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.

Technically described improvements

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 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.

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 probably 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 idle 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 turning 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.

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!

After going over the Pilots Operating Handbook and my old notes from my Columbia-400, we have a new manifold pressure model! This seems to be better than the previous one!!

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 discharge 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 yer good, that is.

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:

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’re 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.

BUT, 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

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!), 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 flight control realism at higher angles of attack then increase the challenge-factor further.

Rotor modelling

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 cyclid 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!

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!

Jet engine modeling

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.

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.

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 cowl flaps can be a joystick axis as well.

Other flight-model improvements

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 and ground service vehicles! 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.