Ok, now it is my turn, what is ground Resonance? I saw it on the news earlier today, and when they stated they suspected that, I am like WTF? Two mil for that eh? Could have bought two Ranger 206b's for that price.

Jim

edit, looked it up on Wikipedia. Interesting about those swat team videos. Hope nobody was seriously hurt. Does remind one of the Keystone cops of yore.

 

Eurocopters are fun to fly, but everything is backwards on them. I do however like the negative G pushovers, watching passengers go weightless for a brief period, frequently followed by their lunch going Weightless.

 

Quick lesson on ground resonance. is when a shock such as a hard landing causes the main rotor system on a helicopter to get out of balance... this happens only in Fully articulated rotor systems.. It usually takes Two or more abrupt shocks to the system, such as a hard landing.. the blades in an articulated system can move in 3 different ways.. by way of hinging systems 1) flapping --allowing the blades to flap or cone up under load and also teeter like a teeter totter, or sag when not under load, 2) lead/lag-- allowing the blades move forward and backward within the tip path plane or rotation of the blade aka speeding up or slowing down, and then finally 3) Pitch-- the twisting or angling of the blade to increase or decrease blade pitch and lift.

Basically one or more but not all blades will droop down very dramatically and will cause an out of balance situation. If the rotor system is under full power the pilot can simply lift off the ground at it will cancel itself, but if the pilot decreases the throttle or Blade RPM to the point where he or she can't lift off, then it will enter into the ground resonance, which is a sound wave that causes the blades to flex like waves on water, and Vibrate uncontrollably until catostrophic failure, as seen on the video...

 

So in the video, the pilot landed too hard? I am surprised he did not just try and take off, it bounced around for a little while before tearing itself apart.

Jim

 

Well it does take two Shocks or bumps to set up the phenomena of Ground resonance, and some pilots upon touch down will reduce throttle settings, which is a bad Idea, but it happens. So the pilot had done a real job of botching things. Once rotor RPM has decayed below a certain RPM it becomes an unrecoverable situation, and cranking the throttle back on again can cause other problems to arise, above and beyond the fact it takes a few moments for the rotor system to spool up and regain RPM. As you can see in that video it all happens very rapidly. Luckily it is not a very common occurrence. As for taking off again it all depends on what the percent of rotor RPM was , if he was at say 85% of his normal flight RPM he probably could regain flight, but as a rule anything much below around 82% and you are pretty such sunk.. Pulling pitch at lower RPM will cause the blades to rapidly stall, Kind of like when you are driveing in a car with your hand out the window playing airplane, if you tilt your hand up to much the wind just pulls back on your hand due to excessive drag.. which will actually exacerbate the problem by causing the then unloaded stalling blades to flail about. It really gets into a full Physics lesson to explain how it all happens, with so many different things going on at once. I hope that helps a bit.

 

another bird shaking apart!

Chinook Helicopter Ground Resonance Test (Mechanical Resonance) - YouTube

 

Thanks for explaining what happened, Jet. I was hoping you would comment. Damn, I can never remember your first name. I almost used the one in your sig.

I know one thing - if I had been filming that, it would have been rapidly diminishing in the viewfinder.

 

Not that guy, but think "apostle"

 

Apostle Paul.

That will stick I think.

 

Matthew?

No, that's not right..........

Pop

 

No, this is Matthew




He doesn't post much.

 

As I recall, that guy had an identity issue too.

 

Cora did, anyway

Paul used to be Saul, right? I don't remember too much of that stuff.

 

Yeah, road to Damascus and all that stuff. I never got the significance of the name change.

 

I shouldn't be encouraging a religious convo, so with apologies I'll change the subject

Nissan Altima (Ultima?). Talk about identity crisis, we've got one sitting in the driveway now. I'm gonna have to ride out to Norco in this thing today. What type of mask should I wear?