Beech19
in response to your last question. Imagine rowing a boat with a blade as thin as a matchstick, how many more strokes, or speed of stroke, you need to move the boat at a given speed. Then put a biger blade on the oar and you need less strokes, or slower stroke speed to make the same boat speed.
Also, for rotor speeds there is another factor. The JT9 fan blade at take off has the equivalent to a 42 ton truck hanging on the end of it due to centrifugal force. Now make a blade that weighs more than 2x the weight, it has to run slower to keep within current blade root technology and strength.
My 2 penneth,
N1 Vibes