Thanks for explaining. I now follow what you were trying to do, but I think you have other errors in your math as well.
If you used 2pid instead of pi*d, you’d get twice the actual circumference and twice the linear velocity at the tip. But even if you used the diameter of the largest wind turbine currently made, you’d get a tip velocity of about 620 mph, or Mach 0.8. This is still not especially close to the speed of sound.
Radians are different from degrees, but they’re not hard. 360 degrees is 2*pi radians. Another way to think about radians is that one radian equals about 57.3 degrees.
Radians are very useful and not conceptually different from degrees; they’re just another unit of measure. It’s a little like the difference between miles and kilometers.