AS350 - Ground Resonance [video]

[CS-Hover]

22/02/2012


Rw9HrR_IuS0

:eek:

[CharlieOneSix]

Jeesh! For some reason it reminds me of that spoof video of the Australian politician and the oil tanker that the front fell off.

[ShyTorque]

Another couple of grams on the red blade should fix that.

[Agaricus bisporus]

Several very lucky, very foolish numpties running around under the blades while that was going on.

[Fareastdriver]

By the time those 'foolish numpties' got there to see if anybody was hurt it was all over.

[Fun Police]

looks like pretty level ground. bad springs or oleos?

[Savoia]

.. bad springs or oleos?

Or a firm landing and which begs the question (if he was at flight idle) why the heck he didn't pull into the hover!

As with the New Zealand crash the driver seems to have been thrown about a bit and, as ever, the Eurocopter cabin stands-up to the trauma with amazing resilience - not!

[Peter3127]

Sobering to say the least!!! :eek:

[Ready2Fly]

I think before he even realised what was going on, it was all over. What a result. :eek:

The rush of adrenalin must have kept him wide awake for a long time.

[TRC]

.... bad springs or oleos?
And a pilot sitting there like a stunned mullet.

[Savoia]

I think before he even realised what was going on, it was all over.

Yes, if he was on drugs perhaps!

In the video (which begins with the onset of GR underway) there are a good 8 seconds (maybe more) before the resonance reaches catastrophic (ie. unrecoverable) proportions. The moment a driver feels any abnormal vibrations (especially of an escalatory nature) instant/immediate action must be taken. That's what you are trained to do!

TRC put it well.

By contrast .. in the clip below (which has done the rounds on PPRuNe I know) the onset occurs at 40 seconds. By 42 seconds the driver has the bird off the ground (probably delayed by the fact that he may have been considering the open door).

6vICf8l-KV0

[Peter-RB]

I ask because I dont know , but what would be the next step when landed and everything gone quiet, would the airframe and such, need to be thoroughly examined with measurments and joints checked,?

Peter R-B

[SASless]

....With Door Flying...

Who's the Dumbass that did not latch the door upon exiting?

[TRC]

Who's the Dumbass that did not latch the door upon exiting?

It was an actor, would we expect otherwise?


are you sure that that the helicopter was at full power to lift off?

As Savoia said, the video starts after the resonance started. There seems to be plenty of Nr so it seems to be at or near t/o RPM.

It's the lack of either lift-off, or throttle chop and rotor brake (but too high an Nr for that probably) that prompted me to suggest the pilot just sat there and watched while his world - and job I expect - disintegrated around him.

I have witnessed an Al II get into resonance and the pilot lifted off immediately as did the Gaz in the video. Maybe the 350 pilot hadn't been trained to recognise what was happening, or what to do about it.


I would like to read the investigation report before blaming the pilot

What would you expect to find in the report that would exonerate the pilot? Lack of training I suppose.

[Savoia]

It's the lack of either lift-off, or throttle chop and rotor brake.

Precisely. He did nothing and which inaction appears to be corroborated by the video footage.


.. throttle chop and rotor brake (but too high an Nr for that probably).

Correct. The throttle chop may have brought things back into balance but, to be effective, would need to be followed by rapid deceleration of the MRB's and which would have meant waiting until Nr came into range before one could apply the brake. Hence the take-off option.

What would you expect to find in the report that would exonerate the pilot? Lack of training I suppose.
Highly probable that this statement will also turn out to be correct.

[Savoia]

I'm sorry, I didn't reply to Eddie's comments:


.. are you sure that that the helicopter was at full power to lift off?

No I am not sure at all but .. the Nr does (as TRC said) appear to be nearer to flight RPM.

When the resonance has put your blades out of symetrie it can be interested to lift off.
Yes it can but, if I remember correctly, the moment the vehicle is taken out of contact with the ground and suspended by the disk, the 'new' loading on the blades combined with the normal dynamic of centrifugal force eliminates the dissymmetry.


Pitch lift regardless off rotor rpm?

If he opted for chopping the throttle then, if it were me personally, I would probably introduce pitch (as a form of braking) until I could use the brake. But .. one needs to know what one is doing. Collective braking can introduce its own risks and one needs to be familiar with the sensitivities peculiar to each type in this area.

Has the pilot tried this or why is the helicopter turning before separating?
To me, there is very little videographic evidence to suggest that pitch was introduced. His 'turning' may well be due to the fact that he is starting to be bouned around in his seat. Conjecture I admit. The yaw does however saying something about the Nr.

[Devil 49]

Does it sound like the OP video was increasing NR?

The answer is simple: when it (ground resonance) starts, stop what you're doing.
If you're running up, NR from ground to flight idle, go back to flight idle (I'd probably shut down...).
If you've just landed and/or are still at flight NR, takeoff. It will fly with the doors open. Getting the helo off the ground with people under the disk is safer than letting the aircraft dynamically disassemble itself overhead/around them.

I've seen guys try to "land through it", especially tempting in a multi with overhead engine condition levers to complicate an eventual hovering auto. That didn't work. I've seen guys try to run up, hoping "it" would stop with a change in NR. Ditto. I know a guy who tried to complete a crosswind running landing with a student, only to end up in a pile of parts.

I'm probably preaching to the choir, but my recollection is that it's clearly a bad thing occurring when "dynamic divergent excitation" commences and one has to react quickly before it's fully developed and destroys the helo- something like 3 turns of he NR? I've been told that the AS350 could do this in flight...

[rotorrookie]

Its pitty that we are missing the first bit, it would be interesting to see the whole sequence.

[Shawn Coyle]

Personally, I'd apply a slight amount of rotor brake even if the rotor RPM was well outside the limits for the brake.
Anything to get the blades / fuselage to behave.

[AdamFrisch]

As I understand it ground resonance is a temporary imbalance in the rotor system caused by external forces. But let's say something actually structural did depart the rotable components, a tip fairing or a blade damper or something, then I assume no amount of lifting off recovery is going to save you. Right?

[Bushrat]

...two ways to stop ground resonance...get off ground...chop throttle...as ground resonance is a function of power... dissipates almost instantly when throttle is cut...no need for weird rotor brake action....have worked on squirrels for over 20 years..lamas as well

[Devil 49]

The theory as I was taught is that articulated systems will diverge at some frequency as the blades 'hunt'. It's usually not noticed because it's transient and low energy anywhere except at operational NR. The airframe is tuned away from most energetic and maximum exposure time, flight NR. That's why the AStar has heel springs, to change the airframe's natural frequency to keep it from being a harmonic to the NR. There are also dampers in the system (forward crosstubes on AS350) to absorb energy and keep it from magnifying the issue. Everything has to be within parameters, from the disk to the ground and the the heel springs should have the actual skid heels clear, a reason to takeoff again and perhaps move and try another SMOOTHER landing so you don't bump a blade out of phase and clear the heel allowing the heel springs to do their thing.

If you stop the process before anything is damaged, a change in the NR arrests the divergence because the excitation is not sympathetic/harmonic. That's a trap to the unwary- you can't accelerate the rotor fast enough from ground to flight idle to keep the airframe intact.

[Savoia]

As I understand it ground resonance is a temporary imbalance in the rotor system caused by external forces.


Posted before in another thread:

Ground resonance is a hazardous condition that can occur any time the rotor of a helicopter or gyroplane is turning while the aircraft is on the ground. Similar in concept to the behaviour of a washing machine when the clothes are concentrated at one point during the spin cycle, ground resonance can occur with a rotorcraft when the spacing between blades become irregular or the damping system, including drag hinge dampers, landing gear oleo struts, or wheel tire pressure, is operating out of limits. Ground resonance occurs at three rotor rpm bands, the first at about one-third of normal rotor speed, the second at a range including normal rotor speed, and the last in a range above normal rotor speed.

Ground resonance is precipitated by a shock to the aircraft arising from excessive motion of a rotor blade in its plane of rotation thereby moving the rotor center of gravity from the axis of rotation. Inadequate damping allows the rotor center of gravity to spiral away from the rotor axis of rotation, causing the rotor to generate unbalanced rotating moments beyond the compensating ability of the damping system. In addition to damping system malfunctions mentioned above, such blade movement can occur as a result of taxying over rough ground with the rotor turning at a speed within a susceptible range, or takeoff/landing in which a shock occurs to one side of the landing gear.

Under extreme conditions, the initial shock can cause violent oscillations that quickly build and result in catastrophic damage of the entire airframe. In some cases, complete destruction occurs, e.g. body panels, fuel tanks, and engines are all ripped about normal rotor speed.

Recovery is possible in some cases. If sufficient rotor RPM exists, immediate takeoff can restore rotor balance. If rotor RPM is low, complete shutdown might be sufficient.

Two bladed semi-rigid see-saw (underslung) type rotor systems are not susceptible to ground resonance because the blades do not lead and lag.
And you may also add Devil49's comments to the above.

But let's say something actually structural did depart the rotable components, a tip fairing or a blade damper or something, then I assume no amount of lifting off recovery is going to save you. Right?
Totally different scenario and almost impossible to predict. Most 'things' that depart dynamic components while in full motion have the potential to cause catastrophe but .. there are always exceptions. I had the 'beanie' hat fly off the head of Hughes 500D in flight and all I got was a mild kick through the cyclic .. but I was lucky!

[AdamFrisch]

Wasn't there a French Squirrel in the Paris-Dakar rally that managed to get down after one MR blade departed, or something? Sounds like a tall tale, but I'm sure I heard it somewhere.

[TRC]

Wasn't there a French Squirrel in the Paris-Dakar rally that managed to get down after one MR blade departed, or something? Sounds like a tall tale, but I'm sure I heard it somewhere.

I very much doubt it...

[birrddog]

There was an EC 130 that had an accident in NY due to partial blade separation. Pilot was Tammy Fein. Landed safely on floats in the Hudson River.

On July 7, 2007, about 1651 EDT, a Eurocopter EC-130-B4 helicopter, N453AE, operated by Liberty Helicopters, Inc., experienced an in-flight separation of a section of one of the main rotor blades during flight and sustained substantial damage during an emergency descent and subsequent autorotation into the Hudson River in New York. The commercial pilot and seven passengers were uninjured. No flight plan was filed with the Federal Aviation Administration (FAA) for the 14 Code of Federal Regulations Part 135 and 136 sightseeing flight, nor was one required. Visual meteorological conditions prevailed at the time of the accident.

[Senior Pilot]

There was a similar incident to an AS350 in Florida, 2006, discussed in Ground Resonance fails AS350 tail boom. (http://www.pprune.org/rotorheads/254906-ground-resonance-fails-as350-tail-boom.html)

[500e]

Tear along the dotted line comes to mind,

[ReverseFlight]

Speaking from personal experience. I once landed AS350 carefully on the flat strip at the rear of the starboard skids, then applied a little too much downward pressure on the collective and ground resonance kicked in immediately - it happens too easily. I lifted off at once and repeated the landing, this time with micro-millimeters of pressure easing on the collective for an uneventful setdown. Never to be repeated again.

[nigelh]

devil49. I think you are wrong . I don't believe you have ever seen anyone try to "land through " this and I don't believe it has ever occurred in flight .
I have been around 350,s and flown them for circa 30 yrs and have never come across this happening in flight . I believe it can ONLY occur when in contact with the ground ...sorry !!!

[fly911]

What would you expect to find in the report that would exonerate the pilot? Lack of training I suppose. That incident went south pretty quickly. My observation only: Right after the start of the video, the pilot had some pitch in, as the squirrel was rotating (hopping) to the left. That would not have happened in flat pitch IMO. The pilot may have been too afraid to bring that heaving mass into the air and figured "almost into the air would do almost as good". Another possibility is that the pilot was too afraid to take his hand off of the collective long enough to reach for the throttle quadrant. Very scary for the pilot. Not a time for indecision.

[ShyTorque]

The sound track on the video gave me the impression that the Nr was increasing but not up to flying speed as the resonance began. It continued after it began decreasing again after the throttle was retarded. I don't therefore think he had a chance to lift off.

(One thing that I'm absolutely sure about is that ground resonance can't occur in the air, btw. The answer's in the name).

[Savoia]

As one suspected (#7) - induced by the landing.

[SASless]

What caught my eye is the way the cabin roof separated....then the tail boom. The 350/355 roof has always been a weak link....and if the tail boom will fail as it did.....well....not very impressed! They are plastic helicopters.

[RVDT]

They are plastic helicopters

Correct - just a plastic Alouette and approaching 40 years old! It meets the standards in force in 1974 so.......................

To get resonance in a 350 something is busted or worn out. They do not just decide to do it just for something to do.............

The more we learn, the more we forget..............

[Gomer Pylot]

Aerospatiale (now Eurocopter) claims (or did) that the AS350 cannot experience ground resonance. It can, however, experience dynamic divergence. A fart by any other name....

I've had it start many times, and lifting to a hover stops it immediately, assuming the throttle is full open and Nr is sufficient. I once had one go into dynamic divergence after landing. Everything was fine until I reduced the throttle to idle, and then it went bonkers. The bystanders (pilots and mechanics) who didn't run and hide said the skids were coming almost a foot off the ground, and the aircraft turned about 45 degrees on the pad. When it started, I had the throttle at idle, and I wasn't about to try to bring the rpm back up, so I put the throttle in cutoff and pulled the rotor brake full on. It only took a few seconds to stop the movement, and there was no further damage. The inspection revealed only a slight amount of wear in the spherical bearings, nothing else. The springs on the heels of the AS350 can be a real problem, especially on wavy concrete. I don't know if that was the problem, but I've had dynamic divergence start many times on concrete pads in the marsh. I always waited a few seconds after landing before retarding the throttle, and that almost always was sufficient to prevent bouncing, but not that time.

[paco]

It would appear that those springy leaf things need to be very precisely fitted for them to work properly.

Phil

[Savoia]

Gomer, thank you for this input verifying the correct action when not using the lift-off option.


I always waited a few seconds after landing before retarding the throttle, ..
This is exactly the correct procedure in the Ecureuil in order to be prepared for the potential onset of GR - especially on level and hard surfaces where forces are communicated more effectively from the ground to the disk.


It would appear that those springy leaf things need to be very precisely fitted for them to work properly.


Ciao Phil! Definately and a lot of the time they become 'flat' and need to be re-aligned in order to restore their 'springiness'!

ps: I was going to drop you a PM after Christmas - I was exposed to non-stop Abba and Boney-M during the festive season the former bringing back memories of John Broome. Boy he was keen on that group. Did you also have to suffer the tape deck in the Towers LongRanger playing 'Dancing Queen' hour after hour, lol! Them' were the days!

[RVDT]

those springy leaf things become worn in thickness as well and need to be replaced regularly after landings on abrasive surfaces.

Watched someone fit "one" new item opposite a worn one due to supply issues from EC on a 355. Bad move!

The other issue that people miss is a worn/loose DU sleeve on the end of the star although there shouldn't be many of those left by now as the new star has "carbide" bushes which are an "improvement" or "not" depending on your point of view.

[unstable load]

I know a guy that had a very similar incident in a Twin Squirrel, on landing the machine went berserk and started thrashing around. Fortunately it was on a large gravel parking lot and it slid around the show while he was chasing the throttles to close them off.
By the time he got them closed the machine had moved about 15 meters on the pad in eccentric circles.

It turned out the starflex had delaminated and threw the blades out of sync once he landed.
A LOT of bits had to be replaced on that ship.

[EN48]

So, any known history of ground resonance with the B407? The aft gear is attached via a pivot which is said to reduce the possibility of ground resonance. It would also seem that the "soft in plane" (hingeless) MR would also lessen the possibility.

[SansAnhedral]

Being soft in plane is what allows a rotor to enter ground resonance. Whats the 350s in plane natural frequency, .4/rev or so? Certainly well within the realm of possibility.

This fact was a major selling factor of the teetering rotor system all those years, as it is inherently immune.

[malabo]

Never heard of or had a problem with ground resonance on a 407. 4 blades better than 3? The 412's that have self-destructed on the ground were something else - loose collective linkage or something like that.

[paco]

Savoia - didn't get Abba, the tape deck wasn't fitted till after I left, but I remember another ear worm - I think it was called music box dancer? Used for the background music for some fountains. Some tinkly thing that drove me mad. Then there was the specially written Alton towers theme tune that blasted out all day at the entry booths.

Sorry for the thread drift!

phil

[Savoia]

Phil: Lol, ah well, he was a special man with special tastes!


.. on the pad in eccentric circles.


US: Sorry, but I have to ask; do you mean eccentric circles (I'm sure it must have looked that way) or did you possibly mean concentric! ;)

.. 4 blades better than 3?
I was told (but I cannot substantiate this) that the most susceptible systems to GR were three-bladed rotors and that the ultimate 'risk' was a three-bladed rotor with wheeled undercarriage. As I understand, it can occur on 4 or more bladed rotors also.

https://lh4.googleusercontent.com/-AZWwZpDGcEk/T0fWshAShgI/AAAAAAAAIFc/Q0g8na9x7-w/s695/AL%2520III%2520N3194K.jpg
In December 2005 this Alouette III landed at Escalante National Monument in Utah and suffered ground resonance apparently shaking itself apart in all of four seconds

Here's a further description of ground resonance in case it helps:


Not all types of helicopters are susceptible to ground resonance. Two-bladed helicopters are exempt because their “teetering” rotors are a single rigid structure, like a see-saw. The only rotors that can produce ground resonance are those with three or more blades. Multi-blade rotors have lead-lag hinges, which allow blades to speed up and slow down at different points as they circle the mast while the helicopter is moving forward. The hinges keep the fluctuating lift and drag forces on each blade from inflicting excessive stresses on the rotor hub. Snubbers and dampers limit the motions of the blades.

Because it is massive and spinning at a high speed, the rotor of a helicopter must be properly balanced. If the lead-lag hinges allow the blades to depart from perfect symmetry, the rotor’s center of gravity shifts slightly to one side of the mast, throwing the system out of balance.

Anything that’s springy has a favorite frequency of vibration—its natural frequency—which is determined in part by its size and mass. That’s why tuning forks always produce a certain tone, and why boats of different sizes rock at different rates. When two things with the same or similar natural frequencies are in contact, or sometimes even merely close to each another, and one of them begins to vibrate, it may “excite” the other to vibrate as well. The ability of one vibrating object to create this sympathetic vibration in another is what enables the rotor blades to gain control of the entire helicopter.

The helicopter’s airframe has its own natural frequency, which can be excited by an out-of-balance rotor. Usually there is a triggering event: a bump or a landing or takeoff on sloping ground or with a little sideways or forward motion. A jolt moves the mast while the blades, because of the freedom of motion allowed by their hinges, lag a little behind. The rotor, now slightly out of balance, begins to wobble like a slowing top. If the characteristic vibration frequency of the airframe is close enough to the rate of rotation of the rotor, it joins the dance, amplifying the rotor wobble.

The destruction is wrought by the considerable energy stored in the rotor blades. The shaking rapidly grows in violence, exceeding the strength of the mast, transmission mounts, and landing gear. The cyclic control in the cockpit flails about so violently that the pilot cannot hold it, the rotor blades strike the tail boom or the cockpit, parts begin falling off, and moments later the helicopter may be a heap of scrap.

If ground resonance begins, the pilot’s best option is to get the helicopter into the air. Once the tires or skids are no longer touching the ground, the vibration fades. If the rotors do not have sufficient speed for flight the next best remedy is to eliminate lift by reducing blade pitch; shut down the engine; and hope for the best while waiting for the rotor to slow.

The wait-and-hope approach is only sometimes successful, so a better solution to ground resonance is to prevent it. Helicopters with multiple-blade rotors have shock-absorbing landing gear with powerful dampers that allow it to soak up the energy that would otherwise set the helicopter shaking. When ground resonance occurs in these craft, it is usually because tires or shock absorbers have been improperly serviced.

[EN48]

Being soft in plane is what allows a rotor to enter ground resonance.


Yes - but how does this design compare to an articulated rotor system with "real" hinges and hydraulic dampers in its susceptibility to GR?

[FairWeatherFlyer]

So, any known history of ground resonance with the B407

It's designed to shed two blades to become B206-like so there's no problem there.

[MartinCh]

I was told (but I cannot substantiate this) that the most susceptible systems to GR were three-bladed rotors and that the ultimate 'risk' was a three-bladed rotor with wheeled undercarriage. As I understand, it can occur on 4 or more bladed rotors also.

Makes sense. Centre of mass moves more significantly in case of one blade not where it should be in 3-bladed MR. No need for rocket scientist (or Shawn Coyle) :)

[Peter3127]

Eccentric in an engineering sense (not the behavioural sense) could be the perfect description of an aircraft spinning around and subject to an increasingly divergent excitation.

Eccentric (mechanism) - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/Eccentric_(mechanism))

Now, where are my lucky socks ......

[Pilot DAR]

If the rotors do not have sufficient speed for flight the next best remedy is to eliminate lift by reducing blade pitch; shut down the engine; and hope for the best while waiting for the rotor to slow.



Question from amature: Would it not be better to lift the collective, and either possibly hop into the air for a second with very low and slowing RPM, or else at least the blades are loaded, and slowing RPM fast?

[Savoia]

Peter: Thank you for continuing my education in the English language. Eccentric it is then. How unique!

http://upload.wikimedia.org/wikipedia/commons/thumb/9/9b/Eccentric_animation.gif/120px-Eccentric_animation.gif
Eccentric circles .. in an engineering sense - what we learn on PPRuNe eh!

Would it not be better to lift the collective, and either possibly hop into the air for a second with very low and slowing RPM, or else at least the blades are loaded, and slowing RPM fast?
I mentioned on the previous page that I 'may' introduce collective braking. I think it would be determined by observing the initial reaction to introducing pitch (ie. whether it exacerbated or diminished the GR).

Re: 'hopping' into the air .. I've only heard of one helicopter capable of doing this .. the Bell 47 (which apparently used to be demonstrated by one of the drivers at Alan Mann after completing an auto). I've pulled near full pitch on 205's and 206's almost instantly after shutting the throttle and they weren't going to get airborne - the best one can achieve (as you say) is to load the disk. The consensus seems to be (with decaying RRPM) to try and stop the blades asap.

Where are all the QHI's who read PPRuNe. What do you teach?

[Peter3127]

To think I survived my first encounter with Savoia ..... :)

Theory is great, and as always CPL(H) students like me (despite having a PhD in Wind Turbines ... that's Autorotation in this forum ... the only place I have found it useful :uhoh: ) hang on the words of the wise.

So when folks like yourself say "I did XYZ when ABC happened", "we" the eager neophytes listen!

Cheers,

[John Eacott]

Before the guessing game gets too far advanced, maybe review Nick Lappos' post (http://www.pprune.org/3002080-post6.html) from Ground Resonance in a 206? (http://www.pprune.org/rotorheads/254896-ground-resonance-206-a.html) thread:

Your source materials are clearly poor in describing ground resonance, even the vaunted Wagtendonk. The number of blades has NOTHING to do with ground resonance, ground resonance is only possible if the blades have lead-lag motion. Most 2 bladed helos do not have any lead-lag, nor does the 3-bladed V22. They cannot experience ground resonance as a result.

If a helo has oleos, they are there to PREVENT ground resonance.

Ground resonance is a condition where the rotor blades hunt back and forth as the aircraft rocks on the ground. The blades and the aircraft motion build up a resonance that makes extremely high vibratory energy that can take the helicopter apart. During rotor runup, the rotor passes through two regions where the rotor and the fuselage can tune to resonance, one at about 30% and the other at about 60% Nr. The rotor blade dampers and the oleos are designed to damp these vibrations and make ground resonance entirely disappear.

If you experience ground resonance, cut the throttles immediately, this makes the blades all go to the lead position, and helps quell the resonance immediately. If you then apply the rotor brake, the blades are also tossed forward and pinned there by the rotor deceleration, a very quick cure. The pilot in this case made a very poor desicion to not cut the throttles, and his helo paid the price. Even if he had to switch hands, or stand on his head, cutting the throttle was #1 on his list. Cutting fuel is NOT a good idea, it takes too much time to starve the engine.

Contemplating what to do while the world is turning to worms around you is likely to be too little, too late. As with many immediate actions it must be ingrained through prior training and planning. An instructor of mine finished up seriously injured when the helicopter thrashed itself to death in ground resonance while chained to a tie down for power checks: it still got GR, and he hadn't prepared himself properly by wearing his shoulder harness :(

Answering the OT bit about 'hopping into the air', the Bristow instructors would do a full spot turn after an engine off to the ground at Redhill in the 206 with the engine still at idle. Plenty of inertia in the Bell main rotor ;)

But heed Nick's advice, and don't second guess a solution :ok:

[CYHeli]

The biggest problem when the blades are slowing and you raise the collective is the ability to cut your own tail off. The blades can flap when there is insufficient momentum to keep them horizontal, the lift created will wash off and the blade flaps down. Snap.
If you are at full flight RPM, just touched down and GR starts, you can get into the air to break contact and stop the GR, but only if you have not slid the fuel flow lever back. Once the FFC has been retarded, then do as John as posted.
It is the same in H269, or an AS350.

[topendtorque]

Know of one incident where the pilot of a '47 G2 contemplated GR.

what happened was he heard a bang and everytime he wound throttle on or off it starting shaking big time. He figured that he had lost a drag brace bolt.

So with care he landed, screwed down the frictions, didn't want to sit in it to shut it down ran it down slowly to low idle, got out, pulled the fuel shut off and - cut - to use an OZ outback term.

Dammit the machine stopped and ran down OK, we figured that as Nick says the deceleration was equal both sides.

The story didn't end there, he who was extremely popular and laid back sort of a dude now passed on long time, was also known as a make do expert and base was a quite aways distant, 2.5 hours or so. So our intrepid pilot wired the drag brace to the blade with a cobb and co hitch with two strands of number eight wire and flew home.

This being a friday afternoon quite late, yeah something else he was noted for, so he rings the engineer about sunday afternoon and said that he had to go out daylight tomorrow could he do a daily on the machine -had a small problem with that head thaang on top friday -, or words to that effect.

I think the engineer still wakes up in the middle of the night thinking about it.

[EN48]

Nick Lappos' post (http://www.pprune.org/3002080-post6.html) from Ground Resonance in a 206? (http://www.pprune.org/rotorheads/254896-ground-resonance-206-a.html)


Thanks for posting this link. As usual, Nick cuts through the fog in an incisive way. :ok:

[Grumpyasever]

Quote: Answering the OT bit about 'hopping into the air', the Bristow instructors would do a full spot turn after an engine off to the ground at Redhill in the 206 with the engine still at idle. Plenty of inertia in the Bell main rotor http://images.ibsrv.net/ibsrv/res/src:www.pprune.org/get/images/smilies/wink2.gif

Don't remember anyone doing that!:confused:

[Landroger]

As an 'interested SLF', with no connection with rotary wing flight other than a semi scale Bell 47 R/C helicopter I built - but never really learned to fly - nearly forty years ago, ground resonance is not entirely unknown to me. However, I always understood that gyroscopic precession played a part?

I can understand the rapid escalation of the phenomenon - positive feedback in the system - and the utter destructiveness if allowed to propagate. I can also understand out of spec' springing/damping being a contributing factor, but I don't really understand exactly what triggers the onset? And does gyroscopic precession - a pretty powerful factor - have any influence?

Roger.

[paco]

Landroger - in brief:

In flight, the assorted rotors, engine(s) and drive shafts transmit their own vibrations to the structure which are generally stabilised because there is no real focal point for them to attach to.

On the ground, however, vibrations can focus through the landing gear - if its natural vibration matches that of the main rotor, every time a blade rotates, the vibrations can receive another reflected pulse to increase their amplitude, which could cause the aircraft to rock back and forth and eventually tip over and be destroyed. This is especially problematical if the C of G of the rotor disc is away from the mast* and creates a wobble, and if any damping effect from the undercarriage is not available (any oleos may be fully extended). Peculiar to helicopters with dragging hinges (which implies articulated heads), Ground Resonance is indicated by an uncontrollable lateral oscillation (because roll inertia is lower than for pitch) increasing rapidly in sympathy with rotor RPM.

*Hooke's Joint effect can cause this.

The official teaching (in the exams anyway) is to lift off if you have flying RPM and to cut the throttle if you haven't. Its best to do this quickly so that the dampers are all snapped back in line at once to stop any eccentric motion. One of Biggin Hill Helicopters' Schwiezers used to suffer from this a lot and they used to do an engine off landing from the hover every time.

Phil

[Landroger]

Thanks for that, although your reply has set me on a path of further reading and not a little brain damage! :eek:

I always prided myself on knowing more about how a helicopter works, than having skills enough to fly it. :sad: An hour in a Robbie for a birthday present, cured me of regretting not learning to fly rotary wing. I've done greasers in America in a 152 when I treat myself to a one hour 'try and see' lesson from time to time. Never really had a 'proper' lesson. But while heliboppers are fascinating and deeply interesting, as far as I'm concerned you need fourteen fingers, rubber legs and reactions like a cat to fly them. :sad:

I need to get my head round why and how the rotor articulation becomes a Hooke joint and why it can't be made into a Constant Velocity joint, which would cure the problem. I suspect greater minds than mine ......... :ugh:

Roger.

[Gemini Twin]

Paco can you show me a main rotor drive with a Hooke's Joint or explain your example a little more?
Thanks GT.

[Brian Abraham]

There is one story circulating in the aviation media that the 350 in question experienced "excessive vibration" in flight prior to landing.

[paco]

The Hooke's joint effect I mentioned above is more in context for forward flight, when the disc is not level, but in the hover an imbalanced rotor mast can come from a sticky damper. However, for those who are interested, here is HJ effect as I understand it:

In the hover, the C of G of a blade will be the same radius from the axis of rotation all the way round the disc. When you move the cyclic to change the disc attitude, the changed radius will make the blade lead or lag about the dragging hinge as it flaps up or down.

In forward flight, the advancing blade speeds up and moves forward on its drag hinge or bearing as it flaps up (if there isn’t a hinge, the blade will just bend). Similarly, the retreating blade moves back slightly as it starts to slow and flap down. Thus, lead and lag arise from the aerodynamic effects that cause flapping.

Image 1 (http://www.electrocution.com/HOOKE2.jpg)

The end result is that both blades (and their Centres of Gravity) lie forward of a line drawn laterally across the rotor hub as they try to reposition themselves, as well as moving towards the retreating side.

So, whenever the driving and driven axes (the shaft axis and axis of rotation) are not in line with each other, the C of G of the whole rotor system is placed forward of the mast in the direction of flight, which will give the mast a tendency to bend - one reason why the Bell 206 has a speed limit above a certain torque setting (mast bending stresses). This imbalance creates Hooke’s Joint Effect, which looks like this from the side:

Image 2 (http://www.electrocution.com/HOOKEB.jpg)

A Hooke's joint (later called a universal joint by Henry Ford) allows a drive shaft to bend whilst it is rotating - in other words, it can allow power transmission to go round corners, within certain limits, by using two rotating joints at 90° to each other. Thus, it can compensate for misalignments and relative movement between the shafts they connect. It would appear that the Hooke's Joint Effect refers to the Bell 47 which had two hinges at 90° to each other in the rotor hub, gimbal fashion, which is strange, because it ought to relate more properly to articulated heads. In other teetering heads, such as the Bell 206, the ring was not used so there was only one hinge (the teetering one) but the expression was retained.

Phil

[Gemini Twin]

Very intersting P but not quite relevent to this incident. Post above your last mentioned vibrations during the flight, I would not be surprised if it had a starflex failure leading to the shackup upon landing. This wouldn't be the first time for a AS350 to seriously damaged after landing with a broken star flex.

[paco]

Somebody asked the question, so I answered! :)

I hear you about the starflex though.

Phil

[Gemini Twin]

No problem paco, I really enjoyed your explaination. Never to old to learn.
I remember a BA landed on a rig deck with a broken starflex causing a massive inbalance condition during rundown. Not quite the classic ground resonance, which can occur with a serviceable helicopter given the right conditions, but the effect was almost the same. Amonst other things, broken tail boom (not due to M/R strike) and lucky it didn't dance over the side.

[HeliStudent]

Explained: AS350 Eurocopter Helicopter Self-Destructs - YouTube

[rottenjohn]

Speaking from personal experience. I once landed AS350 carefully on the flat strip at the rear of the starboard skids, then applied a little too much downward pressure on the collective and ground resonance kicked in immediately - it happens too easily. I lifted off at once and repeated the landing, this time with micro-millimeters of pressure easing on the collective for an uneventful setdown. Never to be repeated again.

phew, close call. Gotta be careful on those strips on the right hand skids.