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"mm" wrote:

You clearly don't know anything about ground resonance. It has little
or
nothing to do with the blade tip Mach number.

I don't pretend to be a fully articulated hub expert, but this is not
something I made up. It comes from an FAA inspector who makes his
living flying all sorts of helicopters. He IS an expert. Are YOU an
FAA inspector? Are you qualified to say that he is mistaken?

Think about the word "resonance". Do resonance and frequency go hand
in hand? By increasing the rotor RPM, do you increase the frequency
of rotation? Of course you do, its the same thing. If you sing a
perfect pitch into a tuning fork, does it also vibrate at resonance?
Yes. If you sing at a non-resonant frequency, does it still vibrate?
No. Are marching soldiers told to break step when crossing a bridge?
Yes. Why? Resonance. Think about this and what ground resonance
actually is.

Before you say this guy doesn't know anything about what he does for a
living, I suggest that you think about it a little while.

Dennis.

Yes, as a matter of fact I am qualified to say he is mistaken. I can
guarantee you that I know MUCH more about ground resonance than you or your
source. I challenge you to find a single reputable reference (text book,
technical paper, certification documentation, FAA training material, etc.)
that relates ground resonance directly to tip Mach number. Your source may
be the worlds ultimate helicopter pilot, but I don't think that this is much
of credential with respect to rotor aeromechanics and structural dynamics.

The original analysis of helicopter ground resonance was by Coleman and
Fiengold, NACA TN 3844. Read it, and will not find any mention of tip Mach
number.

Your tuning fork and marching soldiers examples are essentially meaningless
in this context.