More Anti-vibration Mount for Helicopters

[drawoh]

Our customer wants to mount our system in a helicopter pod, rotated 45[°] about its axis. They want to show if the anti-vibration mounts will work.

Our mounts are the all-attitude Barry B64 series cup mounts with axial:rotational stiffness ratio of 1:1. Our system is well within the load ratings of four mounts. Our centre of mass is approximately in the middle, such that the full static load at 45[°] will approximately be carried by two mounts on one side. My first thought was that this was too much.

My second thought is that static load does not matter. The airframe vibrates. The vibration is transmitted through all four anti-vibration mounts to the mounted mass. If the helicopter crash lands, all four anti-vibration mounts have to tear loose for the system to fail. I am ignoring the fact that this is less critical in a pod than it would be inside the airframe around passengers.

My interpretation of this is that the mounts are okay. I have contacted a local tech rep., but I would like to hear other opinions. Does this sound reasonable. Is there something wrong with my thinking?

JHG

 

[IRstuff]

Not necessarily. While the nominal stiffness ratios may be 1:1, the increased compression may make that less so. Moreover, the increased compression may increase the nominal natural frequency of the two lower isolators, which would decrease the rolloff of the helicopter vibration. The two upper isolators that become less loaded might have a lower natural frequency, which may result in increased low frequency vibration in addition to the increased transmission of higher frequency vibration from the bottom isolators.

Your statement "vibration is transmitted through all four anti-vibration mounts to the mounted mass" seems at odds with the whole notion of isolation mounts. The point of an isolation mount is to "isolate" the payload from the vibration environment. Therefore, if your mounts are actually "transmitting" vibration, then they aren't doing their jobs correctly.

As always, the natural frequency of the isolators should be choosen to be as low as possible, as constrained by sway space, etc., to maximize the attenuation of vibration provided by the isolators.

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize

 

[drawoh]

IRstuff,

Thanks.

Vibration isolators transmit some vibration at all frequencies, and they amplify it at low frequency. Damn those laws of physics anyway. Also, the transmissibilty curves for the Barry B_series mounts do not look much like the transmissibility curves in the textbooks. Damping is weird and wonderful, at least in a mathematical sense.

That is an interesting point about spring rate and compression.

JHG

 

[IRstuff]

"they amplify it at low frequency"

I'm going to give you the benefit of the doubt, in that you are specifically referring to the resonance at the natural frequency.

Yes, isolators transmit, but whole point of the design is to minimize the vibration as much as possible, given all the other constraints. The odds that your isolation system design will function well in a unintended configuration is low. We often have to alter the isolator designs on our systems, even for seemingly trivial changes in mounting.

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize

 

[drawoh]

IRstuff,

One potential problem I have is that we are using aeroplane style mounts that isolate frequencies over 30Hz. In a helicopter, there is a big forcing function that operates well below that. We could be in trouble.

The softer mounts for helicopters are not all-attitude, and do not have 1:1 stiffness ratios, making them non-functional in this application.

This is not the Robinson R44 helicopter from my previous query. We are going to test, and verify that the aeroplane mounts work, in the original, horizontal configuration.

JHG

 

[IRstuff]

We've never used anything over about 20 Hz, and we have applications that go down to about 8 Hz.


TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize