Bounces, Shunts, Verticals and Laterals

[rotorque]

Hey guys,

Lets see if we can head off the downward trend as I still find this forum a great deal of interest. Please be aware that ALL members are free to post a reply as I (and no doubt others) will get a huge amount of information from it, regardless of whether its from an engineer or a pilot.

One question that defines Helicopters has always been a tricky one to explain. Engineers seem to be able to talk it as a second language (as they should) but when it has come for a young pilot to explain to an engineer what is happening to the machine around them, it has been a case of 'baffle with bul5hit' and hope that you have clouded the scenario enough that the engineer has no idea what is going on and therefore has to fix everything to make everyone feel comfortable.

Can any of you out there define the difference between a Vertical bounce and a lateral one? Always enjoyable.

I will try to start by saying that a lateral bounce is a product of mass, while a vertical is a product of aerodynamics. One is more noticeable in the hover while the other is more noticeable with airspeed.

Another bit of info that may be usefull, is whether one or the other is more dangerous.

We all love stories, and I have one about a crack in the main rotor head of a 47. If it wasn't for the bounce it produced the end result would have been spectacular to say the least.

Any way I hope this sparks off some good responses. Don't hesitate to talk about multibladed systems either cos I think the difference between semi and rigid systems should be compared.

Cheers.

[offshoreigor]

Rotorque:

Excellent Thread.

To put it simply, If you think about a lateral vibration or a shuffle, most noticable in the hover, it is generally caused by a weight imbalance in the rotor system. A simple addition of weight or reduction generally will cure this.

A vertical vabration or hump, is generally caused by the way the blades fly or the tracking (which is not to say that the blades have to be in track to be smooth). In a multi-bladed system, you may experience what is known as a 3 per or 5 per rotation vibration. this is usually remedied by either a trim tab adjustment or pitch rod adjustment but may also require a weight adjustment. Most companies use the 8500 vibration analysis equipment and if all adjustments recommended are followed, then you will usually end up with a pretty smooth machine.

Another consideration is whether the Vib is a medium freq (MRH) or a high freq (TR). these vibs can usually be quite distinct and the solution arrived at by any competent crew.

Well that's my two cents worth.

Cheers, OffshoreIgor http://www.pprune.org/ubb/NonCGI/eek.gif

[Hughes500]

Offshore

Don't forget in some multi bladed machines such as a 500D / E you have to balance the head with no blades on first. Highly amusing watching novice engineers/pilots trying to work out why they can't track and balance a 500 as they haven't done the head.

Other things that can effect lateral and verticals are the dampers, if you got a soggy one it will play havoc with the track and can upset the tail rotor. Had before TR balance problem at Ground idle, rotor would not respond to weight adjustments, when it did went from one side of chart to the other, cause dampers.

Other problems can be solved by placing weights in the stabilisers - common on 500.

Sorry this is 500 related, I know the machine best. 300' s seem to be more tolerant although a bad set of friction dampers can be interesting ie difficult to see the world when in auto. However I reckon a good set of friction dampers as good as elastomerics if not better.

Hope this helps, loof forward to a good topic here. Quick question normally use a bog std Chadwick how good is the 8500 worth the money.

Cheers guys

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[Skycop]

Vertical bounce can also occur as a result of autopilot faults. "Spikes" or transient faults can cause sudden inputs to the pitch channel. This can be very difficult to identify and isolate.

SC

[offshoreigor]

Hughes500:

Your right about dampers. I didn't think to mention them on the lateral side because they only tend to have effect on start up or shut down on Sikorsky's. Although the odd 61 damper has siezed in flight giving a memorable ride!

Cheers, OffshoreIgor http://www.pprune.org/ubb/NonCGI/eek.gif

[Lu Zuckerman]

Regarding having to check the balance of a rotor system due to say an overnight change or change from one flight to another it would require some investigation to determine the cause before hooking up the Chadwick system. Depending on the weather in your area the blades can pick up water during a rainstorm and if it freezes during the night the blades depending on their position relative to the rain direction can pick up varying amounts of weight. Blades in most cases are mass balanced to ¼” ounce and any additional weight no matter how much, can effect the balance of the rotor system. If you go directly to the electronic balance you can compensate for the weight change but the next day when it warms up the water will be expelled and then you have to go back to the balance procedures. That is why I would recommend the use of blade sleeves in rainy or freezing weather or at the least, tie down tip boots. If the Hughes rotorhead is that sensitive to mass balance then the same problems with freezing water can manifest itself causing a mass imbalance.

Dampers can at times cause a shuffle due to the imbalance of the rotor disc. They can as described above cause a memorable ride when they seize. In the described case on the S 61 it would require a dual failure to result in the described effect. On a 300C, a damper was improperly assembled and it caused the helicopter to crash killing the pilot. There is another problem lurking in the wings for the operators/pilots of the later Hughes / MD helicopters that have elastomeric dampers. This also includes the AH-64.

When the elastomeric dampers are selected to go onto a rotorhead they must test to within 5% total deviation between the damper that tests highest in resistance to stretch and compression to the one that tests lowest to the same tests. If one or more of the dampers exceed this 5% limit then you can get into (are you ready for this) air resonance.

Air resonance like ground resonance will have the same effect. It can tear you apart. Herein lies the lie about elastomeric elements. If you find that there is sufficient deterioration in an elastomeric element requiring change you should check the new one against the ones already installed on the rotorhead. That means that each operator must have the machine to perform the tests. I do not know if this is the case because I left the Apache program many years ago.

In order to properly perform the test the elements must be removed from the rotorhead and installed in the machine. So much for an improvement of Maintainability when using elastomerics as opposed to metal bearings and viscous dampers. If in the performance of the test, you find that you can’t match the new one to the old ones removed from the head you will have to, either place the old ones on the shelf and start with all new ones, and when you accumulate sufficient dampers on the shelf you can run the test and select those dampers that comply with the stretch/compression limits and the next time a damper goes bad you install the newly selected set on the rotorhead and start over again. This will have limits as some of the dampers will reach mandatory retirement before others in the same set and you must start over again. I don’t know if this is covered in the maintenance manual. I got this straight from Lord over fifteen years ago.

On a fully articulated rotorhead you can test for a bad damper by bringing the helicopter to a hover and rapidly move the cyclic in a circular motion in the same direction as blade rotation. The introduction of cyclic in this manner will cause the disc to tip all around the tip path and cause the blades to continuously lead and lag. This will give an indication of bad damper, as it will cause the disc to shuffle. The rest is up to the mechanics to determine which damper is bad.


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The Cat

[This message has been edited by Lu Zuckerman (edited 04 February 2001).]

[This message has been edited by Lu Zuckerman (edited 04 February 2001).]

[This message has been edited by Lu Zuckerman (edited 04 February 2001).]

[This message has been edited by Lu Zuckerman (edited 04 February 2001).]

[willy the one]

Always thought vertical bounce was out of balance main rotor, due to C of G shifting with out of track flying of blade. Lateral just basic lack of mass balance

[cullcov]

Ref Vertical bounce.
The OK phrase used to be vertical "bounce"
as opposed to lateral "shake".
One very insidious cause for vertical bounce that happened to me on a Bell 206 during a lifting mission was Hydraulic Failure. As an instructor we usually demonstrate hydraulic failure as being a black and white affair, in other words the Heli is in manual or has control assistance. BUT BUT BUT in view of the construction of the splined pump supplying the hydrabolic power there is a very interesting grey period between the black and white. This results in the most baffling flight characteristics as the hydraulics are engaged/disengaged as the spline gradually disintegrates.In my case I was convinced that the vertical bounce lateral shake combination was leading to airframe failure. Without hesitation I released the u/s load and landed. The engineer and I went over the systems with a fine tooth comb, Nothing ,apparently, wrong.We then restarted the hel and realised that the hydraulic pump had failed properly and all was revealed.
MORAL? Even extreme feed-back forces felt on the controls may not indicate imminent breakup of your machine DO NOT PANIC!!! Look for the hydaulic switch (in differing cockpit positions depending on custom isation) and switch the damned thing into manual

[Lu Zuckerman]

To: cullcov


Vertical bounce is quite common on Bell helicopters due to the fact that when the blades are in line with the longitudinal centerline the blades have minimal lift and for an instant the fuselage drops and when the blades leave that position they start to lift and the fuselage rises again. This also causes the blades to flex and generate a traveling wave down the blades. In many Bell blades they incorporate a weight that is internal of the blade and is positioned at the nodal point of the wave thus canceling the wave before it reaches the head and the fuselage. That is why Bell invented the NodaMatic Suspension system. Now the fuselage is highly stable with no more vertical bounce but the driveline including the gearbox is being beaten to death.

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The Cat

[rotorque]

When I first started learning about helicopters, the one thing that I remember being taught was that verticals were bad.

Over time, as stated above, I learned that a vertical was really a tracking problem in the normal course of engineering. The worry about verticals came from a couple of problems with the masts of our KH4's. On one memorable occasion one of our pilots took off from a property and shortly there after started getting a 'bounce'. This bounce was described by the pilot as a short sharp bounce that was not a resonant type of movement (hard to describe I guess). He landed the aircraft and inspected it looking for what ever he could. Nothing was found so he took off again. Now this is where the crash comics would of had a field day, but fortunately for him he got it back down. When he took off the collective jammed. For about 1.5 minutes he tried to get the collective down while the bounce continued to manifest itself.

When the engineers flew out to the aircraft they found that the mast had a crack that covered 80% of the diameter. The only reason that he is alive today is that the crack was situated under the collective sleeve. When it finaly gave way the mast actualy twisted to a figure eight shape, when viewed from above, and jammed against the sides of the collective sleeve. Lucky for him it held till he got it back down.

Anyway it spooked a few people. But it got us all talking about the types of bounces etc that you can get in these types of helicopters. Because of this discussion, I believe it saved the life of another pilot.

He was flying a B47 on stream sampling work when he felt a slight lateral. It was the first flight of the day so he assumed that over night condensation had acumalated in the blade that was tied to the tailboom. (As Lu suggested, this does happen. In the dusty parts of Oz, it can be quite dangerous as the dust accumalates in the blades throught the tip cap. Mixed with water it runs down to the lowest point usually at night with condensation. If it dries and solidifies the machine slowly goes out of balance. Worst case scenario, it can move as a clod in the blade.) Over about 25 minutes the bounce became worse. At this point he bailed out of the job and called the engineers.

To get an increasing bounce means trouble - if you get a bounce for no reason - land.

The young engineer at the time didn't seem overly concerned as the bounce was not that big, he even went on to say fly it back to base as it was due a service anyway. This is where the pilot should probably be applauded cos he said no (a couple of times).

An engineer was sent to the machine. The pilot had a mind set that there was a crack in the mast. The stab bar was changed, the bounce got worse. The mast was changed and the bounce got worse. The swashplate was changed and the bounce got worse. Finally the head was changed and all was fine.

On inspection it was found that the yoke where the blade grip is attached had a crack that ran across two threads and around the yoke covering over two thirds of the radius. When he raised the collective it was literally opening up the crack and allowing one blade to rise. Now this sounds like a tracking problem but it was not felt as a vertical - it was a lateral. I can't help but think that if they had of ran the machine for another 10 minutes the blade would have been seperated as there was absolutely nothing else holding that blade on but an inch of metal. Now that is scary 5hit !!

Long winded I know but it proves the worth of other peoples experiences. The moral I guess is that if you get a bounce in flight there has to be a reason. It could be just blade tape moving or moisture etc, but even so land the machine to find out. You must then be able to tell an engineer (usualy over the phone) what the bounce felt like - either a lateral or a vertical.

You gotta love stories.

cheers.

[Lu Zuckerman]

While we are on the subject of accumulation of dust and the eventual solidification due to moisture, the first place to look when you get a high frequency vibration is the cooling fan on the TR Drive Shaft. It can cause the fan to go out of balance and the imbalance will be reacted by the shaft bearing immediately next to the fan. This will manifest itself in bearing failure.

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The Cat