Destroying a perfectly good Chinook...

Although I can't get the video to play, I think I know this video, and what they're testing here is a phenomenon called ground resonance; engine or transmission parts that are "out of tune" and cause a violent shudder that vibrates the whole machine to death.

One way to solve it, is to lift off. Ground resonance, as the name already suggests, cannot occur when airborne.

I'm not a technical guy though: someone else might clarify this further.

Gertjan

GeejeeZ wrote:

Although I can't get the video to play, I think I know this video, and what they're testing here is a phenomenon called ground resonance; engine or transmission parts that are "out of tune" and cause a violent shudder that vibrates the whole machine to death. One way to solve it, is to lift off. Ground resonance, as the name already suggests, cannot occur when airborne.   I'm not a technical guy though: someone else might clarify this further. Gertjan

Well, you got part of it right. Engine and transmissions "out of tune" (whatever that means) have nothing to do with it

Ground resonance occurs in helicopters with a fully articulated rotor system when an unbalanced situation in the rotor disc occurs. Having the wrong cyclic and collective inputs that result in hard ground contact usually causes the problem, but on aircraft with hydraulic dampers on the rotor blades (to dampen blade lead and lag) that are improperly serviced can contirbute to the problem. Improperly serviced landing gear struts (on aircraft that have them) can also contribute to the problem if they aren't properly serviced

As has been mentioned the solution to getting out of ground resonance is to bring the aircraft to a hover.

I have dial up so I wasn't able to download the video, but I do know that Boeing has run several aircraft through dynamic tests to determine the 'failure' parameters of different components. I suspect that is what this video is of unless someone can shed more light on the issue.

Yeah...

A previously scrapped frame was used in the test, but I think it went a little further than it was supposed. I like the side view where you can see the side door flapping up and down!

Cheers

PHEW!.....I had to wait to get to my offiice at work to see that video. 

I agree....if it was to save lives...sure DO IT!..... It was a bit creepy to see that airframe just come apart like that....untill you have been in a crash or in combat situations its easy to forget that Helos, fixed wing and some ground vehichles are just frames with thin aluminum or metal skins.

Cable High speed this Thursday!! YIPPIE!!!!.....had Verizon High speed till two months ago....we moved and our street does not support phone high speed.....man...going back to dial up...well....it sucks!

Oh yes... Ground Resonance can ruin ones day VERY quickly!!      I remember the photos of #64-13151 when she taxied too close another parked Chinook and dinged her rotor on the aft blades..... #151 literally disassembled herself within a couple seconds because of the out-of-balanced condition.

The only personnel injury happened to the Flight Engineer, Garnett Clark, who's right arm was severed by a piece of the rotor.

Take care,

Frank

Which Go Go 'by name' was that Frank?

Man what a drag ...Ita amazing he was the only serious injury!

Papa-Echo-64 wrote:

Which Go Go 'by name' was that Frank? Man what a drag ...Ita amazing he was the only serious injury!

Troy,

#64-13151 was named "Stump Jumper", due to a forced landing in which she straddled a tree stump.    She was given her name while repairs were being made, and carried it only a couple weeks before being destroyed at Vung Tau.

Here's a 2001 picture of Ed Coberly, John Huddleson, Garnett Clark, and Mike Gegere.    Ed was #2 Gunner on "Stump Jumper" that day, John was Crew Chief, Garnett was the FE, and Mike was #1 Gunner.

Take care,

Frank

Wow, am I glad now that I posted this.  I learned a LOT in this thread; and after a night of googling about and learning about some of the references that I didn't exactly understand, I feel like I just got a crash course! (haw-haw-haw....sorry for the pun!)

Thanks, everyone!

Unknownpharoah wrote:

There is a lot of brilliant minds in the helicopter forum. Lots of experiance to pick from. Not to mention a great bunch to boot!

The most amazing part about that film is if you havn't noticed...is that the forward rotors seemed to stay intact, did not make 'obvious' contact with the aft rotors and where still spining at the end of the film!

Man that film is just creepy to watch.

J.H. Primm wrote:

Ground resonance occurs in helicopters with a fully articulated rotor system when an unbalanced situation in the rotor disc occurs. Having the wrong cyclic and collective inputs that result in hard ground contact usually causes the problem, but on aircraft with hydraulic dampers on the rotor blades (to dampen blade lead and lag) that are improperly serviced can contirbute to the problem. Improperly serviced landing gear struts (on aircraft that have them) can also contribute to the problem if they aren't properly serviced

very true,

there is a similar effect in CH-53E's when main rotor head dampers fail durring turnup.  However, I've never seen it that severe.  

ridleusmc wrote:

J.H. Primm wrote: Ground resonance occurs in helicopters with a fully articulated rotor system when an unbalanced situation in the rotor disc occurs. Having the wrong cyclic and collective inputs that result in hard ground contact usually causes the problem, but on aircraft with hydraulic dampers on the rotor blades (to dampen blade lead and lag) that are improperly serviced can contirbute to the problem. Improperly serviced landing gear struts (on aircraft that have them) can also contribute to the problem if they aren't properly serviced very true, there is a similar effect in CH-53E's when main rotor head dampers fail durring turnup.  However, I've never seen it that severe.

If I am not mistaken it might be due to more blades on the rotor head. I know that more blades on a rotor system make for a smoother ride.

I'm giving my age away here, but I learned (the hard way) how to track blades on a CH-46 using the flag and pole, and masking tape coated pole methods. Then we had the Chadwick-Helmuth strobe set up using a strobe light and reflective tape attached to blade targets. That was pretty much standard on '46s AND '47s up until the mid to late 80s when we started using accelerometers located along three axes where we were more concerned with reducing vibration rather than ensuring that all blades were within certain tolerances in the rotor tip path plane.

The company I work for now has some pretty slam bang stuff that will usually give good results in as little as two or three run ups. (A far cry from what seemed like endless days "chasing" climbing and diving blades...)

ooooops sorry for the rambling....just another old phart who missed his meds

Jonathan Primm

Up to now, I've only read the forum so alert me if I break any rules. I felt compelled to respond because I happen to know about this video. I played a small role in events that lead up to it. Ground resonance was not the object of the test, only the outcome. First off, the a/c was not exactly flyable from the beginning. It was involved in an incident that ended in a hard landing which did some damage, not all of it visible. The a/c could have been repaired but there was a requirement to update ballistic testing on the Chinook, so this a/c made a good test subject. I was reassemble using unserviceable parts but was good enough to perform unmanned ground runs in order to shoot at it to see what happens. I don't know how much was actually fired at it prior to the video but its obvious that it was teathered too tightly to the ground and as they say, the rest is history. Hopefully this quenches some of the curiousity.

Wile E wrote:

Up to now, I've only read the forum so alert me if I break any rules. I felt compelled to respond because I happen to know about this video. I played a small role in events that lead up to it. Ground resonance was not the object of the test, only the outcome. First off, the a/c was not exactly flyable from the beginning. It was involved in an incident that ended in a hard landing which did some damage, not all of it visible. The a/c could have been repaired but there was a requirement to update ballistic testing on the Chinook, so this a/c made a good test subject. I was reassemble using unserviceable parts but was good enough to perform unmanned ground runs in order to shoot at it to see what happens. I don't know how much was actually fired at it prior to the video but its obvious that it was teathered too tightly to the ground and as they say, the rest is history. Hopefully this quenches some of the curiousity.

Thanks for the info on this.

I can't see any problems with your post and I don't think anyone else will.

BTW, where did this testing take place? Philly?, Aberdeen?, Pax River?, Eustis?....

Test was at APG.

I just responded to another string "front end rework" where I saw some of your messages. Your name sounds familiar. Ever here of Ralph Wood?

Wile E wrote:

Test was at APG. I just responded to another string "front end rework" where I saw some of your messages. Your name sounds familiar. Ever here of Ralph Wood?

Hell YES! I remember you, I think we might have gotten drunk together.

Jonathan Primm

There is a lot of brilliant minds in the helicopter forum. Lots of experiance to pick from.

I dunno guys, seems like this thread isn't related to modelling........wonder what heavy arty will think?

phantom works wrote:

There is a lot of brilliant minds in the helicopter forum. Lots of experiance to pick from.   I dunno guys, seems like this thread isn't related to modelling........wonder what heavy arty will think?

J.H. Primm wrote:

BTW, where did this testing take place? Philly?, Aberdeen?, Pax River?, Eustis?....