Here is an interesting post from Austin - delivered in his usual, unique way…but what he is talking about is quite interesting…
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!