RoyJ wrote:
Said fan curves are measured, not calculated. Fan design is a black art!!

Indeed, and it is amazing the efficiency of fans today (some at least,
anyway) compared to a hundred, or even fifty years ago. I suspect the
growth of computational fluid dynamics has helped to advance that black
art, however.

What is amazing to me are these model gas turbine engines with such
dinky centrifugal compressors. I have some old books on GT design that
indicate it would be impossible to design a GT with a compressor smaller
than about 12 inches and even have it sustain itself. What is even more
amazing to me is that these were originally designed by hobbyists.