Never liked the Chinook!!

Post by **Trev Clark** » 24 Jun 2007, 11:35

Tie down tests with a difference!!
http://www.liveleak.com/view?i=a4fae56bff

Post by **DaveB** » 24 Jun 2007, 11:45

Err.. oops! :shock:

I think I'd have closed the throttles and hopped it long before then

ATB

DaveB :tab:

delticbob · Post by **delticbob** » 24 Jun 2007, 11:50

Top Gear's Jeremy Clarkson strikes again

Bob

Post by **Garry Russell** » 24 Jun 2007, 12:42

I've seen that and another angle before

It was a test something to do with resonance/rotor imbalance or something and it was set up and run to see how long it took to self destruct and how what the failure pattern was.

But it was supposed to break up and was a tied down unmanned non flying test specimen according to what I've seen and read in the past on this.

Garry

Kevin Farnell · Post by **Kevin Farnell** » 25 Jun 2007, 00:56

Thatâ€™s one way of dismantling it.

On the other hand, the 'WOKKA' sound would make an awesome ringtone.

Regards

Kevin

Chris Trott · Post by **Chris Trott** » 25 Jun 2007, 01:51

Yes, that was a ground resonance test. All helicopters have to undergo such a test as part of their flight certifications. They basically put weights on the tip of one of the blades (in this case on the tip of one blade on each rotor) to induce resonance and then wait to see what happens. Most Western aircraft have very little tolerance for resonance and the blades have extremely tight tolerances for both static and dynamic balance. Eastern Bloc (i.e. Russian) helicopters however are built to have good dynamic balance, but static balance isn't as highly stressed. This is why you see the famous "Russian Rock" when you watch many Russian helicopters starting up. Once the rotors are turning though, they balance themselves out and in many cases the aircraft actually are smoother than US counterparts. This phenomenon only occurs on mutli-bladed helicopters (i.e. 3 or more blades) as the triggering mechanism is the lead-lag hinges on the rotor hub allowing the rotor blades to get out of synch due to either a weight imbalance or more typically a shock taken by the aircraft (i.e. hard landing or heavy turbulence).

Here's a couple of articles on Ground Resonance -

http://www.helis.com/howflies/groures.php

http://www.heli-chair.com/aerodynamics_102.html

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

Sorry, kinda fresh topic on my mind because I'm studying this stuff as part of training for a museum I volunteer at.

Michael davies · Post by **Michael davies** » 25 Jun 2007, 09:42

I dont think the number of blades has anything to do with it, even on a two bladed rotor if one blade is out of balance its going to resonate to destruction. Dont all modern large helos have lead lag hinges ?, even twin blade rotors, l thought that was necessary to allow effective flight control ?.

Michael

DanKH · Post by **DanKH** » 25 Jun 2007, 09:47

Looks exactly like my Old Toyota trying to start....

Always takes an hour or two bringing the parts back in order with Gaffa-Tape afterwards.... :roll:

Post by **Garry Russell** » 25 Jun 2007, 10:39

Know what you mean Dan..........I had a mini that did that at about 40mph

Garry

Chris Trott · Post by **Chris Trott** » 25 Jun 2007, 11:05

Michael davies wrote:I dont think the number of blades has anything to do with it, even on a two bladed rotor if one blade is out of balance its going to resonate to destruction. Dont all modern large helos have lead lag hinges ?, even twin blade rotors, l thought that was necessary to allow effective flight control ?.

Michael

The number of blades has everything to do with it. Ground Resonance can only occur with aircraft equipped with lead-lag hinges. 2-bladed helicopters don't have them. The purpose of lead-lag hinges is to allow multi (i.e. 3+) bladed rotors to remain spaced properly while moving through a revolution by allowing the advancing blades to slow down slightly and the retreating blades to accelerate. With a twin-blade system, the rotors are directly opposed to each other so they always cancel out that lead-lag movement. With a 3 or more bladed system, the blades are always slightly out of synch in their progression through the rotation cycle (i.e. all 3, 4, 5, 6, 7, or 8 are never exactly offsetting each other's acceleration), thus the need for a lead-lag hinge.

Here's a better description of rotor dynamics -

http://www.helis.com/howflies/bladlift.php