Merlin rotor head question

[Malcolm]

Shock absorbers/dampers? I know the Merlin has an active vibration control system to damp out vibrations in the cabin. Apparantly when it fails, traveling in the back is very uncomfortable, and the pilots can have problems reading the instruments too.

[Gary]

Topic moved to 'The Fighter Control Mess'

[P-3K]

Whilst I'm not saying you are categorically wrong, Malcolm, (I'm not that arrogant) I'd be very surprised if those are to do with avcs. All of the AVCS I've dealt with have been mechanical units, usually bolted to the cabin or airframe itself. Essentially, they are heavy weights that move in compensation to the vibrations generated by the helicopter transmission, coupled with accelerometers.

The two large yellow struts at the front of the gearbox (and you can just see the top of a third one at the rear) I think, are more likely to be the actual lifting struts that impart the lift from the rotor system to the airframe. The Eurocopter Puma and Super Puma family have a very similar system, only there are two at the rear and one at the front. This seems to be a very european approach, as Sikorsky, for one, simply bolt the gearbox to the cabin roof, and translate the lift directly to the airframe, which produces it's own problems (see cracked gearbox feet and airframe cracks on the S-92A). You won't find these struts on an S76 nor an S92.

If I'm right about those large struts (and I'm not saying I am), then I'd imagine that the smaller more horizontal ones are an anti torque device to compensate for the gearboxes natural tendency to rotate. Different from the Eurocopter, which uses a "barbecue plate", a steel or titanium concertina type plate that helps absorb torque of the gearbox.

Hopefully someone with more knowledge of this helicopter will come along and say for sure, but that's what my experience suggests to me.

All the best

Scotty

[Malcolm]

P-3K, You may be correct about those items.

However, I do know the Merlin has an active anti-vibration system. A freind of mine worked on it's development. I don't know how it's actually implemented though.

[P-3K]

The AVCS will take the form of a number of black rectangular boxes (about twice the size of a shoe box, depending on shoe size!), or white double circle (like a figure 8 outline) units bolted to the airframe proper. Usually you find them in the airframe, but the S92 has a set in the nose wheel well. Many S76Cs have a single unit in the nose just behind the radome.

They are linked to a control box somewhere in the electronics or avionics rack which is fed signals from accelerometers dotted about the airframe. The accelerometers pick up the movement and frequency and direction of the vibration, the control box then tells the AVCS units to "throw" their weights in the opposite direction to counteract the vibration from the transmission. (thats in very simplistic terms, but thats it in principle.)