They don't make them like they used to. Chopper distintegrates after landing.

[jollyjacktar]

Shared with the usual caveats.  Full story and video of it at link.

Helicopter disintegrates after landing in Brazil
CBC News Posted: Feb 24, 2012 11:35 AM ET Last Updated: Feb 24, 2012 11:26 AM ET

A rescue helicopter shook itself to pieces after landing in the northern state of Para in Brazil Wednesday.  Brazil's Regional Service for the Investigation and Prevention of Aeronautical Accidents is investigating the reasons. Amateur images taken by the Para Fire Department showed how the aircraft came apart while vigorously vibrating.  One suggestion is that the helicopter may have experienced what is known as "ground resonance," which occurs when the aircraft's rotor is working while the helicopter is on the ground.  Four people that were travelling aboard the helicopter were injured.

http://www.cbc.ca/news/world/story/2012/02/24/helicopter-brazil-breakup.html

 

[Good2Golf]

It's the mechanical equivalent of the screeching feedback you hear when someone brings a microphone too close to a nearby speaker/magnet.  You either roll the throttles off, or pull up into a hover to stop the resonance.  The issue is usually experienced more on helicopters with shock-absorber equipped landing gear, but fixed skid gear can also transmit forces necessary for resonance to occur.

One explanation of Ground Resonance here.  A web page showing an experiment to investigate ground resonance here.


Regards
G2G

 

[OldSolduer]

Very interesting! Thanks!

 

[Jammer]

Wow! Just wow!

 

[GAP]

It's the mechanical equivalent of the screeching feedback you hear when someone brings a microphone too close to a nearby speaker/magnet.  You either roll the throttles off, or pull up into a hover to stop the resonance.  The issue is usually experienced more on helicopters with shock-absorber equipped landing gear, but fixed skid gear can also transmit forces necessary for resonance to occur.

One explanation of Ground Resonance here.  A web page showing an experiment to investigate ground resonance here.

Regards
G2G

Is it part of the normal operation in that it's always there, or is it only when specific frequency vibrations are met?

 

[Strike]

G2G - Isn't that what happened to the Griffon that got stuck in the sand in Pet?  They weren't hovering and weren't fully down either and ended up vibrating themselves into the sand and got stuck.  Or is this something different?

 

[Good2Golf]

Is it part of the normal operation in that it's always there, or is it only when specific frequency vibrations are met?

Yes, the underlying potential is always there, but some conditions amplify the effect.  One of the systems that does so is, ironically, the stability augmentation system (SAS) on many helicopters.  The SAS (SCAS, AFCS, etc...) is meant to stabilize the helicopter from amongst other things, in-flight oscillations.  Each model of helicopter's SAS, if so equipped, is tuned to flight conditions and aerodynamic behaviour.  The problem is when the helicopter's dynamic airborne mode becomes modified by partial ground contact to a degree that the SAS is confused by the helicopter's reactions and tries to compensate for oscillation modes not foreseen (or calculable) by the engineers.  On a wheeled helicopter like the Chinook, the SAS is turned off immediately after landing, and conversely just before taking off.  If a Chinook is ground taxied with the SAS still engaged, particularly over an uneven surface that transmits significant forces enough through the landing gear, the SAS ('thinking' that because the pilots have left it on, it is airborne and needs to stabilize the aircraft) could fight those transmitted forces...perfect conditions for ground resonance.

As you saw in the video, the helicopter was "light on the skids" and there was intermittent contact between the helicopter and the ground.  I don't know about the specific model of the Eurocopter AS350 AStar in the clip, but it could also have been equipped with a HeliSASTM system, which may have worsened the resonance.

G2G - Isn't that what happened to the Griffon that got stuck in the sand in Pet?  They weren't hovering and weren't fully down either and ended up vibrating themselves into the sand and got stuck.  Or is this something different?

Not sure in the specific case, but if light on the skids yet still maintaining partial contact with the ground, it is conceivable that it was a ground resonant type of interaction that bogged the aircraft into the sand/mud.

I've only felt the onset of ground resonance, and that was from being a young co-pilot too quick on engaging the SAS while we were still taxiing over some uneven tarmac.  I felt the rocking starting and turned the SAS back off until we travelled fully to the intended departure area.

Regards
G2G