X-Plane 11 beta3 – скоро

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’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-modelling 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 ARTE 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, so that it 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!

You will get this starting in X-Plane 11 beta-3, and I am really annoyed we have not had this for the last 20 years!

(Because in the version of X-Plane you have right now, all of this is missing which SUUUUUUUUCKS!)

So grab X-Plane 11 beta-3 when it comes out (soon) and try some of these brakes-on, full-power, brakes-off short-field take-offs in light airplanes.. it finally feels real!

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!

One day when it was really hot and I was taxiing with the air conditioning on and I taxied into a tailwind, the ITT start 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!

Now, at this moment in X-Plane, I am making all of this DEFAULT behavior.

It’s SO EASY!

The user ALREADY enters the electrical load in plane-maker for each system.

The user ALREADY enters the generators and what engines they attach to.

The user 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!!!!

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 as well, when applicable.

So we now have accessory drag on the engines.

this is real, and very important.

in free-turbine airplanes, especially, turning on the air conditioning requires you to advance to hi idle BEFORE you turn on the air conditioning or you will destroy the engine!

here is what happens:

-at low idle, the air conditioning drag on the generators slows the compressor

-the compressor slowing causes the ITT to rise

-the ITT ride can DESTROY the engine!

this is problem on lancair evolutions AND EVERY KING AIR IN THE WORLD.

king-airs must go to a VERY HIGH IDLE to be able to run their air conditioning or the air conditioning will snap OFF to prevent it from destroying the engine!

in the jet stream javier you entered a free turbine (this is wrong! that plane has a FIXED turbine!) and the electrical load is so high that it is dragging the compressor down

you need to have lower electrical system load, or a higher idle!

right now, the plane is in an emergency: not enough compressor speed and power to turn those generators!

this is NOT a bug in x-plane.. it is a new system simulation that makes the sim better!

but you and perhaps others will need to tun down you electrical amperage in the systems pages in plane-maker or go to hi idle, or turn up your low or hi idle, to turn the generator (which is hooked to the compressor) fast enough to make your needed generator amperage!

now the story:

in my evo this summer when it was hot, i turned on the ac when i had the plane at a very LO idle with a tailwind during taxi… the ITT in my airplane started to go to redline!

i almost ruined my engine!

i saw it and instantly turned off the generators!

the ITT fell as the generator turned off and freed the compressor to turn as it liked and send the cooling blanket of air through the engine

then i went to HI idle to get plenty of power from the compressor, and generator back on… no problem!

why did this only just happen after i had the plane for 2 years?

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!!!!!!!!!!

so, that is what is going on!