Systems Modeling and Physics Modeling

In past blog posts I have tried to contrast two very different different approaches to simulation, which I would loosely call:

• "Predictive" (or reality-based). Input data comes from real world measurements, and the simulator predicts the expected behavior based on those inputs.
• "Prescriptive" (or specification based). The input data describes the desired output directly through preset formula.

In particular, when it comes to the flight model, X-Plane is predictive, via blade theory (X-Plane adds up all forces) while FS X is prescriptive (a table describes aircraft behavior).

But - not all of X-Plane is predictive. In particular, the "systems model", which is a loose term for the modeling of electrical systems, hydraulic systems, and a number of other parts of the plane, is actually prescriptive in X-Plane, while the aerodynamics of the plane are predictive.

To illustrate: when your plane takes off, it leaves the ground because X-Plane has calculated and summed all forces and torques on the plane and has found that it has positive lift. You don't program this in - you simply provide a number of parts that, under some conditions, influence the lift calculations. (Wings are good for this, btw! :-)

But when you turn off the avionics master switch, X-Plane does not calculate the electron flow through the wiring harness to the glass PFD. Rather, the PFD is tagged with input data saying it requires the avionics master to be on. The PFD's behavior with respect to the avionics switch is prescribed, not derived. (If we had a true "wiring editor" in Plane-Maker, it would be different - X-Plane would trace electrical routes and see if enough current makes it to the PFD.

In truth the distinction is a little bit fuzzier. For example, the gyroscopes actually do model the spin of the internal gyro (fail the vacuum system and watch them slowly pick up error like in a real plane) and the electrical system does internal current calculations. But in general the systems modeling is a bit of a mix, mostly prescriptive. This is in contrast to the physics of flight, which are completely predictive.