I was browsing Stack (great way to kill time at work) and I came across this question. One of the comments confuses me somewhat.
@Zlelik No, helicopter blades cannot exceed the speed of sound - even ignoring the massive damage that would cause, it would cause them to lose all lift, losing control of the craft (don't forget that while one blade is supersonic, the opposing blade isn't). Not everything has a linear relationship, you know - you need to understand how things scale. Neither 14 m @ 392 RPM nor 32 m @ 132 RPM give you a good idea o how 25 m @ 500 RPM would behave. – Luaan
Now I'm merely a layman, but it seems to me that if you're dealing with opposing rotors, they should both travel at the same speed when spinning. (Visually testable with a pencil, both ends should travel at the same speed when then pencil is spun, assuming the rotational center is the center of the pencil)
The only exception I can think of, is that in this particular case the author is specifically describing supersonic speeds. I'm aware that the speed of sound is different depending on the density of the atmosphere. Is it possible that this statement is true because the 'first' blade is leaving a vacuum that the 'second' blade travels through? (Thus changing the speed of sound for the second blade)
My intuition tells me that this is incorrect, as it should make it impossible to fly, much less control a rotor driven aircraft (You'd only generate lift with half of your rotor), in addition to the fact there is no real 'first' or 'second' here as they are equidistantly opposed in both directions of the rotor assembly.
So, do rotor blades really move at different speeds? If so, how is this possible?
Being a layman I will likely not understand any of the math behind this, however if you could include it for the sake of those that may, and summarize for those of us that don't, it would be much appreciated.