I am trying to work out the formula used for this graph, I don't really know where to start, does anyone have any suggestions? Its a bit unclear, its showing FAT, Mach No, CIT, FWD B/P Position.
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First you need to figure out what are the inputs and what are the outputs. For the top graph, I would guess Mach No and FAT (is that some air temperature?) are the inputs and B/P positions and CIT are the outputs-you find a point in the center grid and read off horizontally and vertically. This looks like a reference for flight-you look up the flight conditions and it tells you how to set the aircraft controls. Collect a bunch of data by reading off points-for example at the intersections of the grid. Each point should be (Mach, FAT, B/P position, CIT). Then look at the curves and guess a functional form. You might even want to replot the data as two graphs with Mach and FAT on the axes and each of the other parameters as z and look at what it looks like. Finally feed your form to a multidimensional fitting routine to find the parameters and see how good the fit is. If you're not happy, try another form until you are.
If this sounds non-rigorous, that's because it is.
You could try plotting each of the paths as a separate graph and work out the formulas for each one, then try working out a formula that links the separate graphs together.
Or read all the data off and break it down into a spreadsheet so the data can be analyzed more easily. Then you can re-plot the data in different ways and see how that works out.