I agree that theoretic top speed is limited only be drag (and other resistive forces) if you are able to adjust propeller pitch, gear ratio, or wheel diameter o

I agree that theoretic top speed is limited only be drag (and other resistive forces) if you are able to adjust propeller pitch, gear ratio, or wheel diameter on the fly. But if those values are fixed, as with Blackbird, then you are of course limited to a top speed that is some constant multiple of wind speed.