Ground Resonance Vid

[KBayDog]

When I first learned about this phenomenon, I pressed the "I BELIEVE" button.

After seeing this video, I BELIEVE!

http://www.break.com/index/tiedheli23.html

(The full side and rear views of this video can be seen here (if you have the patience for the large download).)

 

[TheBubba]

Wow...

What causes that? Is it mainly the vibrations from the rotor or...?

 

[Blutonski816]

whoah... remind me to never set foot inside a Chinook...

 

[Jameski]

That is crazy!!!

 

[jbright44]

Wow...

What causes that? Is it mainly the vibrations from the rotor or...?

It looks like the rotors under some certain condition are causing the hellicopter to hit a resonanct frequency (not enough damping). At that point, it's just like feedback on a speaker or amplifier. The vibrations induce energy into themselves and it continues to get worse until it just shakes itself apart.

 

[ZoomByU]

It looks like the rotors under some certain condition are causing the hellicopter to hit a resonanct frequency (not enough damping). At that point, it's just like feedback on a speaker or amplifier. The vibrations induce energy into themselves and it continues to get worse until it just shakes itself apart.

Everything has a natural frequency. When the rotor blades match the natural frequency of the hull of the aircraft it begins to shake like that. It is the same concept when your car starts to shake at a certain speed, which doesn't happen very often with newer cars.

 

[TheBubba]

I know about the natural frequency... was just wondering if it was the vibrations from the spinning rotors that matched the nat. freq of the helicopter and caused it to fall apart.

 

[gaijin6423]

I don't know the history behind the video, but could it have been a track-and-balance problem? An out of balance main rotor blade oscillating at an increasing amplitude, perhaps? The damn thing was chained down and at least partially surrounded by berms, so is that a test video?

 

[gregsivers]

whoah... remind me to never set foot inside a Chinook...

Thats not particular to a Chinook, any helo can get ground resonance and shake until its comes apart. Best thing to do is get in the air so you're not in contact with the ground.

 

[phrogdriver]

I don't know the history behind the video, but could it have been a track-and-balance problem? An out of balance main rotor blade oscillating at an increasing amplitude, perhaps? The damn thing was chained down and at least partially surrounded by berms, so is that a test video?

For some reason, I couldn't view the video, so I can't comment directly to it. Might be some poor interaction with Firefox as a browser. Anyway, chains and tiedowns aggravate ground resonance. When used aboard ship, they are left slack on turning aircraft for that reason. If wheel struts are compressed, the vibes have nothing to dampen them out, plus, a tied-down bird cannot lift up to stop the vibration, which is the first of two possible corrective actions, the other being to immediately shut down.

 

[KBayDog]

I don't know the history behind the video, but could it have been a track-and-balance problem? An out of balance main rotor blade oscillating at an increasing amplitude, perhaps? The damn thing was chained down and at least partially surrounded by berms, so is that a test video?

It was a test at Aberdeen.

 

[macattack]

That or someone was really rockin the casba...

 

[AllAmerican75]

Whoa, I never would've believed that could happen. Mac, rep points.

 

[maxlife]

Ground resonance can only happen in helicopters with fully articulated rotor systems. Two bladed systems like the Cobra and Huey are not subject to this phenomenon. Usually caused by a bad landing gear dampener or blade dampener that allows a blade to get out of phase with the others.

 

[KBayDog]

Ground resonance can only happen in helicopters with fully articulated rotor systems. Two bladed systems like the Cobra and Huey...

...and the mighty Sea Ranger (with a compass in the stock and a thing that tells time...)