EH-101 Merlin Potential problem for maintenance personnel

[Lu Zuckerman]

During the design of the EH-101 I brought a potential problem to the attention of the design engineers for the rotorhead and the flight control systems.

The rotor head on the EH-101 uses elastomeric bearings that allow pitch, flap and lead/lag. In the design stage it was not decided which way the bearings were to be installed. The original concept was to install the bearings so that they operated in flight in and about the neutral position which would minimize the stresses in the bearing. The other concept was to install the bearings in the relaxed state while in low pitch thus increasing the stresses when the bearings were in high pitch while in flight. At the time of my involvement they were going with the former as opposed to the latter.

In this situation when the rotor was stopped
and folded along with the tail pylon being folded the hydraulics were still on. With the hydraulics off the residual torsion in the elastomeric bearings would tend to relax and try to return to the neutral position.
However, with the tail folded these forces would be locked in the system which would place the entire flight control system under a compressive or tensile load ( I can't remember which).

In either case, if the system had to be maintained, requiring a disconnect of the system a control rod could injure the tech and cause a lot of damage to his/her hands and/or face.

I left the program before a final decision had been made. In any case, watch out.

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The Cat

[This message has been edited by Lu Zuckerman (edited 25 November 2000).]

[Genghis the Engineer]

Engessers second law also springs to mind.

G

[mriya225]

So if I'm understanding this correctly:

The elastomeric bearings (installation notwithstanding) have a tendency to accumulate a kind of residual torque, at rest, in the hydraulic system. When the rotors and tail pylon are both folded, the hydraulic system must remain engaged so that the load being put off by the bearings is contained. If the hydraulics are shut down, that load is released, thereby putting the entire flight control system under a compressive load and ultimately putting the control rods under a tensive load.
Some unsuspecting slob like me goes to work on it and (not realizing that the control rods are straining under load) gets beaten bloody when they jerk back around until the bearings have relaxed into their neutral state.
Have I got the jist of this correctly? If so, how do you go about bleeding off the load before you can safely disengage the system?

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Un diva très doué.

[Lu Zuckerman]

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).]