Elastomeric Straps
 

On the R22 rotor head and main rotor assembley there are two of these straps,if the rotor is subjected to any overspeed situation, what is the criteria then for the continued use of that craft ,or should it be grounded untill new Elastomerics are fitted, If flown again what is the possible outcome,would it be dangerous or safe ? and if the elastomerics are affected what else is likly to be affected, I feel that somewhere I have read that the transmision and engine should also be checked is that correct?

Vfrpilotpb :

My notes from the safety course I did say that a 5% overspeed is a simple maintenance check. A 10% overspeed is an expensive check requiring engine removal.

I've not seen engineering diagrams, but have seen elastomeric straps on other machines ( notably Squirrels ). I thought the R22 blades were held on with copious bolts, there being no need for straps since ( my understanding goes ) the elastomeric straps are used to take the flapping and lead/lag loads on rigid heads. Can someone light my confused darkness ?

[This message has been edited by The Nr Fairy (edited 28 March 2001).]

To: Vfrpilotpb and Nr Fairy

According to the R22 tech manual that I have (and it may have been changed) there are no elastomeric straps. As Nr Fairy has indicated the blades are retained by a series of bolts arranged about a circular pattern at the blade root.

Regarding elastomeric straps on a rigid rotor, a rigid rotor is just that and any flapping or lead / lag is a result of blade bending and absorption of the respective loads by the head. The only physical movement of the blade is about the feathering axis and these loads can be absorbed by bearings or a tension torsion strap pack.

Regarding the Squirrel it has elastomeric thrust bearings which allow blade feathering and at the same time they absorb centrifugal loading. The Squirrel rotorhead also has soft in plane elastomeric dampers to allow leading and lagging. This setup is similar to the Bell 412 and the 407.

The correct terminology is elastomeric bearings and not straps. Some helicopters have spherical bearings that allow feathering, flapping and lead / lag. These are used on some Sikorsky and Agusta rotorheads.

Now I’ll sit back and be challenged because I haven’t covered all of the bases. (An American term).


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The Cat

Yup, the R-22 head is definately not elestomeric ...

I would agree they could be called elastomeric bearings, but I like the term torsion straps better, bearings indicates an ability for failure. So far, at least on the Bell 430 in which Bell calls them "yokes" I haven't seen a life limit applied to them, but I have not reviewed the inspection criteria for an overspeed either, it woule be logical to assume they would be replaced, checked, and the same with the blades in such a system.

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Marc

To: RW-1

There is a big difference between tension/torsion packs and elastomeric bearings. A TT pack can be made of a series of flat stainless steel plates that are very thin with these plates placed one on top of the other to form what could look like a long and thin dog bone. One end is attached to the rotorhead and the other to the blade. The TT packs react centrifugal loading and they allow pitch change by bending torsionally. Some TT packs are made of a continuous wire or composite material and these designs perform just like the stainless steel packs described above. The Hughes 500 series and the Apache have strap packs which allow flapping and pitch change and provide a reaction point around which lead and lag take place. These strap packs are also made of thin laminations of stainless steel.

An elastomeric bearing is composed of two parts of which there are many making up the assembly. One part is natural rubber and the other is metal. The assembly is made up of layers of rubber, metal, rubber, metal, rubber … The rubber is vulcanized onto the metal and it becomes one assembly. Each layer of rubber is capable of some deflection. The composite assembly is capable of a great deal of deflection with each layer of rubber contributing to the total deflection. Some elastomeric bearings can react centrifugal, lateral and vertical loads and they are usually used as the main bearings in the rotorhead and allow flapping, leading and lagging and pitch change. Other types absorb only centrifugal load while allowing torsional movement. These are usually used in tail rotors.

The yoke that you referred to is also a part of the structure of the rotorhead. The elastomeric bearing is mounted inside of it and it most likely absorbs centrifugal loading while allowing pitch change. Leading and lagging pivots about the elastomeric bearing and is most likely absorbed by the soft inplane elastomeric dampers at the root of the yoke assembly.

Elastomeric bearings are bearings and are susceptible to failure although the failure manifests itself in different ways as opposed to a metal bearing. In most cases they are limited to 5000 hours but very few ever reach that limit. Elastomeric bearings are susceptible to Ozone damage unless they are impregnated with wax and they can be destroyed if they come in contact with hydrocarbon material and it is not cleaned off. They are also susceptible to solar radiation in that they are limited to an exposure of 160-degrees F for one hour in their total lifetime. Any thing in excess of this can reduce the life by 50%.

Elastomeric dampers used on the Hughes 500 series and the Apache have an even greater problem. All of the dampers must maintain a spread of 5% total difference between the lowest and highest when a tension compression test is performed. If this level is exceeded the helicopter can encounter air resonance which is just as bad as ground resonance. If a damper is replaced then all of the other dampers should be removed and the tension compression test performed. If the 5% limit can not be established then the dampers must be rematched which could require the replacement of several dampers. These dampers would be retained in order to get a match and that set of dampers could be used as a total package replacement for a defective damper. This can go on until it is not financially viable to replace the dampers because of approaching their life limit and then they are discarded. So much for maintainability.

Elastomeric elements are susceptible to the cold as well as heat. They become very stiff but the stiffness will go away after several cycles of the cyclic stick due to the internal friction raising the internal temperature of the rubber material in the bearing or, the damper. The same is true for elastomeric bearings used in the tail rotor.

Elastomeric rod end bearings are used in many applications and they too are susceptible to the same problems as their larger cousins.


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The Cat

[This message has been edited by Lu Zuckerman (edited 28 March 2001).]

[This message has been edited by Lu Zuckerman (edited 28 March 2001).]

The AS350 used to have different dash numbers of the elastomers for low temp ops.
I believe since they changed to the lord ones one size fits all.

Well forgive me for my wrongly named item, but i have not seen the R22 manual, so the actual item could be a bearing , but what I am getting at is this, and I am in a quandry as to whether or not to do anything about it, I was recently up in a SFH R22, I am always very careful on by pre flight sometimes to the annoyance of others, but hay what the hell, any way after about 5 mins of flying in about 18kny gst 22 I was not happy with a one per rev vibe, so 360 back land and turn off, everybody gone home so had no access to write up the obviouse fault, am the day after and I was then told by an other that they thought that R22 had ,had an overspeed but seeming ly everyone was keeping mum , I felt a little like the sheep in the lake district must feel now, so I am sorry if I got the description wrong, but I would just like to kow if some pratt by keeping quiet put my head in a possible bad day situation, and I would like you chaps to try to help me with it,
My regards
Peter VFr

VFR, is this the problem you had from another thread?
1.It is rumoured that this A/C may have suffered an overspeed but all are keeping their heads down.
2.You've flown this A/C since and it has an unusual wumper.
Answer - leave it on the ground until it's thoroughly checked or the next funeral you attend might be yours (if they find all the bits)
You can always print more money but you (anybody) only have one life....that's why we have engineers, to be as SURE AS ANYONE CAN BE that it's safe


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Ground tested, no fault found

To: Vfrpilotpb

Many years ago when I was in flight engineers school they showed us a movie made for the USAAF during WW 2. The movie I believe was titled Joe Instructor and Saint Pete. This AAF instructor was invited to heaven by St. Peter to observe some of his former students that had bought their share of the farm. One was an innocent victim of someone else that had overstressed a P 39 Aircobra and not reported his actions.

During the innocents flight when he was performing normal flight maneuvers his wings folded and he was killed. This sort of thing goes on all the time and like in the movie innocent people get killed. Much later, the USAF initiated a policy that if you had done something wrong and it was unintentional and reported the problem then action to correct the problem would be taken and the offender was let off with a warning.

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The Cat

Sorry for the delay in answering , but I would like to thank all of you who have answered my question, I am sad that some one (pilot, or CFi) can leave a craft in a state of possible corruption ( for want of a better word)without saying anything, for whilst my hours are not massive and therefore my experiance is limited but building I do have the ability to feel something wrong in the cyclic vibes, so that is why I curtailed my flight, the a/c in question previousely had flown like a humming bird, so why did no one else before me feel the problem I have found out that it was flown by two Cfis with students on at least three occassions before me and I think a student on a first solo, with a failure like this was there any danger in the M/R parting company or would it just be an annoying wump per rev, it has made me feel extremly uneasy about the integrety of this flying company, anyway THANK YOU ALL for your informed answers , since I found Prune I have gained a lot of knowledge through reading of all your experiances,

Good info, thank you Lu.

My terms were incorrect, and you are correct on the elesteromeric bearing assemblies in the yokes, they have a 5000 hour life linit, but I haven't seen anything on the yoke life limit, Bell's data only states it is composed of fiberglass composites, no mention of a limit for the yoke itself.

The following is a part of a response to RW-1 and describes the design philosophy used by Bell in their latest soft in-plane rotor heads that also incorporate elastomeric bearings for pitch change.

“The yoke that you referred to is also a part of the structure of the rotorhead. The elastomeric bearing is mounted inside of it and it most likely absorbs centrifugal loading while allowing pitch change. Leading and lagging pivots about the elastomeric bearing and is most likely absorbed by the soft inplane elastomeric dampers at the root of the yoke assembly”.


The A star and Twin Star rotorheads are different and this difference causes a high rate of degradation of the elastomeric elements and increases the maintenance and operational costs.
On these types of rotorheads the outboard elastomeric element instead of being the pitch bearing that accommodates leading and lagging it is the lead and lag bearing which only accommodates lateral displacement. This causes the lead lag bearing to be the pivot point for lead and lag. The tail end of the blade is being restrained by a bearing that allows pitch change and absorbs centrifugal loads but is now being subjected to very high lateral loads which it is not designed to react.

In other words like many French mechanical devices it is built backwards.


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The Cat