Anti-vibration mounts for aircraft and helicopters
Anti-vibration mounts for aircraft and helicopters
(OP)
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?
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





RE: Anti-vibration mounts for aircraft and helicopters
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
FAQ731-376: Eng-Tips.com Forum Policies
Chinese prisoner wins Nobel Peace Prize
RE: Anti-vibration mounts for aircraft and helicopters
Mike Halloran
Pembroke Pines, FL, USA
RE: Anti-vibration mounts for aircraft and helicopters
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
RE: Anti-vibration mounts for aircraft and helicopters
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: Eng-Tips.com Forum Policies
Chinese prisoner wins Nobel Peace Prize
RE: Anti-vibration mounts for aircraft and helicopters
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
RE: Anti-vibration mounts for aircraft and helicopters
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.
RE: Anti-vibration mounts for aircraft and helicopters
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
FAQ731-376: Eng-Tips.com Forum Policies
Chinese prisoner wins Nobel Peace Prize
RE: Anti-vibration mounts for aircraft and helicopters
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: Robinson 44 Landing Skid. There was enough weirdness in that.
RE: Anti-vibration mounts for aircraft and helicopters
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.
RE: Anti-vibration mounts for aircraft and helicopters
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: Anti-vibration mounts for aircraft and helicopters
TTFN
FAQ731-376: Eng-Tips.com Forum Policies
Chinese prisoner wins Nobel Peace Prize
RE: Anti-vibration mounts for aircraft and helicopters
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.
RE: Anti-vibration mounts for aircraft and helicopters
RE: Anti-vibration mounts for aircraft and helicopters
RE: Anti-vibration mounts for aircraft and helicopters
RE: Anti-vibration mounts for aircraft and helicopters
RE: Anti-vibration mounts for aircraft and helicopters