Rotor Vibrations

[heliman500]

Hi all! I'm looking for a real basic explanation of main rotor induced vibrations. I've heard talk of 2 per, 3 per, vertical hop etc....but never fully understood . Thanks in advance

[NickLappos]

heliman500,

The natural per-blade vibrations come from the rotor as each blade creates a part of the total lift the aircraft needs. The number of blades determine the principal frequency of the dominant vibration. We speak of the vibration as "N per revolution" where N is the number of blades. Think of this n per rev as like the 60 cycle hum that audio engineers spend their essence removing from the music we listen to.

Where does it come from? The rotor is really a nervous hum of resonances, the dominant resonance is the 1 per rev flapping oscillation that each blade experiences. For a 4 bladed system, the sum of these 4 individual 1 per revs is a 4 per rev.

Most rotors spin so that the blade tips are going about 725 feet per second, which means that a 20 foot radius rotor spins at 5 turns per second, and so 1 per rev is about 5 cycles per second (5 Hz). A 15 foot rotor spins about 33% faster, so the 1 per rev is about 6.5 Hz.
A 4 bladed 15 foot rotor has a 4 per rev of about 26 Hz, if it had 2 blades it would have a 2 per rev of about 13 Hz

A 20 foot rotor has 4 per rev of about 20 Hz, with 2 blades, it would have a 2 per rev of 10 Hz.

Generally, the fewer the number of blades, the higher the amplitude of the N per rev, so 2 blades have a higher n per rev than 3, and so on.

[Avnx EO]

With 2 and 4 bladed systems, I could never understand where the 3-per-rev came from. Then I got a really good explanantion from a rotor dynamics guy... (I'm going to embarrass myself by trying to relay it as only an avionics puke can.) ....In forward flight when you have an advancing and a retreating blade, the advancing blade is going fast, so fast that the tip generates too much lift, so the blade is actually pitched so that the tip produces very little lift. Given the twist of the blade, most of the lift is being generated about a third of the way out the blade.
For the retreating blade, the part near the hub is actually flying backward, at a certain point down the blade its actually stopped and it is the tip that is producing the lift while the rest of the blade is along for the ride.
As the blade rotates in forward flight, the center of force moves back and forth along the blade setting the blade up an oscillation that works out to three per rev.
He said that there's a magic relationship for 5 blades. Somehow with 5 blades, if you do it right, a lot of these forces and factors cancel out. So theoretically, if a five bladed rotor system is done right, you should require minimal vibration treatment.
.... There's my five cents worth.

I'm sure if the Sultan reads this, I'm sure he can do a much better job of explaining the particulars of vibration, since he's so adept at tuning them out. (Which is why he's the Sultan (of smooth)) .... By the way Sultan...it was JH that gave me the 3-per-rev explanation.

[Matthew Parsons]

Just to add a little, the 1 per rev experienced by the pilot is also due to the mass distribution of the entire rotor system, mostly whether the center of mass is or is not on the axis of rotation.

Vertical hop could mean a lot of things, so I'd be very cautious using it. The vibrations will be dominant in either the vertical or in the lateral plane. For the pilot, the direction doesn't normally make the job any more difficult or uncomfortable, but it is important to know the direction for reducing the vibrations.

If you haven't yet noticed the difference on your seat between the 1per and the Nper, you can normally feel the Nper get worse at high blade pitch settings and high forward speed. 1per is not as easy to produce, but if you feel a vibration during an autorotation that is slower than the Nper (1/N of the frequency) then it might be the 1per vibration.

One problem with learning to feel which are the dominant frequencies, is there are many frequencies throughout the system, some of which lie very close together. For example, a 1per on the tail can be very close to the Nper on the head, especially for larger helicopters (N>=5). Also, frequencies can couple in ways that are difficult to imagine.

Matthew.

[Ascend Charlie]

Remember that there are also lateral vibes, caused by mass imbalances or problems with drag dampers. They are usually 1:1.

An engineer needs to know whether it is a vertical or lateral before he can begin to fix it. One way to determine the difference is to sit behind the crew and watch their heads. If they bounce together, it is a vertical vibe.

If they bounce alternately, it is a lateral. Fun to watch.:}

[nodrama]

Other good indicators of lat or vert vibes:

Which way is the instrument consule dancing?

Which way is your arse shuffling?

[Alloa Akbar]

Alternatively you could always fit the RTB kit...

[Chopper Doc Junior]

If you have a modern track and balancing kit the box will tell you what the lateral and vertical vibrations are. I'm thinking here of the more modern Chadwicks and also the Aces 2020 and 4040.
There are lots of things that could cause a vibration and of you are struggling to achieve a satisfactory result the chances are something is worn or perhaps several things are still within tolerance but all worn.

[chuckolamofola]

heliman500 said:
I'm looking for a real basic explanation of main rotor induced vibrations.

Here's my stab at a basic answer:


1 per rev (aka 1R, 1P, 1 Omega or first harmonic) is usually described as a once per rotor revolution beat.

Causes of the 1 per rev. can be: mass imbalance (lateral), unequal lift between rotor blades (vertical) or dynamic chordwise imbalance (fore/aft)



2 per rev (aka 2R, 2P or 2 Omega) is usually described as a twice per revolution beat.

Causes of the 2 per rev. can be: N per rev. of a 2 bladed rotor or a pair of blades not producing equal lift in a four bladed rotor.


3 per rev ( aka 3R, 3P or 3 Omega) is usually described as a three times per revolution beat.

Causes of the 3 per rev. can be: N per rev. of a 3 bladed rotor or blade mismatch in a four bladed rotor (typically Sikorsky models)


4 per rev (aka 4R, 4P or 4 Omega) usually described as a four times revolution beat.

Causes of the 4 per rev can be: N per rev. of a 4 bladed rotor.

The list can go on, but the 1 through 4 are the primary frequencies most people are concerned with in general discussions. The better RTB equipment will measure typically up to the 12th harmonic of the rotor.


1 & 2 per rev. reduction is usually controlled by adjustments to the pitch links, trim tabs are hub weights. These adjustments have very little effect on N per rev.



N per rev is not reduced by the method above but by adjusting Frahm Dampers, Spring Absorbers, Cantilever Absorbers etc. N per rev. may also be greatly affected by fuel load, loose cowlings, seats and broken structure.



Many people think that tabs only affect vertical and hub weights only affect the lateral vibes. This is often not the case as there can be crosstalk between all of the axis. Therefore it is important to use an RTB system that can acquire data on all three axis at the same time. It is also important that the same system use diagnostics that provide a solution that take into effect the crosstalk between the different axis's. Not all systems are equal...


Regards,


Chuck

[Chopperlover]

What's the adjustment to be taken into account when fighting a fore/aft vibration? Particularly on 412's. And sorry to wander off the original subject from the starter of the thread.

:ok:

[chuckolamofola]

Chopperlover asked:
What's the adjustment to be taken into account when fighting a fore/aft vibration? Particularly on 412's.

Product Balance Wts

[[email protected]]

Unfortunately, all computers are not equal. We changed from Rotortune to RADs a few years ago; it was claimed we would see an increase in serviceability -and we did - but only because vibrations that would have failed on a Rotortune run now pass on a RADs run!

[Alloa Akbar]

Crab

If memory serves, with RADS it can ask you to adjust the chordwise weights at ground and Hover flight conditions... If you are dealing with a 412, I'd recommend leaving these well alone until you have a decent track and mass balance weight on ground and Hover..

however as one poster put on a previous thread.. ask 10 different guys, get 10 different answers...:rolleyes:

[Chopperlover]

I was one of those whose belief was to never touch the product balance whatsoever unless there's been a blade replacement or a repair on a blade. At least that's how I've been taught. I've had low vert and lat vibration levels on ground and hover (good enough to go out and take flight measurements) along with slightly high fore/aft vibrations but as I play with HW, PL and tabs it (the fore/aft vib) dissapears. I even edit the adjustments by turning off the product balance and has worked good so far.

It's always interesting to know other more experienced technician's point of view regarding vibration analysis. Specially on 412's.

BTW, we also use RADS.

Cheers

:ok:

[chuckolamofola]

Chopperlover said:
I was one of those whose belief was to never touch the product balance whatsoever unless there's been a blade replacement or a repair on a blade.

If that was the case then the maintenance manual would have indicated such stipulations. This is a common misnomer, mainly started due to the fact it takes a bit more work to make these adjustments over trim tabs, hub weights and pcl's.

In the initial mode, product balance is used to reduce the track change between ground and hover. Just like it would be done on a whirl tower.

In the flight mode, product balance is used to reduce vibration, mainly f/a, in hover and the slower forward speeds.

An aircraft in good product balance will also exhibit less issues with the ride quality changing with gross weight or altitude.

Chuck

[Chopperlover]

I actually don't see the big deal on adjusting product balance weights working-time wise. Not a big deal, just a few screws and some sealant when done.

Btw, the manual does stipulate something:

BHT-412-MM-2 Ch 18 Pg. 33.
Once a main rotor has been product-balanced, it
should not require additional product balancing until a
blade is replaced, or an existing blade is refinished,
repaired, or otherwise altered. Therefore, when
trimming a previously worked main rotor, use pitch link
adjustments and/or hub balance weight adjustments to
correct balance.

Thnx for the other advises. I just wrote them on my book so I'll follow them the next time I balance a 412 (hopefully next month). :)

:ok:

[chuckolamofola]

Btw, the manual does stipulate something:

BHT-412-MM-2 Ch 18 Pg. 33.

Once a main rotor has been product-balanced, it
should not require additional product balancing until a
blade is replaced, or an existing blade is refinished,
repaired, or otherwise altered. Therefore, when
trimming a previously worked main rotor, use pitch link
adjustments and/or hub balance weight adjustments to
correct balance.


I hadn't seen the manual with that update, go figure. The key word here is shouldn't... I do agree that product balance doesn't have to be done everytime the rotor is worked, usually in the intial mode and in forward flight if f/a vibes are higher than desired.

Chuck

[Munga_3]

Friction dampers on the two rear transmission isolation mounts.
If they're flogged out then you'll get pylon rock which manifests itself as an extreme low frequency vibe, ie less than 1:1.
Booyah :8

[Alloa Akbar]

Munga

The same can be said of a number of parts of the aircraft, some of which, when worn, actually produce vibrations which are the same frequency as say 2R.. I think our original poster may have wanted to focus on MRH vibrations, but a good point from you all the same.:ok:

My own thoughts on chordwise weight adjustment is that I don't do it unless for example I find I have a significant track spread across forward flight speeds, and perhaps adjusting the blades pitching moment is the best way forward. Speaking as one who has worked on the development of script files / smartcharts or datacards dependant on which kit you are familiar with, I can tell you that actually calculating and predicting the outcome of a chordwise weight adjust is rather complicated, and to perfect it, we would need to fly a high number of flight hours on non revenue testing.. which is expensive, therefore, the RTB Kit manufacturers will test a couple of aircraft, and then use data fed back from you guys in the field to evolve the charts / scriptfiles. In essence this means that a mature aircraft should be a piece of cake, however every time you have a new blade modification, new engines or a change to the aircraft's inherent modes of vibration, then the reliability of the files is diminished slightly.

My point being, don't follow the RTB blindly, think about what you are about to do, the kit isn't always right.

[Rotor Whisperer]

I am a Technician working on the 412CF model and my current position is in Vibration Analysis where we use the Health Usage Monitoring System (HUMS) which is similar to RADS but ours is hardmounted on the airframe. Another system of adjustment I will periodically use for F/A vibration is inboard tab to the protest of our Field Service rep; I will only use it providing I do not have a lot of ground bounce in the Flat Pitch on Ground regime or if I have to do a radical move of more than 5 degree of tab on a blade set and I prefer less than 3 degree. One thing I never thought of trying though is product balance as we are taught to use it only from the Flat Pitch to Hover regime and only if there is a blade flying out. My all out preferred method is in blade spacing however, I like to have a nice straight track picture with minimal weight and minimal tab input and have some of our fleet flying with the highest vibration of 0.17 ips at 130Kts forward speed.