Anti-vibration mounts for aircraft and helicopters 1

[drawoh]

I have just finished analyzing a vibration problem on a helicopter, and I have recommended the use of soft, low frequency anti-vibration mounts, suitable for helicopters. I have just been asked why we cannot use these on regular aeroplanes.

I am working from the Barry Controls catalogue here. The cup mounts recommended for aircraft, are all-attitude, with resonant frequencies of 15-30Hz, and 1:1 stiffness in all directions. The cup mounts for helicopters are 7-10Hz, must be mounted horizontal, and have stiffness ratios of around 4:1. They are much looser side to side than up and down.

The helicopter mounts ought to isolate vibration better than the aeroplane ones in any aircraft, not just helicopters. My system is fairly low-slung, with the anti-vibration mounts attached horizontally (i.e. correctly). This is a surveying system, and vibration creates aiming and position feedback issues.

What is the advantage of the aeroplane shocks?

JHG

 

[IRstuff]

You mean, other than that they would work crappy on a helo?

As a general rule, the resonant frequency is supposed to a couple of octaves below the blade frequency, otherwise you're not isolating anything at all. Fixed wing aircraft and helos are not the same animal, so their isolation systems ought to be different.

TTFN

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[MikeHalloran]

I think they're cheaper because of larger volumes, and the 1:1 ratio means you can mount them in any orientation, which reduces the risk of mounting them wrong.



Mike Halloran
Pembroke Pines, FL, USA

 

[FeX32]

Isn't it a different vibration isolation problem?
The Heli has a general or multiple source excitations likely at larger magnitudes and lower frequencies then that of an aero. Is this correct?



Fe

 

[IRstuff]

Depends on the fixed wing, per our buddy MIL-STD-810G. Jets tend to be all random, with most between 100Hz and 2000Hz, although, apparently a 737 has some low frequency content below 20Hz. Props are sine on random, but the lowest sine is in the tens of Hz. Helos are likewise sine on random, but the primary rotor inputs are all less than 10Hz.

So, bottom line, an isolator with a 15Hz resonance will pass the helo main rotor frequency completely unattenuated, and possibly a bit bumped up if it's an OH-6A with an 8.1-Hz rotor frequency.

TTFN

FAQ731-376
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[FeX32]

Thanks. So the idea is there.
I remember on a recent trip I took a CRJ regional jet and noticed that there seemed to be much more vibration at the rear or the plane compared to the front. I wonder if this is normal as the engines were at the rear. Although, the transfer of vibratory mode shapes through the structure should be efficient in an AL structure such as a plane.
Wonder if they isolate the front from the rear or such.
Anyways, sorry to rant a bit


Fe

 

[drawoh]

IRstuff,

On the helicopter I was analyzing, I saw 14Hz, consistent with a two blade rotor at 7Hz, and about 120Hz, consistent with a six cylinder piston engine. I did see anything I could blame on the tail rotor, but maybe it was rotating at 120Hz!

Our stuff goes into all sorts of aircraft, mostly aeroplanes, so I need to be very generic.

JHG

 

[IRstuff]

According to 810, tail rotors are higher frequency (>20 Hz to 52 Hz). In most cases, the blades tend to be dominant, since there's an inherent asymmetry in their rotation.

Most of the helicopter isolators I recall hearing about are in the teens of Hz, and some were way down below even 10Hz.

In some cases, the desired isolation resonance is too low to minimize sway space or shock, and the system just has to tough it out with stiffer than desired isolators.

TTFN

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Chinese prisoner wins Nobel Peace Prize

 

[drawoh]

...

In some cases, the desired isolation resonance is too low to minimize sway space or shock, and the system just has to tough it out with stiffer than desired isolators.

Quite a few years ago, I put low frequency anti-vibration mounts at the bottom of a fairly tall frame. I worked out that the natural frequency was well below ten. The device was observed to rock something like half an inch on takeoff.

This system is nicely configured for anti-vibration mounting, so I do not expect anything weird to happen inside an aeroplane.

The system I just worked out is described in thread16-293739. There was enough weirdness in that.

JHG

 

[plasgears]

Years ago I sat in the rear of a DC-9. After takeoff the port engine was being cycled: 'clunk-clunk-clunk.' They shut it down.
The rest of the flight from DC to Boston was single engine. The crew said nothing.

Rear engine configs are generally noisy in the rear. There's not much that can be done except wall deadening treatment around the rear.

 

[KENAT]

Stuff the noise, it was all those 'blades' twirling around a few inches from vital parts of my anatomy that got to me;-).

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[IRstuff]

Yup. Particularly riding a DC-10 in the back, knowing full well that at least one DC-10 had its rudder-mounted engine self-destruct, and the loose turbine blades sliced through the ONLY non-redundant hydraulic line in the entire plane, causing it to crash.

TTFN

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[WKTaylor]

For Giggles... well maybe NOT so funny...

http://www.youtube.com/watch?v=a4EvVR10AF0

http://www.youtube.com/watch?v=-LFLV47VAbI

http://www.youtube.com/watch?v=6vICf8l-KV0

Regards, Wil Taylor

Trust - But Verify!

We believe to be true what we prefer to be true.

For those who believe, no proof is required; for those who cannot believe, no proof is possible.

 

[plasgears]

It was observed on the early DC-10's that engine 2 was waving on the tail pylon. That twisting motion carried thru to the aft fuselage. It was not a big surprise to me when baggage doors were dislodging. The whole problem was blamed on the door, but the problem was in the engine installation and aft fuselage flexibility. I remember the sensation of standing in the aft cabin in flight. You could feel the yawing.

 

[thruthefence]

Was this fixed with the KC-10 program?

 

[plasgears]

Problem fixed? Can't say with authority. Hope so.

 

[thruthefence]

You'd think that refueling under such circumstances would be "interesting".

 

[flash3780]

I'd imagine the natural frequency changes a bit when add 100,000 lbs or so of fuel below the main deck.