To: Mriya225
First I must make two assumptions.
(1) Agusta would have placed a warning in the maintenance manual if the problem actually exists. However if you read several posts of mine relative to the EH-101 crash in Scotland I stated that there was no communication between the product support department who wrote the tech manuals and the engineering department that designed the system so this is a grey area.
(2)You have not been exposed to elastomeric bearings as used on flex rotors (fully articulated) in you training.
The elastomeric bearing is a multiple sandwiich of metal and natural rubber elements. In the case of the bearings used in the EH-101 rotorhead (5) the metal elements are shaped in a hemisperical form. Each metal element is progressively smaller than the preceeding element. So, when the metal rubber sandwich is cured and the rubber bonds to the metal the bearing is conical in configuration.
In this form any externally applied load can cause the bearing to deform. Each elastomeric element can deform only slightly but the individual element movement combined can amount to twenty to thirty degrees of rotation or flexure. This way, the bearings can acommodate flap, lead lag and pitch change. The bearing also absorbs the centrifugal loading of the blade in flight.
The bearing is in a sense a spring with the attendant spring rate. Any flexing of the bearing requires the application of an external force. If the bearing is flexed and held in that place, it has a reflective load just like a spring under load.
When the helicopter lands, the hydraulic servos are still powered up. If the bearings have been installed in the relaxed position when in high collective then when the helicopter is on the deck the pitch is at the down collective setting and as such the bearings are torsionally wound up. If in this case, the pilot shut down, then the hydraulics would also shut down and the torsional energy in the bearings will unwind and the blades will increase in pitch until the torsional energy is expended or, insufficient to overcome the control system friction.
In the conditioned described in the post above the blades were folded along with the pylon and then the hydraulics were turned off the torsional energy would be locked up in the control system.
That is, if the bearings were installed as I had indicated. If they were installed the other way, this problem would go away, but then the bearing life would be effected.
There is another problem with the bearings or for that matter any elastomeric element in a rotor system.
A common problem is the exposure to solar heating. Solar heating can in some parts of the world increase the temperature of an exposed element to in excess of 160 degrees.
Elastomeric elements in most cases can have no more than a total 1 hour exposure to temperatures of 160 degrees in the life of the part. If it exceeds this temperature /time exposure it can decrease the life of the part by as much as 50%.
Elastomeric dampers as used on the Apache and some MD helicopters as well as the French and Bell helicopters must not vary in spring load more than 5% between the high and low elements. If the difference in load exceeds this level ground or air resonance can occur.
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The Cat
[This message has been edited by Lu Zuckerman (edited 27 November 2000).]