How much more can FSX handle? (be warned, lots of opinion)

[kevinh 409]

This 'debate' arose out of the difference between two assertions, 1) that flight simming as a hobby has a future based on the FSX platform, 2) that flight simming doesn't have a future unless it can simulate the physics of flight.

There is no argument that the Microsoft flight franchise is a platform dependent on lookup tables so it really comes down to whether or not lookup tables can do a good enough job to get the physics right to allow both assertions to be true. Kevin feels there is no problem here, but he has ascribed properties to lookup tables that are, in my opinion, rather generous. (I would use the term fanciful but I am trying to be polite). In particular, he contends "... that any maths model that produces realistic forces and moments is physics based." (which is of course true) and by inference, that lookup tables are, somehow, a mathematical model or can do calculations, which is ridiculous. This is something he has repeatedly suggested, and something that is fundamentally incorrect. Given a key, a lookup returns a value. If it is coded well, then given a key that doesn't match, it will return a 'best fit' guess based on the nearest matches. Kevin surprised me by mentioning the use of interpolation to get the value to use when you don't have an exact key match which is odd because it helps make my argument stronger.

 

Ridiculous eh? More attacks. Are you going to tell Boeing, Airbus, CAE, Thales, etc that their simulation methods are ridiculous while you are at it? Or is it just me you are having a go at in the safety of this forum?

 

If a function generation can accurately calculate, for example, basic lift coefficient from inputs of alpha and flap angle or mach number what else is required? No further complication is necessary because analysis of flight test data has provided you with that mathematical function relationship. Linear interpolation is just one method, but it happens to be the most commonly used.

 

 

 

An example is that all lift and drag calculations involve air density, but air density is not a constant and varies in proportion to compressibility which is associated (in a non-linear manner) to MACH (the ratio to the speed of sound) and this gets really messy from just below to just above MACH 1. The FSX platform the offers space in its tables for 17 Mach modifiers, which sounds great until you realize that they are in steps of 0.2M. In other words, there is only one entry between a slow aircraft at M0.8, and a supersonic one at M1.2 so there is just no way to model transsonic behaviors using FSX's table lookup table based system.

 

FSX is limited in its structure, it only has a limited subset of coefficients and derivatives and some things are simplified but the method it uses is not. In some cases step sizes are fixed as you say. I never argued that FSX "as is" is perfect, but you could expand the model using the "lookup" techniques and produce as accurate a simulation as the CFD method but far more efficiently and, more importantly, with more flexibility. Steps of 0.2 Mach are quite common in Boeing and Airbus aero models. Perhaps you think they aren't good enough too?

 

Even with it's limitations it is still possible in FSX to simulate transonic drag rise, for example, accurately enough for hobby and gaming purposes. More accuracy could be included in a development FSX or P3D. There is no inherent limit to the resolution of a lookup table model.

 

Point your aircraft of choice into a power dive and you have no problems pulling out. Try that in real life and you die. In the real world, when you push an aerofoil into the airstream, the airstream pushes back, causing the aircraft to react. In a physics simulation, you attempt (with simplifications) to determine how hard the airstream is pushing back and respond accordingly.

 

FSX doesn't model aeroelasticity, but the lookup method can do so if required. You only have to add the required functions to the model. A CFD based simulation would not be able to model aeroelasticity at all unless you added a complete structural model to the 3D shape. This is one area where the lookup table method wins hands down.

 

 

In a lookup simulation, you react based on the how much you deflected the control (with some modifiers). Even when you use good physics to determine the values that go into the lookup table, a lookup based sim is not attempting to simulate the physics of the aircraft, only to emulate its behaviors, and that works fine only as long as you remain inside the emulated behavior range.

 

With respect, a CFD based model would also only react according to how much you deflected the control.

 

 

A physics based simulation will break a wing (and the rest of the plane) into smaller parts and calculate the interaction of each part and its contribution to the whole. The more resources available, the smaller the parts that can be simulated and the more frequently they can be sampled, and more accurate the resulting behavior will be (wing tip stall anyone?). A lookup based sim will simply look up the value for the plane and lookup the value of modifiers. If the values in the table are accurate and there are enough of them, it can produce an accurate result, but only as accurate as its tables allow.

 

X-Plane does not calculate interactions between the elements it is broken down into. BTW, you haven't responded to my point that X-Plane aerofoils are based on lookup tables yet. I wonder why?

 

A lookup table simulation can easily account for tip stall. How do you think military flight sims (which also use function tables) manage to simulate such things?

 

In summary, you are arguing from a theoretical point of view with no regard to practicality. Do you seriously think that any game designer would put in the R&D effort required to model things on the lines you suggest?

[scandinavian13 5,192]

How did this thread get locked...?