Friday, April 06, 2007

X-Plane vs. Reality

A few days ago, Austin posted part of an, um, "animated" discussion between himself and an author regarding blade-theory vs. table-based flight models. (You can find it on the xplane-news yahoo group message archive.)

I'd like to ignore the whole "my flight model can beat up your flight model" thread and look at one of the side effects of physics vs. table based flight models.

In this previous post I commented on the nature of specifications in a flight simulator.

- Some data simulates real-world data. ("Reality-based") The sim has open authority to interpret this data for maximum "quality" and the standard is "how close to reality are we". The specification clearly sites reality as the authority on behavior.

- Some data is arbitrary and has a clearly defined interpretation. ("Specification based".) The behavior of the computer program is unambiguous.

What I find interesting is that a blade theory flight model is a "reality-based" flight model; a table-based flight model is a "specification-based" flight model.

What this means is that, just like reality-based specifications in the scenery system, you can't tune your flight model in X-Plane to achieve a desired end result without understanding the real-world meaning of the parameters you are changing, or you risk a compatibility problem with future versions of X-Plane.

Imagine that, for some reason, your plane seems to feel sluggish when turning. So you increase the area of the control surfaces and the problem goes away.

You can't do that with X-Plane's flight model! The area of the control surfaces mean something other than "a variable you can change to affect how the plane turns". They have to match how the real-life airplane is built. If you increase the area, straying from reality, to "fix" a problem, what really happens is you create a new problem later when X-Plane goes to simulate your model.

Simply put, if you put intentional errors into your plane's flight model to compensate for limitations to the sim, any improvement in the simulation accuracy of X-Plane is almost guaranteed to make your plane fly worse in the future.

So one of the important differences between a table vs. blade-theory flight model is how you talk about "bugs". If your plane doesn't fly the way it used to in a table-based flight model, that's probably a bug (well, depending on how interpolation is done). In a blade-theory model it's not a bug per se.

In a table-based flight model, how the real plane flies is moot - the table is king. In a blade-theory flight model, if the real plane flies differently and the input parameters of the plane are the same, it is a bug, or perhaps a design limitation.

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