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Flanor
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Helicopter Falls Apart Caught On Tape

Fri Feb 24, 2012 5:18 pm

http://gma.yahoo.com/video/news-2679...apart-caught-on-tape-28416282.html

How can this happen?

[Edited 2012-02-24 09:21:39]
 
as739x
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RE: Helicopter Falls Apart Caught On Tape

Fri Feb 24, 2012 5:21 pm

Quoting Flanor (Thread starter):

That's why I stick with airplanes. That looks like R2D2 having a seizure. '
"Some pilots avoid storm cells and some play connect the dots!"
 
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Francoflier
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RE: Helicopter Falls Apart Caught On Tape

Fri Feb 24, 2012 6:09 pm

It looks like a rotor blade came out of balance.

But then they mention 'ground resonance'...? Never heard of that. I'd think chopper designers would thoroughly test for destructive resonance frequencies during design...
I'll do my own airline. With Blackjack. And hookers. In fact, forget the airline.
 
dfambro
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RE: Helicopter Falls Apart Caught On Tape

Fri Feb 24, 2012 7:27 pm

That's ground resonance and I don't think there's any design fix around that. There are other great youtube videos of that effect. It's a well known helicopter hazard. You either have to plant to the helicopter on the ground or take off. If you sit there with partial lift, this is what happens.
 
Fly2HMO
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RE: Helicopter Falls Apart Caught On Tape

Fri Feb 24, 2012 7:47 pm

Ground resonance can happen to any helicopter. The reporter incorrectly (as always    ) said it's because "the rotor is still turning after the helicopter has landed" Well no shit!   

A quick google search brings tons of info on it:

http://en.wikipedia.org/wiki/Ground_resonance

http://www.youtube.com/watch?v=0GEj69NANc8
 
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TupolevTu154
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RE: Helicopter Falls Apart Caught On Tape

Fri Feb 24, 2012 8:02 pm

Quoting dfambro (Reply 3):
If you sit there with partial lift, this is what happens.

I watched an A109 a couple of days ago with what I would describe as "partial lift". I guess this only happens under certain conditions and in rare circumstances?
 
dfambro
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RE: Helicopter Falls Apart Caught On Tape

Sat Feb 25, 2012 3:01 am

Quoting tupolevtu154 (Reply 5):
I watched an A109 a couple of days ago with what I would describe as "partial lift". I guess this only happens under certain conditions and in rare circumstances?

my post did overstate it a bit. You need some sort or bump or shock to start the oscillation, and it has to overwhelm the damping systems built in suppress resonance.
 
wjcandee
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RE: Helicopter Falls Apart Caught On Tape

Sat Feb 25, 2012 6:22 am

You don't necessarily *need* a bump to start the resonance, because other failures can cause it.

But pilot error is a major contributor in many incidents.

Most commonly, the pilot bangs the thing down in a rough landing primarily on one skid, and then overcorrects, inducing the off-balance, undampable condition, and then not acting properly to correct it. Here, it might have helped a lot if he came off the power really fast after the hard landing, but it looks as if there was a correction through an upward-but-insufficient movement of the collective, which deprived the rotor system of an opportunity to damp the resonance.

On the other hand, what one may be seeing as the thing twists on one skid is lift induced by a deflection of the rotor from the resonance.

This is another example of one truth about rotorcraft: when certain things go wrong, the pilot has to *do something* very quickly; you have to make the decision to act and then act; you can't pause to analyze, you have to be engaged and focused enough on your job of flying the aircraft to be ready to react and then to do so, otherwise, you are going to get bitten hard.

Examples: Your engine quits in cruise flight, you need to bottom that collective immediately and transition into a descent with forward motion; otherwise, the rotors will slow, stop, fold up, and, as one author described it, cause you to fall like a streamlined anvil.

Antitorque failure in hover, you have to come off the power immediately, without stopping to analyze, because you are going to be 360 degrees around anyway by the time you chop the power, and if you don't the thing is going to thrash itself to death very quickly.

Ground resonance: you need to either get into the air fast, or cut power fast, but sitting there trying to decide which one to do is a certain recipe for rapid disintegration of your airframe.

[Edited 2012-02-24 22:34:17]
 
PlaneInsomniac
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RE: Helicopter Falls Apart Caught On Tape

Sat Feb 25, 2012 8:14 am

It´s not an unkown phenomenon. One incidence of ground resonance - with correct pilot reaction - even made it into a McGyver episode:

http://www.youtube.com/watch?v=6vICf8l-KV0&feature=related
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oksman
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RE: Helicopter Falls Apart Caught On Tape

Sat Feb 25, 2012 12:21 pm

Wow, this is really interesting!

I have never heard of such thing like ground resonance, thank you everyone for your explanations. My first thought would be a fan coming out of balance, which, if I´m not mistaken, has caused some accidents in propellers after wings were torn apart from the fuselage.

Denis
 
contrails15
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RE: Helicopter Falls Apart Caught On Tape

Sat Feb 25, 2012 5:07 pm

Quoting PlaneInsomniac (Reply 8):

It´s not an unkown phenomenon. One incidence of ground resonance - with correct pilot reaction - even made it into a McGyver episode:

http://www.youtube.com/watch?v=6vICf8l-KV0&feature=related

Great example and from one of my fav shows of the past. Yeah, he got the hell out of there. The door was still open as he took off.
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soon7x7
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RE: Helicopter Falls Apart Caught On Tape

Sat Feb 25, 2012 5:18 pm

Quoting as739x (Reply 1):
That's why I stick with airplanes. That looks like R2D2 having a seizure.


Amen to that!...Flying cinder blocks with eggbeaters on top...   
 
aztrainer
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RE: Helicopter Falls Apart Caught On Tape

Sat Feb 25, 2012 7:08 pm

Was the video of the CH-47 a training video as there is a posterior and a lateral view of the same experiment? DAMN the vibration was violent enough to rip the rear rotor assembly to the ground.

I wonder if the McGyver people knew what was happening and that is why it was left in the show as the actors refused to ride in the Gazelle thinking it was a dangerous chopper?
 
PlymSpotter
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RE: Helicopter Falls Apart Caught On Tape

Sat Feb 25, 2012 7:57 pm

This happened to somebody I know. He was very keen on doing his own maintenance but didn't really have a clue, so decided to glue-gun up the vibration dampers. God knows how he didn't kill himself when he tried to take off.


Dan  
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dlednicer
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RE: Helicopter Falls Apart Caught On Tape

Sun Feb 26, 2012 6:55 pm

Ground (or air) resonance happens when something in the rotating system on the helicopter causes a disturbance at a frequency that matches a natural frequency of the airframe. Typically, this happens on the ground when one blade lags just slightly, causing a vibration that happens to match the natural frequency of the landing gear. The ensuing vibrations can destroy the helicopter quickly, if not dealt with immediately.

As an example, the first BO-105 prototype, using a conventional hinged-rotor installation taken from a Westland Scout (not the eventual rigid rotor), encountered ground resonance on a ground run before its first flight and was destroyed..

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