Lessons learned by a "superior" rotor balancer

[C. Beaty]

JC, don’t you think the cyclic control method affects vibration?

 

[Jean Claude]

Yes of course, Chuck. The cyclic control method affects vibration of the stick, but I do not know how that could significantly affect the airframe. Do you believe this possible?

 

[C. Beaty]

I don’t know, JC.

At constant airspeed and fixed stick, rotor motion is the same, Bell-47 or Robinson. But reciprocating mass force input to cyclic stick is higher with Bell-47.
***********
Oops! I made a mistake: the rotor path during cyclic flapping is not identical in both cases.

With Bell-47 cyclic, teeter pivot lies along control axis.

With Robinson cyclic. teeter pivot remains on shaft axis.

 

[Jean Claude]

Oops! I made a mistake: the rotor path during cyclic flapping is not identical in both cases.

With Bell-47 cyclic, teeter pivot lies along control axis.

With Robinson cyclic. teeter pivot remains on shaft axis.

Sorry, I not understand. Can you shows with a sketch, please ?

 

[C. Beaty]

Reference axes of helicopter.

 

[bryancobb]

What???

What???

Reference axes of helicopter.

In my simple world ... TIP PATH PLANE is perpendicular to some arbitrary axis (Tip Path Plane Axis) and is a circular path defined AFTER all flapping has taken place. Tip Path Plane Axis seems like to me, is not an axis of no flapping.

 

[Jean Claude]

Thanks Chuck, Sketch is universal. More easy for me.

Bryan, I do not see any error in Chuck's sketch.
Tip path plane axis is necessarily the axis around which the blades run according a constant angle (i e no flapping)

 

[XXavier]

A memorable sketch by Jean Claude:

And a not less memorable 'unrolling' sketch by Chuck Beaty...


Here’s a plot of JC’s sketch #46 unrolled. There’s nothing mysterious about the path followed by a rotor.

The view from the rotorhead axis is simply that of the blades moving in a simple sine* curve, to a first approximation.

Viewed from the real axis of rotation, the blades don’t do anything except feather periodically.

 

[Jerry_Forest]

Velocity IS Proportional to Dynamic Stress! (Most of the time).

Velocity IS Proportional to Dynamic Stress! (Most of the time).

Thread #20

IPS is not directly representive of mecanical stress, when the frequency is different.

The concept of structural velocity limits have been used in industry for decades. That includes rotor balancing whether its turbojet engines or gyro rotors. It also applies to structures like piping and transmission towers. (I know, because I commissioned all the rotating equipment and piping systems in Ontario's nuclear power stations).

Here's one reference that summarizes the concept:
http://www.vibrationdata.com/tutorials2/SRS_DVA_M.pdf

The "mean discomfort rating" is a totally different concept. And I would be only too happy to see my velocity reading below 0.5 inches per second (ips) in my gyro at any frequency. BTY, the velocity meter reads RMS across the whole frequency range (for multiple frequencies) and all the stress information is included in one simple number.

Example: 1 inch displacement at 1.0 Hz = 1.0 ips. and 0.001 inch at 1000 Hz also = 1.0 ips.

 

[Jean Claude]

Jerry, I agree the speed of a vibration (IPS) is a convenient way to indicate a duration of destruction by fatigue, subject of your interesting document.
But it seems to me that the life of our airframes is much more often shortened by the wear ou excessive momentary constraints than by fatigue.
I used the word "stress" in the sense of momentary constraint, and this constraint comes from accelération (F = mass x Accélération)

 

[Mike G]

Bryan (Cobb)
Where does this:
"3.0 IPS vibration (unairworthy by FAA standards) and a 0.5 IPS vibration (well within acceptable standards of airworthiness)."
come from?

I think my general statement :
" the average Euro gyro will have between 2.5 and 5 IPS at 2/rev" needs some clarification.

These are typical values measured right next to the rotor bearing. This drawing shows where I measure it.



In fact the accelerometers (there are 2) are mounted on the other side but it's the only drawing I have at hand. I am therefore trying to measure the vibration at its scource where I expect it to be greatest.

With the PB4 I now systematically (if possible) measure the vibration in the cockpit at the same time as at the rotor. I did it occasionally with the PB3 but it was a more awkward, it's now standard on the PB4. Here is a sketch of the installation I use.



Here you can see the spectrum of a typical eurogyro AT THE ROTOR that was well reasonably balanced (1/rev = 0.19 IPS at the rotor) and the 2/rev is around 3.0 IPS.



Here is the spectrum AT THE COCKPIT FLOOR taken at exactly the same time as that above and you can see the 2/rev is much lower.



The moral :violinf the story is

If you are talking about or comparing levels of vibration you must say where you are measuring it otherwise you're wasting your time.

Mike G

 

[chipchap42]

Mike,

great article and a serious amount of work just putting it together - many thanks.

Re the 2/rev figure, are you saying there is nothing we can do about that? So it's down to the manufacturer?

Thx,
Paul.

 

[Mike G]

Paul
I've said all this before,

I'd say "no" there isn't any modification or adjustment you can make, especially if you're flying a factory gyro and your local rules don't allow you to make major changes.
If you are allowed to change things and if you're an experienced gyro builder like CB or Norm (Phantom) then there have been discussions on this forum before and I won't repeat them here, they include such mods as a slider roll pivot, stiffer in-plane rotors, softer ("limber") masts etc.

The one thing you CAN do that I would encourage every owner of a factory built gyro to do is to find somebody with a balancer that includes a frequency analyser, record the vibration at the mast and see how high the 2/rev is. If it's over say 1.0 IPS (I chose that arbitrarily, that doesn't mean that less than is acceptable) ask your manufacturer or his rep why it is so high and what are the acceptable limits, and report back here.

The manufacturers don't want to do anything about it and you can't blame them, they are commercial operations that are trying to make money (the American dream) and there is no incentive for them to spend any time and money trying to resolve a problem that only Mike G seems to be concerned about. If I was a manufacturer trying to stay profitable I wouldn't, would you?

If you do measure the vibrations of your gyro and the 1/rev is reasonable and the 2/rev is higher, then you either accept it or you complain and it's only when enough owners complain that the manufacturers might consider doing something.

Mike

 

[chipchap42]

Paul
I've said all this before,

I'd say "no" there isn't any modification or adjustment you can make, especially if you're flying a factory gyro and your local rules don't allow you to make major changes.
If you are allowed to change things and if you're an experienced gyro builder like CB or Norm (Phantom) then there have been discussions on this forum before and I won't repeat them here, they include such mods as a slider roll pivot, stiffer in-plane rotors, softer ("limber") masts etc.

The one thing you CAN do that I would encourage every owner of a factory built gyro to do is to find somebody with a balancer that includes a frequency analyser, record the vibration at the mast and see how high the 2/rev is. If it's over say 1.0 IPS (I chose that arbitrarily, that doesn't mean that less than is acceptable) ask your manufacturer or his rep why it is so high and what are the acceptable limits, and report back here.

The manufacturers don't want to do anything about it and you can't blame them, they are commercial operations that are trying to make money (the American dream) and there is no incentive for them to spend any time and money trying to resolve a problem that only Mike G seems to be concerned about. If I was a manufacturer trying to stay profitable I wouldn't, would you?

If you do measure the vibrations of your gyro and the 1/rev is reasonable and the 2/rev is higher, then you either accept it or you complain and it's only when enough owners complain that the manufacturers might consider doing something.

Mike

Mike, sorry if this is "known". All new to me. I will do the measurements.

Regards, Paul.

 

[Mike G]

Paul
No need to apologize, we all had to start at the begining by asking basic questions.
Mike G

 

[Joe Pires]

Thanks for the comments.

I just realised that I left out the text about tracking adjustments under "Dynamic tracking" so I've edited the text above.

Joe
I've promised myself for the last couple of years to go to Bensen Days or Mentone to do a forum about balancing and each year something has cropped up, this year is no exception but I'm waiting for a confirmation of a meeting, if it falls thro I might just come over, I owe you and Laura a dinner from when I was last there and I really want to meet CB.
Now I'm retired money's a bit tight so you might have to let me sleep in your hanger
I would really like to balance your Aviomania, the French one is still grounded due to engine problems.

Wolfy OK perhaps we'd be better doing by email or Skype or something so as not to clutter up this thread.

Brian
You understood perfectly.

Rambler thanks for the link I'll edit it into the text.

Mike G

Mike sorry I missed this before. I am careful who i give unattended access to my hangar, so you will have to accept accommodations in the bedroom we have set aside for visiting Gyro pilots. And as far as a Dinner if you make the effort to come all the way across the pond, the least we can do is provide the hamburger helper casserole.

Hope to see you.

 

[Mike G]

Joe
I really had hoped to attend Bensen Days this year, I wanted to balance your rotor for you and get some data on the Aviomania and Ernie's rotor system for my personal data base. I also wanted to meet up with CB and the others I "know" from the forum. I put out a couple of feelers to see if there were others interested apart from yourself and got no feedback. I got yours but by then it was too late because I was under pressure to attend another balancing meeting in the UK on the same dates and with no feedback from Florida I accepted the other meeting.

I feel really bad for you and frustrated because I wanted to get data from your gyro and other American models to add to my knowledge.

What I propose is the following. If you have the time to spend setting up a PB3 and doing all the test flying to establish moves lines etc ( it's long, tedious and boring) and climb the learning curve I'll "lend" (I want it back) you a spare PB3 that I have and talk you through the setup etc via email, skype, telephone or even this forum. I'm interested in doing this to help you and to get the data so you'll have to have the time to do a couple of days flying circuits gathering data while I explain things to you.

It's the best I can do for you, pm me your address if you want to try this.

Mike G

 

[All_In]

Hi Mike
As I said in my PM to you. PRA feels as you do it needs a person who does them all the time.

PRA would like to buy the tools and build a team for the community and have them trained by you. If the team showed up at most of the chapter fly-in's no one person would be stuck balancing blades full time.
I'm only one vote but if we have the funding, I would vote to pay for the team to travel to fly-in's and help other chapters and this would be retraining for the team.

I've asked you in the PM and I asked Jake if PRA could help and pay for part or all of your travel expenses and we would have a raffle and fund raising also asking those who wish to have there blades balanced make a donation of any amount and schedule your time the best we can.

PRA would help pay for expenses as you perfer to BD's and or a trip to Oshkosh for a real vacation for you and then Mentone for a 1st or 2nd training visit.
I would like to create a team of PRA balances with a member from every PRA chapter being invited to join the team and share the use of PRA balancer that we would buy for chapters use and learn from you balancing blades for a week or so?

I will not know how much I can raise until I try and see the demand for balancing at the fly-in's.
But I bet we would all love to have our blades balances at PRA fly-in's for FREE as a member benefit or tax deductible donation through PRA that goes to the local chapter for the service?

Great way for PRA to support our mission to our Chapters and grow the grassroots for them while making the world a better place.

The offer is still open... Or we will still build the team and do it by internet etc as you wish.
This is just too good an opportunity for PRA to let go by.

What would it cost to get you over here? Maybe it's way out of our budget but I would like to try at your earliest convenience.

PS:
We would also create a video documentary of your balancing and turn it into a training video. It occurs to me that we might reach out to the manufactures of balancers for a travel donations for you and maybe a discount to buy the tools as we will be kind of selling the equipment by demonstration at all the fly-in's has it's all we know. Great advertising a training video and demonstrations.

 

[Joe Pires]

Joe
I really had hoped to attend Bensen Days this year, I wanted to balance your rotor for you and get some data on the Aviomania and Ernie's rotor system for my personal data base. I also wanted to meet up with CB and the others I "know" from the forum. I put out a couple of feelers to see if there were others interested apart from yourself and got no feedback. I got yours but by then it was too late because I was under pressure to attend another balancing meeting in the UK on the same dates and with no feedback from Florida I accepted the other meeting.

I feel really bad for you and frustrated because I wanted to get data from your gyro and other American models to add to my knowledge.

What I propose is the following. If you have the time to spend setting up a PB3 and doing all the test flying to establish moves lines etc ( it's long, tedious and boring) and climb the learning curve I'll "lend" (I want it back) you a spare PB3 that I have and talk you through the setup etc via email, skype, telephone or even this forum. I'm interested in doing this to help you and to get the data so you'll have to have the time to do a couple of days flying circuits gathering data while I explain things to you.

It's the best I can do for you, pm me your address if you want to try this.

Mike G

Hi Mike, I think I would like that. I did make a bit of progress this weekend. I discovered that we had put some washers on the hat bushing that actually caused their own problem, I have backed out of that misstep and made a pretty good reduction in shake. I have two more things yet to do till I resolve how much shake I actually have so I will do everything non technical that I have at my disposal then make an evaluation of how much of an issue I still have and let you know.

Thanks for your most generous offer.

 

[Mike G]

I was recently asked about balancing a gyro in Germany. I did not want the owner to suffer the expense of my trip only to find that he was experiencing 2/rev vibration and I could do nothing for him.

I have said some of this before but here we go again.

Balancing and tracking are only good for reducing 1/rev or synchronous vibrations and cannot help reduce 2/rev vibrations.

I find that when I talk to people about frequency analysis and vibrations at different frequencies, their eyes glaze over and I can see that I've lost them. I know this feeling, its like when somebody explains to me how simple it is to update my computer, my eyes glaze over and they might as well be talking Chinese.

I will try to explain.

In my experience with eurogyros, mostly tandems, there are three vibrations that are usually dominant.

1. 1/rev or synchronous at about 6 hz or 360 rpm. This vibration is mainly due to unbalance or tracking or both. These vibrations can be reduced by balancing and tracking either statically or dynamically.
2. 2/rev vibration at about 12 hz or 720 rpm. These vibrations are usually due to four potential causes.
   a) Rotating drag, or the difference in drag between when the rotor is at 3-9 o’clock and 12–6 o’clock. This is inherent in all 2 bladed Bensen type rotors. Solutions (or rather methods to reduce) include slider roll pivots and “limber” masts.
   b) In-plane resonance. Explained better by CB but basically the in-plane (or xylophone) natural frequency of the rotor is about 6 hz which is the same as the rotor rpm and this creates a 2/rev (yes surprisingly) vibration. In my opinion this probably isn’t applicable to all of our gyros because while we experience high (in my opinion) 2/rev vibrations I don’t think they are high enough to be true resonance. The published examples I know of are CB’s gyro and the video of the early Bell helicopters. In both cases the pilots reported that the machines were unflyable. While most eurogyros have high 2/rev vibrations I wouldn’t say they were unflyable. I would guess that we are near the natural frequency but probably not on it. Solutions are stiffer in plane rotors, slider roll pivots and “limber” masts.
   c) Inertia around the spanwise blade to blade C of G axis, better explained by CB or J.C.Debrayer. Due to the fact that the blade tip plane is not perpendicular to the rotor bearing axis there is a 2/rev moment at the teeter bolt acting in the bearing shaft. With an optimum undersling the 2/rev vibration caused by this moment is minimised but there is always some. Incorrect undersling (either too large or too small) increases the moment of inertia around the spanwise axis and increases the resulting 2/rev vibration. The best solution is to optimise undersling.
   d) High friction in teeter bearings. This is seldom a problem on Eurogyros because they usually use needle bearings. This could be a problem if these bearings were badly corroded and started to seize up. This problem seems to be more common with smaller gyros with teeter bushings rather than bearings.
3. Prop unbalance. This is usually about 33 hz or 2000 rpm.

Typically the 2/rev is dominant, the 1/rev is more uncomfortable and the prop needs to really bad (it sometimes happens) before it is worse than the other two.

I wanted to find a simple method so that owners (like my German owner) could carry out a trial to see if a tracking/balance would be valuable.

1/rev is pretty easy to simulate, I’ve explained before my “hands on” (thanks Juergan) method. Take a bottle of ketchup and shake it as fast as you can (make sure the top is well screwed on). That’s 5 to 6 hz. If you feel that sort of vibration then a track and balance is probably interesting. I’ve noticed that the 1/rev vibration feels more like a 1/rev pulse than a smooth backwards and forwards cyclic motion. High 1/rev vertical vibration is usually a sign of tracking error rather than unbalance.

2/rev is a bit more difficult to find a simple example so I propose the following.

Measure the height of the centre of your car wheel from the ground.



Get some tyre balance weights, you’ll need a lot. I had to put 130 grams (4.5 ounces) on mine to get a decent vibration. Stick them on the inside of the rim so that centrifugal force will help keep them in place.

Find a smooth bit of straight road and drive at the speed from the graph that corresponds to 2 times your rotor rpm.



The vibration you feel will be your 2/rev vibration. Get used to what it feels like and then go fly after shaking a bottle of ketchup.

This should help you decide if you need to find someone with a balancer or not.

Another solution is to download an app onto your smartphone that gives you a frequency spectrum.

I hope this helps.

Mike G