Scaling (the change of material properties as you change the size) is behind all the great physics disasters. It has played a big part in my problem with carbon warming.
But here, we ask ourselves -- why is there a limit to bird size? Why haven't flying birds evolved to be monsters, even during the ice age?
Nobody asks that question, but it's a good one. It's because the concept of a feathered wing flapping away, has a scaling limit. As you scale larger, the Raleigh number changes, and turbulence dominates over laminar flow.
For the wing to work, you need just the right amount of 'thick' and 'thin' air. A wing can only work at the smaller scales. If you go smaller, to insects, you find they struggle to generate turbulence, and have to use tricks, like smashing the wing tips together.
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