It's fairly easy to code up the ISO standard atmosphere to 20-30 km in excel. When we get to that stage, we will probably build a test kit of some sort with a various tools for data reduction. Eventually we'll get a load of test data points from different sources and then plot them all onto a single large chart for each set of parameters of interest (e.g. one TAS vs altitude chart, one ROC vs altitude chart etc.).
It would still be useful to have tables from another source for crosschecking, but don't feel under any pressure because:
- There's not much point in testing anything until the sim settles down, which I expect will be a couple of patches after the US release.
- Obviously until then the sim is in a constant state of flux, so our results would probably be obsolete before we could publish them.
- The object of the exercise is to make the sim better. That means that it's only worth producing test results if the devs are likely to have time to read them, which they obviously don't at the moment, and won't until the other game engine related bugs are fixed.
- Additionally, a lot of the effort required comes from the fact that at the moment we don't have a lot of information about the assumptions underlying the model. We also don't have testing tools (like the old devicelink autopilot in IL2). In the longer term, when the devs are under less pressure, they might provide us with some helpful information, and indeed somebody might even come up with a new autopilot.
- There are therefore very strong arguments for waiting a while before we invest serious effort into this project.
- The way to actually attack the problem will be to code up a model and then interrogate it to derive the correct standard values for our exact test points. We don't want to be interpolating from tables if we can possibly avoid it because this leads to error and argument.
We're still probably going to have issues of course, because you'll generally find that standard atmospheres use geopotential altitude (it makes the maths easier; there's a nice discussion in the document which sets out the 1976 US standard atmosphere), whereas I somehow suspect that this sim might just use geometric altitude.
Indicated altitude will almost certainly come out as geopotential, because it's referenced to ambient pressure (if the model doesn't differentiate between geometric and geopotential altitude then the most likely fudge would be to just use the geometric altitude as input to a standard, geopotential atmosphere model, which is a small source of error), but "wonder woman" altitude will probably be geometric (WW alt was effectively radar alt in IL2, and thus geometric, but indicated altitude was true altitude above MSL because the altimeter pressure setting was fixed at QNH for the map - but this was probably also geometric because IL2's model was quite simple and it doesn't make much difference at low altitude anyway).
Converting between geopotential and geometric altitude isn't a problem, but explaining the differences to certain sections of the community could be a pain (it's only a tiny difference at the sort of altitudes we're going to be working at, but
if a job's worth doing then it's worth doing properly... and also, if people see a difference they're likely to make accusations of error and/or bias, because that's how the cookie crumbles - spot the jaded realist...).
06-15-2011, 12:03 PM
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