View Full Version : Good (Or Bad) Vibrations
20th Jul 2012, 21:57
I recall from my days on Lynx (AH1) the umpteen problems with regard to airframe vibrations, however age, and probably the vibration, has dulled my brain and I am now seeking an explanation on the difference between 1R and 4R vibrations.
I confess, I was always impressed when guys in the crewroom confidently suggested that airframe xyz had a 4R vibration, or a 1R vibration, and always wondered exactly how these different vibrations manifested themselves through the airframe/controls/backside.
To me it was just a vibration! :confused:
20th Jul 2012, 22:02
With a 4R your bottom vibrates. With a 1R your d**k flops up and down.
20th Jul 2012, 22:12
maybe the british military never ran to coffee-in-flight, that's a good way to read the vibes. One of our machines I remember had so many coffee stains that worked like glue on the centre console that it was impossible for the venerable mug to be displaced in any type of gyration. used to be quite amusing though to tool up alongside the usual driver and watch his helmet bobbing around whilst the eyes stayed steely still. his coffee though as thick as, had been labled by the boss as 'UBoat fuel".
21st Jul 2012, 00:02
You wish Shy.....you wish!
"Flop" connotes movement of measurable dimensions.
21st Jul 2012, 00:56
There are different sources of the vibrations transmitted to an airframe. Some come from the main rotor, some from the gearbox, etc. The 1P, 2P, etc. main rotor vibrations are low frequency but are the ones you feel "in the seat of your pants". The vibrations produced by the gearbox are of much higher frequencies and can be quite loud. You don't really feel these vibrations "in the seat of your pants", but the high noise levels can be just as bad because they are mentally fatiguing.
The in-plane rotor modes (lead/lag) can be mitigated pretty well with hydraulic dampers or bifilars. The out-of-plane rotor modes (flap) are now being addressed with active vibration control systems or with individual blade control. However, the gearbox vibration problem still needs to be worked on.
21st Jul 2012, 07:13
For sure we had coffee - if for no other reason than for use as an AI if ever the instruments failed................. ;)
21st Jul 2012, 07:30
Try looking up www.rwas.com.au (http://www.rwas.com.au) for a better explaination for various vibes....in particular n per rev (4per in this case)
21st Jul 2012, 14:03
Further to what Shytorque said,
1R vert you will feel with an itchy nose, and is usually solved with Pitch rod adjustments.or transposing the blades
4R you will feel in your ass when using "tin blades" was fixed with trim tab adjustments.
Now with all the Berps, 4rR is done also with pitch rod adjustments.
The Rads kit is normally pretty spot on in telling you what frequency you are exceeding, the trick is to figure out where these frequencies occur.
Loose seat pins, loose door pins, avionics rack etc.
Not too unusual to take the winchman along to put his feet against the door to keep the levels down.
21st Jul 2012, 16:40
WS55/3 pilot, Ken Smith DFC.............Das Island 1967.
I've got a "wumper", you had better go and check the dampers.............he was always right, two full of oil, third one empty!
21st Jul 2012, 21:37
Using a loosly hanging appendage is an easy way to determine problems.
If you are "flopping" then it may not be so bad, a bug under your protective skin say (blade tape) causing an erosion of lift and causing a blade to go out of track.
But if you're 'swinging', and that malady worsens it is a very seious time to consider 'going down', dampers or mast breaking or who knows?
22nd Jul 2012, 02:31
Used a Bic Pen held between fingers resting on a knee....then watched how the end of the pen moved around....works for Main Rotor Vibs.
Take a knuckle and and press against the airframe near structural beams, stringers, etc.....and feel for hi-freq Vibs.
Any Chinook FE can explain this to you in detail as they should be constantly walking up and down the cabin checking the old girls pulse.