Huey "bounce"
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Huey "bounce"
Has anyone out there had a bad vertical bounce develop very quickly with an external load in a UH-1H? Happened to me today. Came on in turns only and developed very quickly into a resonance that feels like the airframe is about to tear herself apart. Corrective action seems to be lowering the collective and then flying straight and level. Obviously this isn't ideal as a turn is usually required at some stage??!!
Last straw was when on a very shallow approach to the landing site with 1.6T load on a 30 foot line, as I started to ease in collective at about 150 feet the "bounce" started again and was off the scale in a few seconds. Which left me with an interesting decision..... Lower collective to get out of the "bounce", ( and spud in) or raise collective and make the resonance worse!??!! I did the later by the way and rode out a horrendous resonance til load was clear of everyone and buttoned.
This problem doesn't happen with a 1.5T internal load, but external is a b--tch!
Any feedback from anyone who knows the ole Huey appreciated....
Last straw was when on a very shallow approach to the landing site with 1.6T load on a 30 foot line, as I started to ease in collective at about 150 feet the "bounce" started again and was off the scale in a few seconds. Which left me with an interesting decision..... Lower collective to get out of the "bounce", ( and spud in) or raise collective and make the resonance worse!??!! I did the later by the way and rode out a horrendous resonance til load was clear of everyone and buttoned.
This problem doesn't happen with a 1.5T internal load, but external is a b--tch!
Any feedback from anyone who knows the ole Huey appreciated....
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See this regarding collective bounce.
Transportation Safety Board | Home
Report is about a Longranger, but the Huey/205 is susceptible as well.
However, judging by your description, I would venture a guess that the 5th mount on your machine is hooped. Really easy to get a vicious bounce when the cyclic is overcontrolled, even a little bit. Turns scary in a hurry.
Transportation Safety Board | Home
Report is about a Longranger, but the Huey/205 is susceptible as well.
However, judging by your description, I would venture a guess that the 5th mount on your machine is hooped. Really easy to get a vicious bounce when the cyclic is overcontrolled, even a little bit. Turns scary in a hurry.
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Huey "bounce"
You don't say what troubleshooting was conducted, what was found and the corrective action was taken, nevertheless, here are my initial thoughts:
1. Check all flight controls for integrity, particularly rod-end wear.
2. Check integrity of the fifth mount and beam.
3. Check the transmission mounts for deterioration.
4. Check the transmission dampers both for friction and attachment - they have a habit of pulling the attachment brackes off the pylon structure.
5. Check the friction setting of the synchronized elevator torque shaft (in the tailboom, connecting the L/H and R/H elevators).
Hope this helps, let us know the eventual outcome.
1. Check all flight controls for integrity, particularly rod-end wear.
2. Check integrity of the fifth mount and beam.
3. Check the transmission mounts for deterioration.
4. Check the transmission dampers both for friction and attachment - they have a habit of pulling the attachment brackes off the pylon structure.
5. Check the friction setting of the synchronized elevator torque shaft (in the tailboom, connecting the L/H and R/H elevators).
Hope this helps, let us know the eventual outcome.
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Thanks....
Hey thanks so far guys. Our on staff engineer was stumped. Cheif piot has had collective bounce and it isnt that, but I'll follow up on the rest of the thoughts you have passed on. Just seemed strange that she flys nicely without the heavy external load.
We were wondering about transmission mounts?....
We were wondering about transmission mounts?....
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Could be a combination of things,
How's the track and balance in the turn? Is it worse one way or the other?
How steep a turn are we talking? eg would there be an increase of g's and therefore an increase in the effective load felt by the machine?
What sort of line are you using? Length? Is the load spinning?
Not having a go but does this just happen to you or are the others who fly it experiencing the same problem? Has it been happening for a while or has it come on suddenly?
Sorry for all the questions but I'm sure it will help someone come up with a solution for you.
Any extra bounce could very easily turn into collective bounce with the collective friction a little loose or as others have said the transmission dampeners on the way out.
I have had collective bounce a couple of times in different machines and each time it was induced by my being either too stiff on the collective or a bit rough on the controls.
How's the track and balance in the turn? Is it worse one way or the other?
How steep a turn are we talking? eg would there be an increase of g's and therefore an increase in the effective load felt by the machine?
What sort of line are you using? Length? Is the load spinning?
Not having a go but does this just happen to you or are the others who fly it experiencing the same problem? Has it been happening for a while or has it come on suddenly?
Sorry for all the questions but I'm sure it will help someone come up with a solution for you.
Any extra bounce could very easily turn into collective bounce with the collective friction a little loose or as others have said the transmission dampeners on the way out.
I have had collective bounce a couple of times in different machines and each time it was induced by my being either too stiff on the collective or a bit rough on the controls.
If you are going to check one thing, go for collective minimum friction. Collective movement feeding the oscillation.
Does it all the time
Minimum frictions (all axis) are important and very much overlooked
Have a great day!
Does it all the time
Minimum frictions (all axis) are important and very much overlooked
Have a great day!
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You should clearly understand the differences between Collective Bounce and Pylon Rock.
Collective Bounce can and will kill you if left unattended. Before further flight, I would ensure that the collective minimum friction check is complied with and rectified if neccessary. This is absolutely required to prevent any rotor forces, not contained by the hydraulic system, reaching the flight controls. You as the pilot should be the only one who inputs control change to the collective servo. However, in a damaged system (loose rod ends, universals, servo uniballs, etc., rotor forces can cause an uncommanded input to the servo, increasing in amplitude and spoiling your day.
And definitely ensure that you are using approved cables for your loads, nothing that provides elasticity in the line.
Pylon Rock does not increase in amplitude to a destructive force on it's own but it can become quite uncomfortable for pax and crew.
Pylon rocking of two to three cps is inherent in the 205 rotor, mast and transmission system. A rubber mounting and dampening system is used to keep this to a manageable level. Rocking can be magnified on fixed floats or heavy internal/external loads and is more noticeable when hovering IGE/OGE and during T/O and approach.
You can verify rocking in the hover and in forward flight. (These checks are available in your manuals). You simply do the cyclic inputs and check to see that the system is dampening the rock in the required time. If it does not, there is a problem in the dampening system.
And please do not forget the collective minimum friction and always know that it is available to you. If it isn't, get it fixed. I believe that it is still a 100 hr. requirement.
Gerry
Collective Bounce can and will kill you if left unattended. Before further flight, I would ensure that the collective minimum friction check is complied with and rectified if neccessary. This is absolutely required to prevent any rotor forces, not contained by the hydraulic system, reaching the flight controls. You as the pilot should be the only one who inputs control change to the collective servo. However, in a damaged system (loose rod ends, universals, servo uniballs, etc., rotor forces can cause an uncommanded input to the servo, increasing in amplitude and spoiling your day.
And definitely ensure that you are using approved cables for your loads, nothing that provides elasticity in the line.
Pylon Rock does not increase in amplitude to a destructive force on it's own but it can become quite uncomfortable for pax and crew.
Pylon rocking of two to three cps is inherent in the 205 rotor, mast and transmission system. A rubber mounting and dampening system is used to keep this to a manageable level. Rocking can be magnified on fixed floats or heavy internal/external loads and is more noticeable when hovering IGE/OGE and during T/O and approach.
You can verify rocking in the hover and in forward flight. (These checks are available in your manuals). You simply do the cyclic inputs and check to see that the system is dampening the rock in the required time. If it does not, there is a problem in the dampening system.
And please do not forget the collective minimum friction and always know that it is available to you. If it isn't, get it fixed. I believe that it is still a 100 hr. requirement.
Gerry
I would look at the "Lift Link"...and transmission mounts first....then consider the Collective Friction system. If the oscillation decreases with your hand off the Lever....it is probably a lack of friction on the collective.
The Maintenance Manual has a section on how to air test for pylon rock. It involves a stick stir while in climbing turns....the oscillation should dampen out after no more than 2-3 cycles. (Memory tells me anyway....)
What you are describing does not sound like Pylon Rock. Also interesting is it only occurs while slinging a load and not doing just internal load carrying.
That points to the lift link or a load rigging problem it would seem.
The Maintenance Manual has a section on how to air test for pylon rock. It involves a stick stir while in climbing turns....the oscillation should dampen out after no more than 2-3 cycles. (Memory tells me anyway....)
What you are describing does not sound like Pylon Rock. Also interesting is it only occurs while slinging a load and not doing just internal load carrying.
That points to the lift link or a load rigging problem it would seem.
Last edited by SASless; 9th Nov 2012 at 12:28.
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I've no great experience on the Huey, although my first flight in one was carrying a netted USL.
However, collective/ vertical bounce is a known phenomena on another aircraft I've operated; for example:
On the Blackhawk it can occur where the pilot himself is vertically "bounced" by the aircraft when lifting loads. The initial bouncing he suffers causes him to inadvertently induce small, but in-phase, collective inputs. This may result in the rapid amplification of naturally occurring vertical vibrations when picking up external loads. It can rapidly become a runaway situation causing major or even terminal airframe damage. The cure is to briefly let go of the collective and to screw on some more friction, which breaks the cycle.
I recall Sikorsky sending out a bulletin on this in the mid 1990s. Around that time our unit lost a big firebucket to the bottom of a deep lake, after a trainee pilot froze on the controls whilst experiencing the phenomenon during load pickup (6,000 lbs of water). The instructor found it necessary to jettison the load to recover the aircraft from a rapidly deteriorating situation.
However, collective/ vertical bounce is a known phenomena on another aircraft I've operated; for example:
On the Blackhawk it can occur where the pilot himself is vertically "bounced" by the aircraft when lifting loads. The initial bouncing he suffers causes him to inadvertently induce small, but in-phase, collective inputs. This may result in the rapid amplification of naturally occurring vertical vibrations when picking up external loads. It can rapidly become a runaway situation causing major or even terminal airframe damage. The cure is to briefly let go of the collective and to screw on some more friction, which breaks the cycle.
I recall Sikorsky sending out a bulletin on this in the mid 1990s. Around that time our unit lost a big firebucket to the bottom of a deep lake, after a trainee pilot froze on the controls whilst experiencing the phenomenon during load pickup (6,000 lbs of water). The instructor found it necessary to jettison the load to recover the aircraft from a rapidly deteriorating situation.
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Mast Rock/Pylon Rock
WARNING:- MAST ROCK/PYLON ROCK – BELL 212
This is the name given to the situation that occurs caused by inappropriate Pilot input to the controls or the effects of wind or turbulence on the aircraft flight path resulting in a violent rocking of the transmission and main rotor system. This condition is aggravated by equal and opposite AFCS input, worn mounts, bearings or dampers. Occurrence is invariably in or close to the hover. Avoidance relies on extremely smooth cyclic input at all times especially at low speeds or in the hover. Should mast rock occur it is imperative that normal flight conditions are regained as soon as possible to prevent damage to the airframe and its’ components.
I would agree with SASLESS - this is what I put in a sling load PPT Presentation about 20 years ago after having scared the **** out of myself on several occasions and having discussed the phenomena with a bunch of extremely experienced Canadian long liners. Doesn't sound the same as what you experienced!
Trog
Trog
I might add....a flight with Trog would scare the bejesus out of me too!
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I checked the thread this morning and was surprised and happy to see the replies. Thanks again guys. I don't claim to know everything about the Huey, (I have 500 hrs on type. All ag work and lifting) and I am open to the thought that it may have been "pilot induced", but after it happened a few times I double checked everything I was doing on the sticks and pretty much ruled that out. It wasnt a problem pulling 50psi getting the loads off the ground, and transition was ok.
We pushed her back in the hangar and the engineer is going to have a good think about things over the weekend...... Hopefully we can sort something next week. We do have a set of trans mounts coming from the states so I was thinking that was the problem. But I will show him the thread on the office puter on Monday. Maybe there will be things that will jog his memory.
Sat down with a bottle of James B Beams finest last night to settle the nerves and the world seems like a much better place this morning
Thanks again guys. Fly safe out there.
We pushed her back in the hangar and the engineer is going to have a good think about things over the weekend...... Hopefully we can sort something next week. We do have a set of trans mounts coming from the states so I was thinking that was the problem. But I will show him the thread on the office puter on Monday. Maybe there will be things that will jog his memory.
Sat down with a bottle of James B Beams finest last night to settle the nerves and the world seems like a much better place this morning
Thanks again guys. Fly safe out there.
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Had something very similar in a 212 awhile ago. I was an engineer working on a seismic job, we had a 212 going into what felt like an airborne ground resonance when coming into a hover with a heavy long line load on, not a nice feeling. Only happens with a load on.
Tried everything, new mounts, rigging etc. Replaced the transmission and the problem was fixed. The transmission had been assembled with mastinox between the splitlines . With a load the transmission was being pulled apart, the matinox was then a lubricant and the cases started moving.
Have the engineers check torque the splitline bolts, the washer stackup is an alloy then a steel, the alloy tends to get squashed and you loose torque on the bolts. If you have lost the torque retouquing may help but the splitlines may need cleaning to totally fix the problem.
Hope this helps
Tried everything, new mounts, rigging etc. Replaced the transmission and the problem was fixed. The transmission had been assembled with mastinox between the splitlines . With a load the transmission was being pulled apart, the matinox was then a lubricant and the cases started moving.
Have the engineers check torque the splitline bolts, the washer stackup is an alloy then a steel, the alloy tends to get squashed and you loose torque on the bolts. If you have lost the torque retouquing may help but the splitlines may need cleaning to totally fix the problem.
Hope this helps
Off beat as this may sound.....check the your seat for vertical movement. That could generate the PIO in the Collective if there are "friction" problems or worn rod ends. Along with the rod ends themselves...check for wear in the bolt holes where they attach.
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While there have been a few novel ideas suggested so far that I will be sure to remember in the future if the usual suspects don't pan out. As has already been said this is most likely a collective minimum friction problem causing collective bounce. The load bounces on the line inducing more of a vertical into the airframe than the aircraft on its own would normally see with just an internal load. Pylon rock is more of a cyclical lateral vibration not a vertical one. A bad 5th mount usually is the cause of pylon rock.
Let me re-iterate: check and adjust the collective minimum friction clamp before further slinging ops, aircraft have been lost because of this! While you're at it (i.e. your engineer) remove and clean up the friction clamps with some scotch-brite, it does wonders for collective feel-no more notchiness for the next several hundred hours hopefully.
Let me re-iterate: check and adjust the collective minimum friction clamp before further slinging ops, aircraft have been lost because of this! While you're at it (i.e. your engineer) remove and clean up the friction clamps with some scotch-brite, it does wonders for collective feel-no more notchiness for the next several hundred hours hopefully.
Last edited by cbox chip; 11th Nov 2012 at 04:55.
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What type of line?
Possibly way off the mark but ...
Could it be that the length and elasticity of the line is just right for your load? Creates a oscillation at natural frequency (suspended mass on a spring) that may coincide with the excitations caused by collective bounce etc.?
A longer or shorter line would test that pretty quickly.
Could it be that the length and elasticity of the line is just right for your load? Creates a oscillation at natural frequency (suspended mass on a spring) that may coincide with the excitations caused by collective bounce etc.?
A longer or shorter line would test that pretty quickly.
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Thanks for that cbox chip. The engineer did mention collective friction. I guess tomorrow will start with a sit down and big discussion (including all of the suggestions on here, even the seat) I'll let you all know how it goes.
Cheers
Cheers
The engineer 'mentioned' the collective friction. That suggests to me that it hasn't been checked. My experience is with the 205A-1, we had a case of collective bounce with an underslung load, which resulted in a heavy landing. IIRC there was very little in the MM about the collective friction adjuster apart from the static friction value. I add the following in case the MM is still deficient, please forgive me if the following is patronising it is not intended to be.
It is easy to check, remove the co-pilot's seat and the floor panel, disconnect the collective bellcrank and place a spring balance in the bellcrank and pull the collective up. Until then we had not ever checked this but we did every 100 hrs after that.
It is easy to check, remove the co-pilot's seat and the floor panel, disconnect the collective bellcrank and place a spring balance in the bellcrank and pull the collective up. Until then we had not ever checked this but we did every 100 hrs after that.
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Here's what the UH1H Pilot's Operator's Manual aka "dash 10" says:
8-46.
Collective Bounce
Collective bounce is a pilot Induced vertical oscillation of the collective control system when an absolute friction (either pilot applied or control rigged) is less than seven pounds. It may be encountered in any flight condition by a rapid buildup of vertical bounce at approximately three cycles per second. The severity of the oscillation is such that effective control of the helicopter may become difficult to maintain. The pilot should apply and maintain adequate collective friction In all flight conditions.
9-28. Collective Bounce
If collective bounce occurs.
1. Relax pressure on collective. (Do not ’stiff arm’ the collective.)
2. Make a significant collective application either up or down.
3. Increase collective friction.
8-46.
Collective Bounce
Collective bounce is a pilot Induced vertical oscillation of the collective control system when an absolute friction (either pilot applied or control rigged) is less than seven pounds. It may be encountered in any flight condition by a rapid buildup of vertical bounce at approximately three cycles per second. The severity of the oscillation is such that effective control of the helicopter may become difficult to maintain. The pilot should apply and maintain adequate collective friction In all flight conditions.
9-28. Collective Bounce
If collective bounce occurs.
1. Relax pressure on collective. (Do not ’stiff arm’ the collective.)
2. Make a significant collective application either up or down.
3. Increase collective friction.
