If I'm not mistaken a scale of 1:500 means that for every 500 feet of real runway is represented by 1 foot in the scale model.
Therefore, for an 11,000 foot runway at a scale of 1:500 you must divide the 11,000 feet into 500 foot segments, the result will be the number of 1 foot segments you need to represent in your model.
Hence: 11000/500=22 feet on your model;
or else at a scale 1:1000: 11000/1000=11.
I might be wrong but I think thats the way to do it, hope this helps.
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