New flight-modeling stuff done recently for 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!
PT-6 engine modeling:
Thanks to LOTS OF FLIGHT-test IN N844X, which has a PT6, I am getting that engine just right in X-Plane!
I got the RECIP engine performance dialed in when flying recip engine airplanes like the Cirrus SR-22 and Columbia-400, and now I’m dialing in the TURBOPROP performance now that I’m flying a turboprop!
Not only are the fuel flow and efficiency now correct.. but the engine just FEELS right.
But how does an engine FEEL right in a simulator?
How does that make any sense?
Well, the PT-6 engine has a distinct feel to it to operate.
That engine is all about having enough AIRFLOW moving through it to balance the FUEL FLOW to keep the temperatures under control.
The turbine is slow to respond at low power settings, since not enough air is racing through to rapidly change the turbine speed.
The turbine feels HEAVY to slow-moving air with little dynamic pressure, so the turbine is at first sluggish, slow to spin up!
But as the speed picks up and the airflow with it, the turbine seems comparatively lighter compared to the airflow moving through it and responds much more quickly!
So, adding fuel from a low power setting, temperatures spike as you add fuel at low turbine rpm, and cool as the turbine spins up.
The turbine sound is one sound and feel, the prop completely another since there is no connection between the turbine and prop! Only air between turbines connects the prop to the rest of the engine!
So a PT-6 engine has a real feel and response in turbine RPM, torque, fuel flow and temperature over time as you adjust the throttle, prop, and idle lever at different altitudes and temperatures,
always spinning up or down at varying speeds and temperatures as the fuel and air moving through it change.. it is really very Steam-Punk in its’ operation.
Now, TECHNICALLY described improvemens for X-Plane 11:
Much better engine ITT modeling for those turboprops, including response speeds on power, prop, or condition lever changes.
Better torque and fuel flow modeling as well.
The dynamic pressure through the power turbine across varying rpm ranges controls you can exchanging torque and rpm to get the same power,
for accurate cruise performance as you dial back the prop rpm but hold constant fuel flow, and glide with the prop feathered or not as well.
The new turbine model includes, by the way, compressor stalls, which I have gone through first-hand!
As well, I have done a lot of engine failure simulation tuning for hot start, ITT runway, compressor stalls and the like.
As well, the turbine idle is now floating point like everything in a real PT-6.
Move the red knob to move smoothly from low to idle, or hold it partway if you want to keep the engine temps JUST where you want, as you would in reality.
Just remember to tweak those idle speeds in Plane-Maker now to get your idles just right!
For turbines, you will want to enter a higher high idle than low idle now… X-Plane does not do this for you any more,
since you can now tune those fuel flows at idle as you like for yourself!
At low idle, you need to be above 52% Ng for PT6 engines.
At high idle, you want to be higher… the King Airs like to spin about 70% Ng to have enough speed to turn their air conditioner compressors!
The shop adjusts the idle stop points per-engine, per-aircraft, based on pilot or company desires.
I like my idle speeds a bit low in 844X so the plane taxis almost like it was carefully designed, and not trying to race away by taxiing at 50 knots!
So the low and high idle adjusts, set in engine window of PLANE-MAKER, should be set according to how your airplane is dialed in in reality.
For a king air you prolly want 55% Ng for low idle, 70% for high idle, which work out to about:
HI idle: 1.70
LO idle: 1.00
in Plane-Maker.
In other words, our low idle is just enough to run the engine at the lowest Ng recommend, and we bump that up by 70% at high idle to spin the generators and air conditioning compressors.
This is fun to do, since you can get that idle Ng just right for YOUR PT-6 setup!
NOTE: THIS WAY OF ADJUSTING IDLE SPEED IS A HAIR DIFFERENT FROM VERSION 10.
In version 10, the high idle was automatically boosted inside of X-Plane, but now it is not: You have to boost that high idle manually by entering a higher value for high idle in Plane-Maker.
Also our whole turbine model is different now which also means the idle speeds need to be re-tweaked in your planes.
In my REAL airplane, when you turn ON electrical system stuff, it DRAGS DOWN the speed of the compressor.
The compressor provides the cooling air that keeps the engine from destroying itself!
This is a BIG DEAL for pt-6 airplanes!
So as the generator load comes up, the turbine Ng does DOWN, and the ITT comes up… possibly enough to destroy the engine!
One day when it was really hot and I was taxiing with the air conditioning on and I taxied into a tailwind, the ITT started to go to REDLINE!!!!!
I quickly turned off the air conditioning and the ITT quickly came down!
I said “whew”, advanced from LO IDLE to HI IDLE to get enough compressor speed to support the electrical draw of the air conditioner, and THEN turned the AC back on!
So why was the idel speed set too low to handle the air conditioning on a hot South Carolina summer day?
Because I had just had the idle adjusted to be lower… IN OREGON, WHERE IT IS COLD, AND THEY DON’T NEED AIR CONDITIONING! HAR!!!!!!!!!!
Now, at this moment in X-Plane, I am making all of this DEFAULT behavior
It’s SO EASY!
The aircraft author ALREADY enters the electrical load in Plane-Maker for each system.
The aircraft author ALREADY enters the generators and what engines they attach to.
The aircraft author ALREADY enters the horsepower of the engine.
So now, as I am coding right this moment, X-Plane looks at the amperage and voltage of each generator hooked to each engine.
What is amperage times voltage? POWER!
So we can MATHEMATICALLY FIND the power that is sucked from the compressor by the generator!
And, yes, as you turn stuff on, the generator load will go up, the Ng will come down from the drag, and the ITT will go up!
So it is all baked right into the model.. no user-mods needed.
As you increase electrical load on the airplane, that load will be passed to any turned-on generators which will drag down any engine attached to those generators,
changing power output and temperatures accordingly! COOL!!!!
All of this is done by the direct application of load to the engine.. there is no ‘faking it’! If you have more electrical load on a smaller engine, then truning on your various electrical devices will have a greater impact on the engine,
especially noticeable at idle!
While this is surely most noticeable on the PT-6, I have applied the affect to the pure jets and recip-engine airplanes as well, and also applied the drag on the engine from the pressurization system, when applicable.
So we now have accessory drag on the engines.
This is real, and very important.
Going over the Pilots Operating Handbook and my old notes from my Columbia-400:
OK we got the new PT6 model now lets take a look at the RECIP engines.
I’ve gone back over all my old flight-test data from my previous airplane (Columbia-400) and overhauled the manifold pressure model quite a bit.
This seems to be quite a good bit better than the previous one!
As well, other reciprocating engine improvements:
We track the fuel in the cylinders or carb from the prime or simply running the fuel pump when the engine is not running!
So the engine starts with a bang if you primed it enough or too much, or just barely rumbles to life if not.
And, yes, in an emergency, you can fly the thing on the primer if the engine driven pump fails!
That was not custom-coded.. it works out because I coded the dynamics of the system!
And finally rather than just using ‘rules of thumb’ for carburetor ice, I now actually find the air temperature in the carburetor based on adiabatic expansion and the ideal gas law for air.
Then, any carb heat is applied to raise that temperature if you have applied carb heat.
Once I know the temperature of the air OUTSIDE of the carburetor (is it above freezing?) and the air inside the carburetor (is it below freezing?) I add or melt ice as appropriate.
There are NO rules of thumb here… this is adiabatic gas expansion in first-principles! Cool!
So here is where this gets so interesting: The ideal gas law says that it lower pressures, we get lower temperatures.
When the throttle butterfly valve is closed or partially closed, a low pressure exists in the manifold which is what keeps the engine running slower.
That low pressure, and adiabaitc expansion under the ideal gas law, are what give the low density and temperature inside the carburetor.
Interestingly, at IDLE, the temperature in the carburetor is SO LOW that ice will not form! It is TOO COLD, because any moisture in the air has already gone to ice crystals that will not adhere to anything.
It is at PARTIAL POWER that there is JUST ENOUGH pressure loss to give just enough temperature reduction to form that carb ice.
Now, here is the next interesting thing: On a cold day, carb ice will NOT form because the cold air outside will hold little moisture! It is on HOT days that the air holds moisture that can turn into ice in the carb!
So, carb ice feeds on the differential between a HOT day with plenty of moisture in the warn air outside, JUST THE RIGHT restriction from a partial throttle to cool that air right down to the freezing point on the INSIDE of the carb.
At that point: BANG. Carb ice. Hit the carb heat to throw that equation out of balance with some warn air into the carb.
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.
Now, X-Plane has already PRECISELY determined the lift and drag on each little bit of each propeller blade, in the 12 o’clock, 3 o’clock, 6 o’clock, and 9 o’clock positions to get all the right p-factor and other effects.
Now imagine one of these propeller blades spinning for a second putting out both LIFT and DRAG.
What happens to the air in this case?
Clearly, the LIFT acts as THRUST, pulling the plane forwards (and kicking the AIR AFT!)
Clearly, the DRAG opposes ROTATION, retarding the rotation of the propeller… but in so doing it is guaranteed to DRAG THE AIR ALONG WITH IT DUE TO FRICTION BETWEEN THE PROP AND THE AIR!
So, how MUCH does the prop drag the air with it as it rotates, forming this spiral stream of wash behind the prop?
Here is my theory:
The if the PROPWASH is proportional to the LIFT from each propeller blade,
then the SPIRAL is proportional to the DRAG from each propeller blade!
Put another way, if the propeller is putting out TEN pounds of lift and ONE pound of drag,
then the equal and opposite reactions from the air will be in equal proportion: TEN knots of propwash for every ONE knot of spiral!
You see the ratio here? If LIFT pushes the AIR AFT (conservation of momentum! every reaction has an equal and opposite reaction!), then DRAG drags the air along behind the prop in a spiral pattern…
and the ratio of drag spiral speed to the propwash is the same as the same ratio of drag to lift of the prop (whew!)
Equal and opposite reaction happens just the same for drag as for lift!
Another example:
A small bit of a propeller blade puts out 20 knots of lift (thrust) and 2 pounds of drag (opposing rotation).
Thats 20 pounds of lift for every 2 pounds of drag.
Now, due to conservation of momentum, we might prove that this prop has 50 knots of propwash.
So what will the side component of propwash, or the spiral sideways motion of the air be? 5 knots, because the drag is 2/20ths, or 1/10th, of the lift,
so the sideways drag on the air 2/20ths, or 1/10th, of the propwash!
If I am understanding conservation of momentum here, then I think that this is the key to understand spiral slipstream from the prop:
The more drag on the prop (the less efficient it is) the more the spiraling slip-stream!
The less drag on the prop (the more efficient it is) the less the spiraling slip-stream!
Specifically the ratio of spiral to propwash is the ratio of drag to lift on each bit of the prop, since that is simply the direction and magnitude of the forces on the prop, and the displacement-rate of air that MUST exist to cause those forces!
(EQUAL and opposite reaction!)
So, in X-Plane, we of course break the prop down into tiny little pieces and add up the effects from all of them to get a weighted average of the spiraling-slipstream speed for the entire prop, and scale that rotational speed from zero right at the axis of the prop hub to maximum out at the prop tip, and bang, we got a spiral slipstream with math that I believe proves that we are making a very good approximation.
(And, doing the math in this new way results in propeller spiral slipstream that runs about 45% higher than the previous model… a positive indication since the spiral slip-stream was previously under-represented!)
So, now we have better propeller spiraling slip-stream, with the needed rudder effects, and it feels GREAT to fly.
Rotor modelling:
AAAAAND now that we are on PROPS, lets really test and tune that ROTOR model!
This is a modest upgrade to the rotor model for helos. We have some small internal re-organization, and significant tuning, to really nail the performance in:
->Climb and cruise,
->Effective Translational Lift,
->Setting with power,
->Settling WITHOUT power
->mast-bumping, with mast-bumping limits in disc tilt set in the Plane-Maker window where you enter your cyclic deflections.
These tweaks really dial in the rotor performance to another level of refinement, which has been really fun to flight-test in the sim, for sure!
Jet engine modeling:
OK so why stop at props?
Let’s go FASTER!
OK I overhauled and upgraded the jet engine model as well.
So here is a thing about jets: They get thrust based on the amount that the ACCELERATE the air coming out the back of the engine.
Here is the problem, though: All non-supersonic jet engines can only accelerate the air up to near the speed of sound… a little bit under Mach-1.
So, as the aircraft accelerates, the thrust of the engine deteriorates because as the INCOMING speed gets closer to the OUTLET speed, the thrust goes approaches zero!
So jets LOSE thrust as they accelerate, since the speed differential between input and output deteriorates… the jet can’t air accelerate the air as much since it already moving!
Now, if this were all there was to it, then jets would become pretty useless around 300 knots, like propellers are.
BUT, jets have a trick up their sleeve: Inlets. The INLET to the jet engine can SLOW the air and PRESSURIZE it, carefully using ram-air effect, to deliver slow-moving, high-density air to the engine!
So it’s all about the inlet slowing and pressurizing the air.
And now X-Plane does that starting with version 11, and here is how you control all that in Plane-Maker as you design YOUR jet airplane!
Fire up Plane-Maker 11 and go to the ENGINES window, and then the JET CURVES tab.
Though JET CURVES sounds like a really really bad James-Bond spinoff movie, what it ACTUALLY is is the thrust that the jet engine will put out in X-Plane, and when you understand the text above
you will see why the curves are shaped the way they are in Plane-Maker… go to that window right now so you see the curves as you continue reading.
First you see the many curves for many altitudes, and of course the thrust at higher altitudes is lower since the air is thinner.
Now, look at the left side of the curve to see the thrust at it’s maximum.
Then as you speed up, the thrust deteriorates. OOPS! That is the incoming speed building up, so the difference between entry and exit speed (the thrust!) falling apart.
BUT WAIT, THERE’S MORE! As you speed up towards Mach-1, you see the falloff start to ease, and then the thrust actually INCREASES! YAH! That is the INLET doing its’ part to capture the
RAM-air effect and deliver some nice high-density air to the engine compressor.
Look at the lower left where you enter the bypass ratio. Higher bypass ratios are more affected by the speed-based falloff since they have slower exhaust.
Then look at the inlet compression efficiency: This is how much of the energy is kept as the air is slowed and compressed by the RAM-air effect. Usually, most of it, for a properly-designed inlet.
Then see the normal-shock Mach number. This is the speed at which normal shocks start to form across the inlet, and efficiency is lost. An inlet designed for subsonic flight is likely to have a value of a bit less than 1.0 here,
since it is not designed to accept air near or above the speed of sound where those shock-waves start forming.
Only a supersonic airplane is likely to have a Mach number greater than 1 here!
As the inlet is dragged by an over-speeding airplane above it’s critical Mach number, normal shocks will now form across the inlet, DECIMATING the efficiency of the engine and robbing you of thrust.
The normal shock, only a few atoms thick, slows all air that hits it across the space of a few atoms, dumping a huge amount of the incoming streams valuable kinetic energy and turning it instead into HEAT.. the last thing you want coming into the front of your engine.
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:
constxflt gamma =1.4 ;
constxflt gamma_m1=1.4-1.0;
constxflt 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)// https://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, in addition to the subsonic thrust effects mentioned above, which is nice.
Also we don’t JUST want the engine to work properly at idle and full thrust… we want all the N1 settings in between!
SOOO I got a local charter company to run their LearJet from minimum-loiter speed all the way up to redline speed in steps, carefully measuring the N1 and speed at each step.
Then, in case they made any mistakes, do the whole thing all over again SLOWING DOWN in steps, from redline down to loiter.
By carefully looking at the speed associated with each N1 value, I mapped the thrust as a function of N1… this is the thrust CURVE, indicating what FRACTION of your max thrust you get at each N1 setting.
The result?
In flight, a jet engine like the ones found on the Lear-35, you get about 50% of your thrust at about 82% of your N1.
At an N1 of 50%, you only get about 9% of your max thrust!
So armed with that information, as well as thrust-versus-N1 curves for a number of engines, I found the following:
At very LOW speeds (like for idle and taxi) the thrust from the engine goes with the SQUARE of N1. This is not at all surprising, since DYNAMIC PRESSURE goes with the SQUARE of the speed.
And of course the thrust is scaling with the dynamic pressure on the system. So if you run the fan twice as fast, you get four times the thrust.
But, at high speed, something DIFFERENT starts happening: The thrust starts running with the CUBE of the N1!
This must have something to do with the way the engine is only operating in its’ performance envelope as it approaches 100% N1, though I welcome a more technical evaluation on this from any reader.
Now, while I found from experimentation that the power curve of thrust versus N1 is about SQUARED for low N1, and CUBED for high N1, there will surely be SOME variation across engine type.
So, in Plane-Maker, ENGINE window, DESCRIPTION tab, right side of the screen, we have “thrust power curves with N1”, which you can enter.
We DEFAULT to 2 at low N1 and 3 at high N1 (with a smooth interpolation in between of course) but you can tweak these numbers as desired if you want to really dial in the thrust across a wide range of power settings!
So, jet simulation has been improved now for V11, especially in the supersonic regime… because getting that F-4 PERFECT is just going to be soooooooo cool!
Tire-force modeling.. WHEN THE PLANE IS NOT MOVING!!!
OK maybe my sense of humor is all screwed up after 2 weeks of straight coding but this one is just too funny.
After all this SPEED, let’s perfect the flight model for when we are NOT MOVING AT ALL!!!! HAR!!!!
For X-Plane 11 beta-3, I just solved a bug (pointed out by Vit Zenisek) that has actually been in X-Plane for 20 years… and only affects the motion of the airplane when it isn’t moving.
Got it?
Here’s the dynamics of the non-dynamic situation:
The tire force model in X-Plane is good enough to use in a driving racing simulator, as it actually gets right down to the vector along which the rubber is dragged across the pavement on the contact-patch of the tire.
The dynamics are really quite good, especially in X-Plane 11 where I have taken tire-modeling updates from Stradale.
BUUT, this physical model has fatal flaw: The model that simulates the detail right down to how the rubber interacts as it is being dragged across the pavements….
only works WHEN THE RUBBER IS BEING DRAGGED ACROSS THE PAVEMENT! DUH! So when does it NOT work? WHEN YOU ARE STOPPED!!! HAR!!!
So, whenever an aircraft in X-Plane has been STOPPED, I simply ‘locked the airplane down’, bypassing the tire model altogether.
No motion? No flight model!
This SOUNDS fine, right?
WRONG!!!!!!
During the run-up, the plane is indeed motionless, but the forces acting on the airplane, via the landing gear, are HUGELY important!
As you add power, for example, the force opposing propeller thrust is COMING FROM THE TIRE CONTACT PATCH FAR BELOW THE PROPELLER!
This aft force, far BELOW the prop, opposing the forward motion of the prop, creates a torque that LOWERS THE NOSE when power is applied with the brakes on!
You sure feel this on short-field take-offs, when you add power, holding the brakes, and the nose hunkers DOWN
Then, when you release the brakes, the nose POPS up as the nose-gear strut is unloaded and it is off you go!
So, even though the airplane is NOT EVEN MOVING during the run-up or power application before brake-release, the forces on the landing gear and resulting aircraft dynamics are CRITICAL to making the X-Plane aircraft behave, and feel, like the real airplane!
SOOOO, how do we BUILD a tire model that is based on MOTION, but still works when the plane is STOPPED?
SIMPLE!
We simulate a WELD!
When the plane is stopped and the tire forces are adequate to HOLD it there, we imagine that the tire contact patch is WELDED DOWN TO THE GROUND right at the center of the tire contact patch! The force on the airplane from the tires is a damped spring that opposes any displacement of the aircraft from that welded-down spot! Any (small) displacement from that world-point of the tire contact patch is due to the flexing of the tire sidewall, allowing the axle to move ever so slightly fore and aft as the tire flexes under the loads of the engine, wind, a sloped runway, or whatever else it is that is trying to move the airplane!
SO, when STOPPED, we weld the tire contact patch to the ground with a damped spring simulating the tire sidewall that holds you in place with, indeed, some FLEX!
Then, as the brake are released OR the forces on the aircraft EXCEED the braking allowed by the tires… we switch over to the rolling or dragging dynamic tire models as needed!
Cool!
The whole thing happens seamlessly, and the effect is really quite amazing.
With the Cessna 172, for a short-field take-off, get all the way on the brakes and go to full power.. the nose starts to dive under the thrust!
Then, get OFF the brakes and the nose POPS up and oscillates as the nose strut unloads, over-extends from the aircraft inertia, and oscillates a few times until the motion is damped out, as the airplane starts to accelerate down the runway!
It feels JUST like the real plane!
Control-effectiveness improvements:
Andd all these engine and tire effects are useless without the control-effects to GUIDE them!
SOOOOOO…..
Control effectiveness at high AOA reduced according to wind tunnel results.. you lose it all by around 45 deg AOA… and a good solid 30% of it around 20 deg AOA
(this is in addition to losses due to dynamic pressure and local flight path no longer being aligned with the airplane, of course!)
So, this makes the stalls a good bit scarier… that control deflection comes down for the recovery!
And, if the stall is ICE-induced, where the ice lowers the stalling angle of attack, well, that plus reduced control effectiveness in the stall makes for some pretty scary stalls!
Based on information from a TBM-850 pilot that has done some stalls in his airplane when iced (by ACCIDENT!):
And these control effects do not always work properly in REALITY…. not when covered in ICE!
So, ICE is QUITE a different experience now.
QUITE different.
A customer sent me a video of him stalling a TBM-850 with ice on its’ wings… it stalled WAY earlier than he planned.
So now, rather than just adding weight and drag and reducing lift, which is what they teach you and what X-Plane used to do, we NOW lower the stall angle of attack as the ice builds as well.
This can lead you to think that everything is mostly ok with only a bit of ice, and then WHAM! That stall bites fast and hard, sooner than expected! A nasty stall at a much lower AOA than you expected!
Then you have to recover without exceeding a much lower-than-expected AOA, with limited lift and extra weight and drag… which means you need to re-evaluate your new stall AOA from that first stall
and not let yourself get up to that AOA level again to hit a SECONDARY stall!
This is where the skill requirement shoots up through the roof.
So the ice is much more realistic.. which results in it being more terrifying, by far.
The new improvements in flgith control realism at higher angles of attack then increase the challenge-factor further.
And then on to plenty of systems modeling to get the whole airplane right:
Pressurization modeling:
Also thanks to my experience in the Evolution N844X, which has has several pressurization failures in reality, in X-Plane I now have a whole new pressurization model!
In reality as well as now in X-Plane, you have to carry enough Ng (gas generator RPM) to hold up that pressure, and now we have fractional pressurization available to hold SOME pressure, but maybe not ALL pressure, as in the real airplane!
We look at the ratio of bleed air available and what part of the engine it comes from to see if we have adequate bleed air inflow to the cabin based on the current engine RPM, and local atmospheric pressure!
The higher you are, the more power you better carry to keep cabin pressure, as in the real airplane!
The cabin altitude will climb if you don’t.. and how much it climbs will depend on the air density outside the plane and the gas generator speed on a turbine!
How much power you need to hold pressurization depends on the altitude and even baro pressure setting, since this is hooked to air pressure! Cool!
Also a more efficient inlet pressure recovery and more speed gives more pressurization.. because the INLET pressurizes the air before handing it to the engine to pressurize further, as in reality! NICE!
Electrical system modeling:
The electrical system code is overhauled, with new models for generators and batteries, all connected though the various buses and cross-ties.
You can hop in a plane, take it up high, fail the engines, see the generator output sag based on the low engine RPM while gliding,
watch the batteries in partial dis-charge due to low generator output, start the APU, turn on the APU generator, watch it power systems and charge the batteries,
turn on too many systems and watch the APU get overloaded and have it’s amperage sag and the avionics flicker on and off, load-shed to get within
the APU amperage, turn off the batteries to avoid trying to charge them, and bring the airplane home on APU power only.
If y’er good, that is.
Hydraulic system modeling:
Hydraulic systems have a bit more oomph, delivering at or near full actuation power at idle when engine-driven, as they should.
So really dialing in these physical systems models here for 11.00.
Pitot-static modelling:
Now we have more realism in the LAG of the airspeed indicator, which is really noticeable in a Columbia-400 doing a short-field take-off,
and also the correct reactions when the pitot tube, static port, both, and neither are iced over to infinitely-variable fractions as well.
Other systems modeling:
For roll with with elevator, yaw with rudder, aileron with pitch, the TRIMS now apply there as well!
So if you use those controls, X-Plane now gives you the TRIM as well. (it did not before).
Updated electric motor dynamics as well!
Now more accurate with battery depletion.
I have a sense of how electric motors and re-generative braking work now from (wait for it) our family Tesla!
Now with cowl-flap drag!
Set it in Plane-Maker!
Drag scales with cowl flap deployment! Cool!
I’m told it makes 15 knot difference on the Mooney Encore (!)
Set the drag as need for your plane!
Now with cowl flaps can be a joystick axis as well.
There is now a BUTTON for boost, so in the engines page where you enter water injection or NOX or other boost, you gotta turn it on with the button to get it at max throttle.
The nosewheel steering model is a hair refined: We go from max to min nosewheel steering as the speed picks up as always,
but if we have a tiller axis assigned, then we add the tiller and nosewheel steer, like real airliners. Cool!
Engine specific fuel consumption now scales with density not altitude, which is more accurate.
Propeller gyroscopic forces now fixed.. they were not quite right before but are now.
Even though prop gyroscopic forces are fairly small in most cases, we have them mathematically perfect now!
Other features and refinements as requested:
New view option: Lock to point.
Lock onto any spot on the ground and you TRACK it!
Nice for VFR pattern, etc.
Slung loads now rotate as they should on the end of the cable, so they look pretty decent now.
Radio altimeter has +/- 40 degree scan, so up to 40 deg of aircraft bank it is correct.. then it goes slant range… and of course useless at 90 deg bank or more.
More cockpit instruments and controls transmitted to external cockpits to really keep those external panels in sync with the master machine.
Unlimited weapons attachable to the aircraft in Plane-Maker, with that weapon action saved in replays as well, and a more efficient snapshot structure to recall the replays also.
Major new features:
Pushback trucks!
Ground service vehicle!
All with dynamic flight models and stuff and paths that they drive that are different for every airport so they never do the same thing twice… it’s all dynamic.
Overall:
These are just tiny little bits of notes I took while coding.
The real exciting thing here is the internal file formats:
Everything is now designed, internally, to be object oriented, extensible and flexible, so we can add stuff in the future without breaking file formats.