The Chinook Cha-Cha

Different Helicopters
come with different types of Rotor Systems:
Rigid,
Semi-Rigid,
Fully Articulated,
and hybrids of the above.

As you might expect,
each of these systems has advantages and disadvantages.

The U.S. Army's big Chinook
has a fully articulated rotor system.
This rotor can be compared
to the connection of your arm to your shoulder.......
the individual rotors can move fore and aft,
up and down, all at the same time,
similar to your arms moving independently.

The fully articulated system is a complicated one,
with MANY moving parts,
but when properly balanced and tracked,
it provides a "Cadillac on the boulevard" ride.

One of the disadvantages of this system
is that if you shock it with a hard bump on landing or takeoff,
the rotor can get dramatically out of balance,
and that unbalanced condition can amplify and worsen catastrophically.

Videos
http://www.chinook-helicopter.com/video/Ground_Resonance_Rear_View.mpg


and
http://www.chinook-helicopter.com/video/Ground_Resonance_Side_View.mpg

show two views of the same helicopter experiencing
this phenomenon,
called "Ground Resonance".

I assume the fact that the helicopter is restrained on a test bed
with two detailed videos of the incident
is an indication this was an Army test.
Notice the puffs of smoke from the engines,
indicating they have been shut down
as the Ground Resonance begins to destroy the airframe.

I have no information on what this test was analyzing,
or if Ground Resonance was expected.

I'd like to know more.