of other systems simulations as well, which is always fun.

So, here is the new stuff done for 11.00 so far in the flight and systems modeling area! Read More

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