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whats the technical cause of the massive two-per vibrations of an airworthy Huey

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whats the technical cause of the massive two-per vibrations of an airworthy Huey

Old 20th Apr 2021, 16:07
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whats the technical cause of the massive two-per vibrations of an airworthy Huey

There is a nice vid about the Huey the Nelson-D'Anona foundation is operating.
And I noticed the two per revolution they are famous for, while bringing the MR up to speed.

Question:
How can a rotor that would feel rather balanced at nominal rpms cause that enormous amount of oscillations during spool up?

Centrifugal force cause by an imbamace is proportional to rpms squared !

I could understand if there would exist a specific rpm where the system and its dampeners enter resonance,
but then slightly off that rpm everything should be rock solid.
Given the structural stress rotational imbalance such a heavy rotorsystem would cause,
I presume the Huey would have some rather tight balancing limits to be cleared as airworthy, not?

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Old 20th Apr 2021, 17:17
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It has more to do with the resonance frequency of the airframe and various other components. Everything is designed to have a resonance frequency outside the normal operating frequencies of the main rotor, tail rotor, main gearbox, and engine. So when you are NOT at the normal operating frequencies, other components can resonate.
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Old 20th Apr 2021, 18:59
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Originally Posted by Reely340 View Post
There is a nice vid about the Huey the Nelson-D'Anona foundation is operating. https://www.youtube.com/watch?v=zunO6iUVUT0
...
I could understand if there would exist a specific rpm where the system and its dampeners enter resonance,
but then slightly off that rpm everything should be rock solid.
Given the structural stress rotational imbalance such a heavy rotorsystem would cause,
I presume the Huey would have some rather tight balancing limits to be cleared as airworthy, not?
Once the rotor is out of a pure hover and has edgewise airflow, there are significant harmonic air loads on each blade. 1 per rev, 2 per rev, 3 per rev and so forth. These forces variously sum or negate themselves at the rotor hub and transmit certain frequencies to the fixed system. 2 per rev vertical blade loads sum to a 2P vertical airframe vibration while 1P and 3P horizontal forces in the blades sum to a 2P horizontal vibration in the airframe. Higher frequency harmonic air loads also sum, but the magnitude of the forces generally drop as the ďper revĒ increases, unless the blades themselves are poorly tuned and get excited near a resonance.

The 1P and 2P harmonic air loads in a rotor are large enough, that even a very well tuned structure sees large loads without a hinge in the relevant direction, but of course, hinges introduce some new problems, particularly hinges in the rotor plane.
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Old 21st Apr 2021, 00:36
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Originally Posted by SplineDrive View Post
Once the rotor is out of a pure hover and has edgewise airflow, there are significant harmonic air loads on each blade. 1 per rev, 2 per rev, 3 per rev and so forth. These forces variously sum or negate themselves at the rotor hub and transmit certain frequencies to the fixed system. 2 per rev vertical blade loads sum to a 2P vertical airframe vibration while 1P and 3P horizontal forces in the blades sum to a 2P horizontal vibration in the airframe. Higher frequency harmonic air loads also sum, but the magnitude of the forces generally drop as the “per rev” increases, unless the blades themselves are poorly tuned and get excited near a resonance.

The 1P and 2P harmonic air loads in a rotor are large enough, that even a very well tuned structure sees large loads without a hinge in the relevant direction, but of course, hinges introduce some new problems, particularly hinges in the rotor plane.
Thx for the detailed reply, but those "vibrations" occurred way before translational lift, even before hovering, they were pratically still on the ground when everything was rocking wildly. So no lateral airflow, everything should be perfectly balanced at that stage.

I find it truly amazing that these vibrations happen early on, at low rpms, stationary, at or before hovering and precisely NOT in horizontal flight with all it asymmetry.
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Old 21st Apr 2021, 10:46
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Great stuff, Roscoe! True of all 2 bladed machines. I observe all those conditions and effects on the R44.
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Old 21st Apr 2021, 14:02
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I am damn sure it has nothing to do with downwash on the tail boom or aerodynamic effects caused by wind......
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Old 21st Apr 2021, 15:32
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Originally Posted by 212man View Post
I am damn sure it has nothing to do with downwash on the tail boom or aerodynamic effects caused by wind......
Pheewh, that was close. I was astonished to hear that explanation (because I'd figure at 100% and max. torque the downwash in a (near vibrationless) hover at the tail boom would be much stronger, but what do I know), and was just about to thank for the enlightenment.

So, if not MR downwash, what on earth causes a heavy structure like a Huey to rock as heavily as seen in this and other videos,
to the brink of rattling the pilots out of their seats, when an AS350 sits very calmly, while spooling up?
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Old 21st Apr 2021, 22:38
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Originally Posted by Reely340 View Post
Thx for the detailed reply, but those "vibrations" occurred way before translational lift, even before hovering, they were pratically still on the ground when everything was rocking wildly. So no lateral airflow, everything should be perfectly balanced at that stage.

I find it truly amazing that these vibrations happen early on, at low rpms, stationary, at or before hovering and precisely NOT in horizontal flight with all it asymmetry.
Itís a long video so Iím not sure which minute mark youíre referring to, but I did scrub through and see the cockpit video when the ship was at some ground idle condition with the crew clearly bouncing in their seats. At that condition, the rotor isnít tuned well to that condition and quite possibly the airframe/gear/ground system isnít either. Often, a helicopter is tuned for only a few specific rpm/operating conditions and away from those targeted conditions structure is nearer or in resonance, etc. The UH-1 is a tad before my time so I donít know much about the airframe/gear frequency placement but I do know something about the rotor.
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Old 22nd Apr 2021, 00:50
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Possibly a little negative pitch in the rigging at full down collective. Our 412's bounce a bit at idle and a smidge of collective sorts it out.
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Old 22nd Apr 2021, 10:09
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I don`t know the huey rotor system, but I assume that the rotation frequency crosses the blade lead-lag resonance.

The lead-lag resonance frequency increases with increasing rotor speed, but not as fast, as the rotor speed itself. Therefore there is a wide frequency band, where both rotation frequency and lead-lag resonance frequency are near each other.

look at the bottom fat dashed line, and its intersection with the first thin dashed line (BO105 chart):
https://www.researchgate.net/figure/...fig1_268529297

There are also many other resonances that are crossed by the rotor frequency or multiples of it, but I think the huey does not have a lead-lag damper.
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Old 22nd Apr 2021, 10:18
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Curious, MM: does that chart date from back when Boeing licensed the MBB-105 rotor technology for their UTTAS prototypes ( YUH-61 )? Notation refers to Boeing.
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Old 22nd Apr 2021, 10:45
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I just picked a familiar looking graph from google :-)

Thank you for the pointer to the YUH-61. I did not know about that one.
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Old 22nd Apr 2021, 12:54
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Originally Posted by Reely340 View Post
Question: How can a rotor that would feel rather balanced at nominal rpms cause that enormous amount of oscillations during spool up?
What minute mark of the video are you referring to?

If I'm following your narrative, you are asking about "oscillations" in the M/R up to the ground idle RPM, which can be caused by several things. If vertical, usually means the minimum blade angle is still set, i.e., the autorotation RPM is off a bit, which is "common" in some heavy inertia blades systems. If lateral, usually points to the static blade alignment is off a bit, or the M/R hub trunnion centering is off a bit. Both of the lateral possibilities can be balanced out during the dynamic track/balance procedure and are usually only noticed at very low M/R RPMs.
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Old 22nd Apr 2021, 14:53
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Been a long time but some things to possibly check include loose skid saddle mounts - these can set up a 2 per rev bounce on a tarmac...lift to hover or jack and get a maintainer to give the skids a good bang and shake to check security of mounts and a good check.

As Evil Twin states, similar to the B412, holding about 1/2inch or so of collective can also markedly reduce this if saddle mounts are a problem as well.

Another possible cause of 2 per rev in huey/teetering head systems with transmission mounts, can possibly be worn pylon damper and/or transmission mounts. There is a hover check for the pylon damper but unfortunately I don't have the old test flight schedule handy with me at the moment to check. Something along the lines of a smart 2inch cyclic displacement and the pylon rock should stop within 2-3 oscillations of the Tx (but don't quote me on that).

See if any of the above may be a culprit.

cheers

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Old 22nd Apr 2021, 15:29
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I would agree with all of the comments above related to mechanical origins for these vibes, but some vibes outside of operating RPM are always going to be there as the rotor system is tuned for 100%. Certainly, all control rod bearing rod ends should be inspected, and the skid gear attachments, but much of this is the nature of the beast. If it's smooth in flight regimes, then it's doing its job.

There is a Bell Engineering explanation to this as well, and it ties into a couple of comments above, with the same solution, pull a small amount of collective to smooth is out. https://www.uh1ops.com/amp/2015/06/10/collective-bounce

Collective Bounce



Pilots have asked me to please do something about aircraft that have a bounce with flat pitch running on the ground. I tell them to pull up slightly on the collective until the bounce is reduced. I could adjust the pitch change links to make the aircraft smoother, the problem is I have just changed the auto rotation RPM. I try to get them to understand auto rotation RPM is much more important than their short term comfort. The following article from Rotor Breeze explains why this happens. Peter Frinchaboy



From Bell Helicopters Rotorbreeze 2007:

LEADING EDGE PITCH LINK MAIN ROTOR TO MAST COUPLING PHENOMENON



Many Bell Helicopter models have leading edge pitch link installations on the main rotor hub. Each of these is subject to the vibration or ground bounce caused by the Leading Edge Pitch Link Main Rotor to Mast Coupling Phenomenon. This has often been mistakenly identified as being caused by Negative Pitch in the blades. This article explains why leading edge pitch link rotors can have this bounce.


1. The main rotor mast bearing of all Bell aircraft has axial play by design. This play is usually only observable when the mast does not have any weight from the rotating controls or main rotor hub attached to it. This allows the mast to move up and down axially on the mast bearing. It also allows some angular/side-to-side movement at the top of the mast. For example, the model 407 has maximum allowable axial play (up & down) of 0.065 inch.


2. The swashplate and support are mounted fixed to the top of the transmission. These controls have no vertical movement unless commanded by the control inputs from the pilot.


3. When the main rotor attains sufficient rpm and blade pitch, it lifts the mast and rotating controls up the distance of the axial play in the mast bearing from the static position (at the down position of the axial play) to the operating position (at the top of the axial play). The point at which this happens is variable due to rpm and the basic minimum pitch angle (Auto-Rotation Speed Adjustment) set in the blades.


4. If the basic minimum pitch angle set in the blades is such that lift occurs as the main rotor reaches operating rpm, the main rotor lifts. The leading edge pitch links are fixed and as the rotor lifts, the pitch links decrease the blade pitch angle and the rotor loses lift and moves back down the amount of the axial play.

When the rotor moves down, the pitch links now increase the blade pitch and the rotor lifts back up and then the whole process keeps repeating. The process repeats at a frequency of the rotor rpm times the number of blades. This vibration or bounce in the aircraft can be quite severe and show a high lateral vibration or a high vertical vibration on test equipment on the ground.


5. Usually a slight increase in collective pitch will cause the rotor to sustain its lift and the vibration will go away. In many cases this indicates that the auto- repeats at a frequency of the rotor rpm times the number of blades. This vibration or bounce in the aircraft can be quite severe and show a high lateral vibration or a high vertical vibration on test equipment on the ground. rotation rpm is set on the high side of the adjustment range. Normally, the auto-rotation rpm should be decreased, which will increase the basic minimum pitch angle set in the blades. This will cause the rotor to sustain lift at flat pitch and at operational rpm (100%). Alternatively, the pilot must input a slight amount of collective pitch at operational rpm (100%) to prevent the bounce.

Either method will stop the vibration and bounce.

The following Bell Helicopter aircraft have leading-edge pitch links. The table shows the normal maximum axial play you will see in their respective mast bearings as a guide to how much vertical movement the mast has.



Model Max Mast Axial Play Model Max Mast Axial Play

206A/B/B-3 0.037 inch 206L/L-1/L- 3 0.037/0.039 inch.

206L-3/L-4 0.058 inch OH-58A/C 0.037 inch

UH-1 (204 rotor) 0.038/0.040 inch 204/B 0.038/0.040 inch

205/A/A-1/B 0.038/0.040 inch 212 0.038/0.040 inch

210 0.038 inch 407 0.065 inch

412 0.040 inch


The 427 mast bearing has 0.085 inch axial play but is limited by design in its movement.

Note: the models UH-1(540 style rotor), all AH-1’s, 222/B/UT, 230, 214A/B/C, 214 ST, and 430 have trailing edge pitch links and are not subject

These numbers are for reference only and not for use as an inspection requirement for the bearings. Note that some models have alternate bearings with different normal maximum axial play. Always consult the appropriate maintenance manual for current allowables.

If you have questions on this subject, feel free to contact me.

David C. Burch

Senior Customer Support Representative

Bell Helicopter Textron, Inc.

P.O. Box 460640

Aurora, CO 80046-0640

Phone: +1-(720)-870-7414

Facsimile: +1-(817)-278-0096

[email protected]


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Old 23rd Apr 2021, 12:52
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Originally Posted by SplineDrive View Post
Itís a long video so Iím not sure which minute mark youíre referring to, but I did scrub through and see the cockpit video when the ship was at some ground idle condition with the crew clearly bouncing in their seats. At that condition, the rotor isnít tuned well to that condition and quite possibly the airframe/gear/ground system isnít either. Often, a helicopter is tuned for only a few specific rpm/operating conditions and away from those targeted conditions structure is nearer or in resonance, etc. The UH-1 is a tad before my time so I donít know much about the airframe/gear frequency placement but I do know something about the rotor.
Tune? It should be balanced right. If something is balanced it is so at 100 rpms as well as 400, not?
Considering that the centrifugal force increases proportional to rotational speed squared (!), I'd be very nervous when something is already rocking visibly at 100 rpms to go on and crank it up 400ish rpms, as that would create 16 times the force working against bearings and frame than one could witness rocking the scene at 100 rpms.
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Old 23rd Apr 2021, 12:55
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Originally Posted by Evil Twin View Post
Possibly a little negative pitch in the rigging at full down collective. Our 412's bounce a bit at idle and a smidge of collective sorts it out.
You mean asymmertical negative pitch at full down collective?
Well I can see how the rigid head of a BO would translate uneven lift into mast torque and vibration,
but a teethering head would simple have one blade tip lower than the other, tilt accordingly and not "tell the frame" about it, no?
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Old 23rd Apr 2021, 13:05
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Originally Posted by Cyclic Hotline View Post
4. If the basic minimum pitch angle set in the blades is such that lift occurs as the main rotor reaches operating rpm, the main rotor lifts. The leading edge pitch links are fixed and as the rotor lifts, the pitch links decrease the blade pitch angle and the rotor loses lift and moves back down the amount of the axial play.

When the rotor moves down, the pitch links now increase the blade pitch and the rotor lifts back up and then the whole process keeps repeating. The process repeats at a frequency of the rotor rpm times the number of blades. This vibration or bounce in the aircraft can be quite severe and show a high lateral vibration or a high vertical vibration on test equipment on the ground.
SWEET!
That is a very amazing revelation! That description sounds like the condition of "bouncing rotorshaft" should be avoided at all cost, because it stresses everything, from MR bearings to pitch links and blade bearings.
But wouldn't that rotorhead + mast banging up and down create a clearly noticeable and rather frightening noise?
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Old 23rd Apr 2021, 13:07
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Originally Posted by Reely340 View Post
Tune? It should be balanced right. If something is balanced it is so at 100 rpms as well as 400, not?
Considering that the centrifugal force increases proportional to rotational speed squared (!), I'd be very nervous when something is already rocking visibly at 100 rpms to go on and crank it up 400ish rpms, as that would create 16 times the force working against bearings and frame than one could witness rocking the scene at 100 rpms.
This is not correct. Balanced does not equate to zero vibration. Vibration at 100Hz may excite other aircraft structures whereas vibration at 400Hz may not. In fact, small vibrations at the resonant frequency of a structure can easily result in the damage or destruction of that structure. This is why certain helicopters have regions of rotor RPM where operation for more than a few seconds is prohibited. The 206 comes to mind as a good example (which in that example I believe is related to tail rotor driveshaft whirl modes but I could be mistaken).
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Old 23rd Apr 2021, 13:19
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Originally Posted by Cyclic Hotline View Post
I would agree with all of the comments above related to mechanical origins for these vibes, but some vibes outside of operating RPM are always going to be there as the rotor system is tuned for 100%. Certainly, all control rod bearing rod ends should be inspected, and the skid gear attachments, but much of this is the nature of the beast. If it's smooth in flight regimes, then it's doing its job.

There is a Bell Engineering explanation to this as well, and it ties into a couple of comments above, with the same solution, pull a small amount of collective to smooth is out. https://www.uh1ops.com/amp/2015/06/10/collective-bounce

Collective Bounce



Pilots have asked me to please do something about aircraft that have a bounce with flat pitch running on the ground. I tell them to pull up slightly on the collective until the bounce is reduced. I could adjust the pitch change links to make the aircraft smoother, the problem is I have just changed the auto rotation RPM. I try to get them to understand auto rotation RPM is much more important than their short term comfort. The following article from Rotor Breeze explains why this happens. Peter Frinchaboy



From Bell Helicopters Rotorbreeze 2007:

LEADING EDGE PITCH LINK MAIN ROTOR TO MAST COUPLING PHENOMENON



Many Bell Helicopter models have leading edge pitch link installations on the main rotor hub. Each of these is subject to the vibration or ground bounce caused by the Leading Edge Pitch Link Main Rotor to Mast Coupling Phenomenon. This has often been mistakenly identified as being caused by Negative Pitch in the blades. This article explains why leading edge pitch link rotors can have this bounce.


1. The main rotor mast bearing of all Bell aircraft has axial play by design. This play is usually only observable when the mast does not have any weight from the rotating controls or main rotor hub attached to it. This allows the mast to move up and down axially on the mast bearing. It also allows some angular/side-to-side movement at the top of the mast. For example, the model 407 has maximum allowable axial play (up & down) of 0.065 inch.


2. The swashplate and support are mounted fixed to the top of the transmission. These controls have no vertical movement unless commanded by the control inputs from the pilot.


3. When the main rotor attains sufficient rpm and blade pitch, it lifts the mast and rotating controls up the distance of the axial play in the mast bearing from the static position (at the down position of the axial play) to the operating position (at the top of the axial play). The point at which this happens is variable due to rpm and the basic minimum pitch angle (Auto-Rotation Speed Adjustment) set in the blades.


4. If the basic minimum pitch angle set in the blades is such that lift occurs as the main rotor reaches operating rpm, the main rotor lifts. The leading edge pitch links are fixed and as the rotor lifts, the pitch links decrease the blade pitch angle and the rotor loses lift and moves back down the amount of the axial play.

When the rotor moves down, the pitch links now increase the blade pitch and the rotor lifts back up and then the whole process keeps repeating. The process repeats at a frequency of the rotor rpm times the number of blades. This vibration or bounce in the aircraft can be quite severe and show a high lateral vibration or a high vertical vibration on test equipment on the ground.


5. Usually a slight increase in collective pitch will cause the rotor to sustain its lift and the vibration will go away. In many cases this indicates that the auto- repeats at a frequency of the rotor rpm times the number of blades. This vibration or bounce in the aircraft can be quite severe and show a high lateral vibration or a high vertical vibration on test equipment on the ground. rotation rpm is set on the high side of the adjustment range. Normally, the auto-rotation rpm should be decreased, which will increase the basic minimum pitch angle set in the blades. This will cause the rotor to sustain lift at flat pitch and at operational rpm (100%). Alternatively, the pilot must input a slight amount of collective pitch at operational rpm (100%) to prevent the bounce.

Either method will stop the vibration and bounce.

The following Bell Helicopter aircraft have leading-edge pitch links. The table shows the normal maximum axial play you will see in their respective mast bearings as a guide to how much vertical movement the mast has.



Model Max Mast Axial Play Model Max Mast Axial Play

206A/B/B-3 0.037 inch 206L/L-1/L- 3 0.037/0.039 inch.

206L-3/L-4 0.058 inch OH-58A/C 0.037 inch

UH-1 (204 rotor) 0.038/0.040 inch 204/B 0.038/0.040 inch

205/A/A-1/B 0.038/0.040 inch 212 0.038/0.040 inch

210 0.038 inch 407 0.065 inch

412 0.040 inch


The 427 mast bearing has 0.085 inch axial play but is limited by design in its movement.

Note: the models UH-1(540 style rotor), all AH-1ís, 222/B/UT, 230, 214A/B/C, 214 ST, and 430 have trailing edge pitch links and are not subject

These numbers are for reference only and not for use as an inspection requirement for the bearings. Note that some models have alternate bearings with different normal maximum axial play. Always consult the appropriate maintenance manual for current allowables.

If you have questions on this subject, feel free to contact me.

David C. Burch

Senior Customer Support Representative

Bell Helicopter Textron, Inc.

P.O. Box 460640

Aurora, CO 80046-0640

Phone: +1-(720)-870-7414

Facsimile: +1-(817)-278-0096

[email protected]

That's an excellent description, and makes a lot of sense. Interestingly our newer airframe was the worst culprit with the older machine (5 years wish between them) much smoother. Since the older airframe had it's 5 yearly they're both the same in terms of bounce.
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