Go Back  PPRuNe Forums > Aircrew Forums > Rotorheads
Reload this Page >

AS350 - Ground Resonance [video]

Rotorheads A haven for helicopter professionals to discuss the things that affect them

AS350 - Ground Resonance [video]

Old 24th Feb 2012, 16:27
  #41 (permalink)  
 
Join Date: Apr 2007
Location: Center of the Universe
Posts: 644
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.
EN48 is offline  
Old 24th Feb 2012, 16:53
  #42 (permalink)  
 
Join Date: Nov 2010
Location: Earth
Posts: 604
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.
SansAnhedral is offline  
Old 24th Feb 2012, 16:54
  #43 (permalink)  
 
Join Date: Sep 2005
Location: Montreal
Posts: 568
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.
malabo is online now  
Old 24th Feb 2012, 17:31
  #44 (permalink)  

 
Join Date: Nov 2000
Location: White Waltham, Prestwick & Calgary
Age: 67
Posts: 3,743
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
paco is offline  
Old 24th Feb 2012, 17:43
  #45 (permalink)  
 
Join Date: Jun 2010
Location: Milano, Italia
Posts: 2,425
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.


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.
Savoia is offline  
Old 24th Feb 2012, 17:54
  #46 (permalink)  
 
Join Date: Apr 2007
Location: Center of the Universe
Posts: 644
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?
EN48 is offline  
Old 24th Feb 2012, 23:21
  #47 (permalink)  
 
Join Date: Aug 2004
Location: London, UK
Posts: 216
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.
FairWeatherFlyer is offline  
Old 25th Feb 2012, 00:52
  #48 (permalink)  
 
Join Date: Jul 2007
Location: UK, US, now more ɐıןɐɹʇsn∀
Age: 36
Posts: 894
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)
MartinCh is offline  
Old 25th Feb 2012, 02:10
  #49 (permalink)  
 
Join Date: Feb 2010
Location: 'Stralia
Age: 53
Posts: 227
It's not eccentric to be eccentric

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

Now, where are my lucky socks ......
Peter3127 is offline  
Old 25th Feb 2012, 02:44
  #50 (permalink)  
Moderator
 
Join Date: Aug 2006
Location: Ontario, Canada
Age: 58
Posts: 4,161
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?
Pilot DAR is offline  
Old 25th Feb 2012, 03:42
  #51 (permalink)  
 
Join Date: Jun 2010
Location: Milano, Italia
Posts: 2,425
Peter: Thank you for continuing my education in the English language. Eccentric it is then. How unique!


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?
Savoia is offline  
Old 25th Feb 2012, 07:53
  #52 (permalink)  
 
Join Date: Feb 2010
Location: 'Stralia
Age: 53
Posts: 227
Phew

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 ) 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,
Peter3127 is offline  
Old 25th Feb 2012, 08:18
  #53 (permalink)  
 
Join Date: Aug 1999
Location: Gold Coast, Australia
Age: 71
Posts: 4,092
Before the guessing game gets too far advanced, maybe review Nick Lappos' post from Ground Resonance in a 206? 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
John Eacott is offline  
Old 25th Feb 2012, 09:13
  #54 (permalink)  
 
Join Date: Mar 2006
Location: On the move...
Age: 53
Posts: 359
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.
CYHeli is offline  
Old 25th Feb 2012, 09:24
  #55 (permalink)  
 
Join Date: Feb 2005
Location: Australia
Posts: 1,958
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.
topendtorque is offline  
Old 25th Feb 2012, 12:20
  #56 (permalink)  
 
Join Date: Apr 2007
Location: Center of the Universe
Posts: 644
Originally Posted by John Eacott View Post
Thanks for posting this link. As usual, Nick cuts through the fog in an incisive way.
EN48 is offline  
Old 25th Feb 2012, 20:10
  #57 (permalink)  
 
Join Date: Nov 2007
Location: Normandy
Posts: 20
AS 350 Ground resonance

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

Don't remember anyone doing that!
Grumpyasever is offline  
Old 26th Feb 2012, 17:28
  #58 (permalink)  
 
Join Date: Feb 2009
Location: Jungles of SW London
Age: 72
Posts: 354
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.
Landroger is offline  
Old 27th Feb 2012, 03:55
  #59 (permalink)  

 
Join Date: Nov 2000
Location: White Waltham, Prestwick & Calgary
Age: 67
Posts: 3,743
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
paco is offline  
Old 27th Feb 2012, 22:16
  #60 (permalink)  
 
Join Date: Feb 2009
Location: Jungles of SW London
Age: 72
Posts: 354
paco

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

I always prided myself on knowing more about how a helicopter works, than having skills enough to fly it. 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.

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 .........

Roger.
Landroger is offline  

Thread Tools
Search this Thread

Contact Us Archive Advertising Cookie Policy Privacy Statement Terms of Service

Copyright © 2018 MH Sub I, LLC dba Internet Brands. All rights reserved. Use of this site indicates your consent to the Terms of Use.