3 Per Rev Vibration in Helicopter Rotor System 1

[RotorHead269]

Hello Everyone,
Needing some help/ideas on locating abnormal vibration in Hughes 269 rotor system. Have been dealing with the "experts" in the industry, but absolutely no success in correcting this. I have a lot of vibration data logged with a MicrovibeII analyzer that can be emailed if necessary. If anyone has an interest in assisting me with this, your effort would be greatly appreciated. Thanks in advance.
Jerry

 

[GregLocock]

A 269 has 3 rotorblades. I'd expect 3*rotor speed to be the dominant frequency in the entire drivetrain.

Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[RotorHead269]

Greg,
Thanks for the reply. This vibration is "new" and not normal. It registers as high as 1.5ips (inches per second)in the cabin at about 1350 cpm. The blades turn at approximately 450rpm. The rotor system itself registers about .07ips vertical and around .14 lateral. Anything under .2 is within spec. This vibration until recently would "come and go", but is now present about 99% of the time. I've taken spectrum analysis readings in many various locations/situations. Pretty much anything and everything that would be "typical" according to 269 mechanics has been checked, tried, or replaced. No luck.
Jerry

 

[TPL]

This vibration is "new" and not normal. It registers as high as 1.5ips (inches per second)in the cabin at about 1350 cpm.

The blades turn at approximately 450rpm

The use of the words 'about' and 'approximately' suggests that you cannot be entirely sure the the problem vibration is 3X rotor turning speed - you need to confirm this before going off on tangents. There is a world of difference in trying to figure out a problem vibration that is exactly 3X turning speed and one that is close to, but not exactly 3X running speed.

Is the problem '3X' vibration present at all rotor speeds or just at 450rpm? Whereabouts are you measuring the problem vibration - is it at all measurement locations or just specific points in the cabin - is it possible that you are seeing a structural resonance local to your measurement points (e.g. panel) that is casing this grief?

 

[RotorHead269]

TPL
The operational rpm of the enging during flight is 2700-2900 rpm, which varies the rotor rpm proportionately. Unfortunately the analysis equipment is not 100% accurate at the lower frequencies (rotor), so I've used 'about'. I have several pdf plots with notes (location of sensor, hovering, forward flight, etc.), if you have an interest in looking at them.
Jerry

 

[TPL]

When posting a message - Step 3 on the LHS of the dialog box shows Attachment - can you put the data here?

 

[RotorHead269]

Here's the data.

 

[TPL]

Not a lot to say really - you've just posted a bunch of plots. Doesn't provide anything of real use.

As a starting point, look at the plot labelled 'Cooling Fan Sensor on Top of Engine' - the peak at 1373cpm is followed by many low amplitude multiples, with the 4th multiple being quite high - I'd be looking for a) a worn bushing or b) a damaged rolling element bearing.

For b) I would want to see acceleration data out to at least 5kHz

 

[RotorHead269]

I've attached a spec sheet to the analyzer I have and a link to a parts catalog that will show you exploded views of the various systems on the helicopter. Any suggestions you might have as far as analyzer settings, sensor locations, etc. would be helpful. I am a mechanic/pilot, not an engineer, so I really appreciate your patience.
Jerry

http://www.airfliteok.com/parts.asp

 

[TPL]

Jerry - I think your expectations are a little high, I for one don't have the time to wade though the info/manuals.

On thing did strike me - what is the speed ratio between the main rotor and the tail rotor drive?

 

[TPL]

Also, are there any MCDs on this engine? It might be worth pulling any oil filters and having their contents analysed for wear metals - similarly, it might be worth taking a sample of oil for metal particle analysis

 

[RotorHead269]

I know I'm reaching for a solution, as I've been dealing with this for quite some time. The service center's solution is to start buying/swapping parts. If there were a true "engineer" available to me locally, I would've already hired them! When the engine/lower pulley is at 2800rpm, the upper pulley/tail rotor driveshaft operates at 2150rpm (also the input to the main rotor transmission). The tail rotor is at 3100rpm and the main transmission output/main rotor rpm is at 460. Again, Thanks for your time.
Jerry

 

[GBor]

Jerry,

Where are you? Perhaps there is someone on this forum that is closer than you think. It is a very international group.

 

[RotorHead269]

Some additional information: This vibration was noticeable for the first time about 400 hours ago (hobbs time). It has "come and gone" since then, but is now present about 99% of the time. When it initially appeared, it would gradually "peak" and "fall", with about 4-5 seconds between peaks with absolute predictability if rpm was held constant. Over time, the vibration became more intense and more frequent, and is now almost always present. Since it first appeared, I have installed new main rotor blades, new main rotor driveshaft and hub, new upper pulley bearings, new rear engine mounts, and new pitch grip bearings in all 3 blade grips.

 

[RotorHead269]

I'm in Tulsa, OK.

 

[BobM3]

I don't have helicoptor experience but one thing to consider is that 2 or more components might be vibrating. With so many different speeds going on at the same time and the fact you mentioned that the vibration seems to come and go periodically makes me wonder if you have a beat frequency problem.

 

[TPL]

This vibration was noticeable for the first time about 400 hours ago (hobbs time). It has "come and gone" since then, but is now present about 99% of the time.

This sounds like a problem that is getting worse with time -if a fatality results, how are you going to defend yourself?

Despite your efforts, I think you would be culpable. You knew there was a problem; the problem got worse with time, you were unable to solve the problem; you allowed the aircraft to continue flying: someone got hurt. You will be in the firing line

 

[RotorHead269]

It's on the ground until this is solved.

 

[RotorHead269]

I'm the only one flying it to "trouble shoot" the vibration.

 

[seymours2571]

Rotorhead269,

There are "true" engineers available to you locally. You could try DRD technology. They are located at approximately 51st and Lewis on the West side of the road. That is just one off the top of my head.

You could also contact some of the engineering professors at Oklahoma State University in Stillwater, which is only 1 hour away. They regularly consult with people in industry. Try contacting the Mechanical and Aerospace Engineering (MAE) school.

Tulsa University (TU) has an engineering school you could contact them. You could also do a search for PEs in Tulsa that work for consulting firms on the Oklahoma State Board of Engineering website

http://www.ok.gov/pels/

You could even google for consulting engineers in Tulsa.

Help is closer than you think.

Steve