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CH-47 Chinook - DESTRUCTIVE TESTING

combatmech

combatmech

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Was FCF'ing an SH-60F that had a bit of a lateral shuffle. Couldnt get the PCL's all the way to fly. We almost rolled over......
Watching the transmission come off that Chinook is pretty impressive.
Vortex ring state is no joke either
 
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tim292stro

tim292stro

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Al Harvey said:
Yeah yeah I checked the balanced. Man, think I'm an idiot or something??? rofl

Amazing to watch that rip apart.
Click to expand...
Ground Resonance can "just happen", and with catestophic results as seen.

The main rotor blades are not simply rigid like you might think, there is some up and down, forwards and backwards (relative to rotation) play - think of it like suspension for the rotor blades. The helicopter's landing gear (skids or spung wheels) also have some play. When a helicopter is on the ground, the mass of the fusalage is locked in a battle between the landing gear "play" and the rotor "play". If these two spring frequencies creat a poitive feedback (feed eachother), the forces on the fusalage will eventually exceed the structural design - and find the weak spot in the airframe.

Really the only way to get out of it is to take off immediately. You can hear in the video that they tried to shut down the engines when the resonance started, but it was too late - and it appears that they had the gear stpapped to the deck for the run.

Think of ground resonance like the wobbles on a motorcycle, or death wobble in a 4x4 steering.

There is another video on YouTube of a helicopter on a MacGyver episode - you can see the pilot just saved it...
https://www.youtube.com/watch?v=6vICf8l-KV0
 
flynavy80

flynavy80

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Well, I just learned something new. I've seen the CH-47 video before, but it was years ago. I can honestly say I wasn't aware of ground resonance. Pretty incredible. But I am certain Macgyver could fix that if he had to.
 
tim292stro

tim292stro

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Generally if you have a main rotor with dampers or a landing gear set with tires and shock absorbers you need to make sure they are serviced properly and regularly. If you've have a eurocopter with the elastomer damper in the main hub, you need to strictly follow the airframe manufacturer's service intervals religiously. This may mean taking the whole rotor head apart to replace the elastomer damper at the core - otherwise your two day unbillable down time and a $10K part will turn your $10Million airframe into this:
https://www.youtube.com/watch?v=0FeXjhUEXlc


The Chinook has a hydraulic damper (shown in brown) to control lead-lag which doesn't require total disassembly of the rotor head to service or replace - incidentally if you were to remove the rotors to put the airframe in an airplane for long-haul relocation this would be one of the connection points to the blades:
Chinook_helicopter_rotor_head.jpg

Eurocopters do things a bit different to save weight and package space (which improves the aerodynamics of the head), but putting a hinge (in red) for lead-lag, and then using a flexible damper in orange in the core to control the movement of the blade relative to the rotormast (both lead-lag and flapping) - this picture is not the best, is has some parts mislabelled :roll::
EC_flex_damper.gif
 
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flynavy80

flynavy80

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So what you're saying is, if Mcgyver had used FRESH rubber bands, none of this would have been an issue? (or at least less likely)

Seriously though, great info. Seeing how complicated the rotor head alone is really clarifies why helicopters have so much more down time for service compared to fixed wing aircraft (on a whole).
 
85CUCVtom

85CUCVtom

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combatmech said:
Was FCF'ing an SH-60F that had a bit of a lateral shuffle. Couldnt get the PCL's all the way to fly. We almost rolled over......
Watching the transmission come off that Chinook is pretty impressive.
Vortex ring state is no joke either
Click to expand...
I'm not savvy to helo talk, what are PCL's?
 
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