Fast helicopters

"Bart" wrote in message
...
I don't mean to further yet another moronic discussion about yet
another pie in the sky (lol) rotorcraft, but MM, regardless of
who he is, is correct. Ground resonance has nothing to do with
rotor tip speed. Its mainly germane to helicopters which have
shock-struts as the main rotor rotational rate crosses the
resonant frequency(ies) of the struts. It tends to be exaggerated
and more dangerous in machines with four struts and three blades.
Nothing has ever caused my Jetranger to enter the onset of
ground resonance. Know why?; Two blades, No struts....Hmmm.

Mach number my eye, and who the heck cares about ground resonance
in a discussion on increasing Vmax?

Bart

What a nice change, someone who does actually understands something about
the issue!

The reason that a JetRanger can not have ground resonance is that it does
not have lag hinges or other source of flexibility in-plane. If the rotor's
natural lag frequency is higher than the rotor speed, as is the case with
the Bell teetering rotors, it is impossible for it to have a ground
resonance instability. Rotors with this characteristic are called
"stiff-inplane". You can have a stiff-inplane rotor with more than 2 blades
(BO105, BK117) and they too are immune from ground resonance. (If look at a
BO105 or BK117 hub you will not see any lag dampers.)

The struts are associated with ground resonance, but not in the way that you
seem to think. They do not cause the problem; they are there to provide the
damping needed to stabilize the system.

If anyone is interested, we could have a nice little productive thread on
ground resonance. If you do care about this subject, though, please, ignore
that guy "Dennis". He really, really doesn't know what he is talking about
on this subject.