Jump to content

Dynamic Prop Balancing


Recommended Posts

Did you see the bit where he demonstrates on a test rig that uses ball bearings as a Mercury substitute on an out of balance wheel and a strobe to light it up?

Couldn't find that. Can you post a link to it pls?

 

 

Link to post
Share on other sites
  • Replies 65
  • Created
  • Last Reply

Top Posters In This Topic

Check post #6 it's about halfway through the video in question, a bit annoying watching the lot though. The guy uses "actually " way too often.

 

 

Link to post
Share on other sites
Check post #6 it's about halfway through the video in question, a bit annoying watching the lot though. The guy uses "actually " way too often.

Thanks for the quick reply. Of all the videos on that page, it is the only one that won't play on my iPad

 

 

Link to post
Share on other sites
Did you see the bit where he demonstrates on a test rig that uses ball bearings as a Mercury substitute on an out of balance wheel and a strobe to light it up?

Yep, saw that.

 

 

Link to post
Share on other sites
Yep, saw that.

And that didn't make things any clearer?

If I am understanding correctly, they wok on the same principle as the bifilar weights on a Sikorsky S-70 rotor head.

 

Here is an engineer's explanation of bifilar weights to some others. not sure if it will help...

 

The principal of the bifilar has been used widely in automotive engines for some time - as Nick says. Those who remember the Astron engine in Mitsubishi cars of the '80's would remember the "revolutionary" silent shafts which were simply shafts with hollow compartments containing masses free to find their own equilibrium and tuned to roughly the operating regime of the crankshaft. The silent shafts were located close to the crank shaft in order that the countering (absorbing) effect of the free-to-move masses operated closer to the source of excitation. I'm sure there are many other models/manufacturers using the same principal.

 

 

  • Informative 1
Link to post
Share on other sites
And that didn't make things any clearer?If I am understanding correctly, they wok on the same principle as the bifilar weights on a Sikorsky S-70 rotor head.

Here is an engineer's explanation of bifilar weights to some others. not sure if it will help...

 

The principal of the bifilar has been used widely in automotive engines for some time - as Nick says. Those who remember the Astron engine in Mitsubishi cars of the '80's would remember the "revolutionary" silent shafts which were simply shafts with hollow compartments containing masses free to find their own equilibrium and tuned to roughly the operating regime of the crankshaft. The silent shafts were located close to the crank shaft in order that the countering (absorbing) effect of the free-to-move masses operated closer to the source of excitation. I'm sure there are many other models/manufacturers using the same principal.

No it didn't. He has an offset mass that happens to be then balanced by a specific number of ballbearings, this occurs during acceleration. There are a large number of mechanical differences between this and a concentric gallery containing some mercury.

 

I'll detail one: in a vibration free situation, I think we can agree that the mercury would form an even layer round the entire concentric gallery? Pop a dozen ball bearings in there instead, and the forces that cause that mercury to spread are no longer there to equally spread the ball bearings.

 

As for bifilar: that means, broadly, suspended at two points: it's a kind of pendulum action, but modified by being suspended at two points. Nothing in a concentric gallery of mercury introduces a bifilar modification to the forces involved.

 

But the most obvious question to me: if it can be done that simply, then why are all the highly trained vibration specialists mucking around with difficult and wear prone mechanisms on helicopters???

 

 

Link to post
Share on other sites
And that didn't make things any clearer?If I am understanding correctly, they wok on the same principle as the bifilar weights on a Sikorsky S-70 rotor head.

Here is an engineer's explanation of bifilar weights to some others. not sure if it will help...

 

The principal of the bifilar has been used widely in automotive engines for some time - as Nick says. Those who remember the Astron engine in Mitsubishi cars of the '80's would remember the "revolutionary" silent shafts which were simply shafts with hollow compartments containing masses free to find their own equilibrium and tuned to roughly the operating regime of the crankshaft. The silent shafts were located close to the crank shaft in order that the countering (absorbing) effect of the free-to-move masses operated closer to the source of excitation. I'm sure there are many other models/manufacturers using the same principal.

I'm not saying this thing doesn't have some benefits.

 

What I'm saying is that I'm still trying to understand how it might work.

 

I am less than reassured by the website, in several ways.

 

I am very interested in some form of objective testing, along the lines of that suggested by Kyle Communications.

 

 

  • Caution 1
Link to post
Share on other sites

Worries, trying to help understand, nothing more.

 

I agree, setting up a deliberate imbalance and carrying out tests with measuring equipment would be more objective.

 

 

Link to post
Share on other sites
No it didn't. He has an offset mass that happens to be then balanced by a specific number of ballbearings, this occurs during acceleration. There are a large number of mechanical differences between this and a concentric gallery containing some mercury.I'll detail one: in a vibration free situation, I think we can agree that the mercury would form an even layer round the entire concentric gallery? Pop a dozen ball bearings in there instead, and the forces that cause that mercury to spread are no longer there to equally spread the ball bearings.

As for bifilar: that means, broadly, suspended at two points: it's a kind of pendulum action, but modified by being suspended at two points. Nothing in a concentric gallery of mercury introduces a bifilar modification to the forces involved.

 

But the most obvious question to me: if it can be done that simply, then why are all the highly trained vibration specialists mucking around with difficult and wear prone mechanisms on helicopters???

That's a good question about the helicopters....I'd love to give it a go, chasing vibes on helis is part of my job, and it can be very frustrating. That said, a lot of heli vibes are in relation to blade tracking at different speed regimes, the mercury thing would only be good for weight imbalances.

As for the ball bearing comment...nope, there are systems in use on some vehicles that actually use the same system, only using ball bearings instead of mercury.

 

As for the bifilar weights, yes you've explained what they are, but not how they work, and the resultant action is very similar to the above mentioned.

 

 

Link to post
Share on other sites

I'm a firm believer in the science through related experience. My 1923 vintage car has wood spoke wheels and 3 1/2 inch wide tyres. When first restored there was a real problem with wheel balance. Not having any shock absorbers made it worse. The front wheels in particular would jump up and down while driving. The traditional approach was to locate an old on-the-car wheel balancer and use sheet lead wrapped around the spokes as weights.

 

Some web trawling revealed that many members of the Model T Ford club in the US were using a system of glass beads inside the tubes as a dynamic balancer. I bought a kit from the States as an experiment, and yes, it works superbly. With the car on a hoist (so no weight on the wheel) the beads are all at the bottom of the tyre and if you spin up the wheel by hand the up and down motion reduces as the speed increases to the point where their is no vibration. The beads are distributed around the circumference as needed to balance the rotating mass and are then held there by centrifugal force until the rotation stops. About 5,000 km later the tyres are worn in a completely uniform manner. From memory, it was only 6 ounces of beads per wheel.

 

I have since heard that steering wheels on prime movers use the same system.

 

A plastic bag of beads is placed in the tubeless tyre when fitted to the rim and it breaks open as soon as the wheel rotates.

 

 

  • Informative 1
Link to post
Share on other sites

Here is a good pictorial explanation of the beads-in-tyres application, I thought. Don't be put off by the title, the author concludes that the beads will work for a given type of imbalance:

 

Dyna Beads - Miracle Balancing Cure or Tire Snake Oil? | RideApart

 

There are a number of outfits in the US making varieties of this, and they have been around for 50yrs, probably longer.

 

Wheel Balancers for many trucks, buses, trailers, and motorcycles from Centramatic

 

 

Link to post
Share on other sites

Here's some good bifilar stuff:

 

Foundations of Helicopter Flight

 

As used on helicopter rotors, it's a pendulum device used to help reduce torsional vibration, I think.

 

The way it works is that, as the rotor momentarily tries to slow, the pendulum swings forward, tending to pull the rotor with it, and vice versa when the rotor tries to speed up. Typically, the pendulum frequency is tuned or set up to help counter specific torsion vibrations associated with the rotor blade frequency. This tuned frequency will alter as the rotor speed alters.

 

The reason it is a bifilar arrangement (two suspension points for the pendulum) is that this achieves the required pendulum effect in a more compact way than a single suspension point would.

 

 

Link to post
Share on other sites

That's used in larger aircraft piston engines. Dynamic counterweights that absorb torsional vibrations. Those engines still have "do not use" RPM's listed. MAGIC is great stuff if you can get the right magic.Nev

 

 

Link to post
Share on other sites
That's used in larger aircraft piston engines. Dynamic counterweights that absorb torsional vibrations. Those engines still have "do not use" RPM's listed. MAGIC is great stuff if you can get the right magic.Nev

Neatly put, Nev. I've had a fair bit to do with trying to get a variety of industrial machinery and systems to work well over the years, including both passive and active setups.

 

Just sometimes someone found the magic, and we ended up with a brilliant piece of simplicity.

 

More often we made things that only worked well within fairly tight parameters, if they worked at all.

 

I also worked twice for outfits that made okay gear, but whose owners engaged in 'magical thinking' (okay, I'm a slow learner). It took the form of believing that if we tried enough possibilities, some previously undiscovered physical principal would accidentally materialise.

 

Given the number of engineers, from capable to brilliant now spread across this earth, and all looking for the mythical free lunch, that is naive. And I learned not to go there.

 

 

  • Helpful 1
Link to post
Share on other sites
It would be interesting to try one on a totally unbalanced prop first to see how it feels. With my dynavibe I should be able to test this. Setup the dynavibe on the prop and run it and take the measurement at a few different rpm settins this is easy. Then fit the balancemaster and run again the same test and measure any difference with the dynavibe. That would be conclusive proof that it works. I am sure these guys would have done this at balancemaster to confirm it works. Then I would balance the prop at cruise without the balancemaster to best I could get then fit the balancemaster and then use the dynavibe to confirm that the result was better again. Would be an interesting test. Anyone know of a balancemaster around that is for a 912ULS that would be free for this trial?. I am willing to give it a go. If it makes a improvement it has to be a good solutionMark

From post 2 the link HOME opens to a price list. There is a price for a 912 but who knows what variations.

 

 

Link to post
Share on other sites

Going back to the original question of getting your prop dynamically balanced - I have had props on my Jabiru 6 balanced a few times by Brian Rebbech.

 

See SonexAus - Dynamic Propeller Balancing for details.

 

The web site for Balance Master claims up to a 50% reduction in the balance factor (out of balance factor?), but the first time Brian did my prop on which I had previously done a static balance, he managed to achieve an 80% reduction in the out-of-balance factor. I think there are a couple of advantages to having the prop dynamically balanced: The obvious one is reducing vibration in the cockpit but the other is reduction of stresses on the engine. It could be that your engine mount rubbers are doing a terrific job of isolating you from prop vibration. If you read through the linked document you will notice that vibration in the cockpit was about 1/3 of the vibration at the engine.

 

 

Link to post
Share on other sites
Going back to the original question of getting your prop dynamically balanced - I have had props on my Jabiru 6 balanced a few times by Brian Rebbech.See SonexAus - Dynamic Propeller Balancing for details.

The web site for Balance Master claims up to a 50% reduction in the balance factor (out of balance factor?), but the first time Brian did my prop on which I had previously done a static balance, he managed to achieve an 80% reduction in the out-of-balance factor. I think there are a couple of advantages to having the prop dynamically balanced: The obvious one is reducing vibration in the cockpit but the other is reduction of stresses on the engine. It could be that your engine mount rubbers are doing a terrific job of isolating you from prop vibration. If you read through the linked document you will notice that vibration in the cockpit was about 1/3 of the vibration at the engine.

And that is why I kept my dynamic balance weights on when I fitted the Balancemaster thingy.

 

 

Link to post
Share on other sites
I borrowed a dynamic balancer from Mark, of Kyle Communications on this forum. I then spent hours balancing and rebalancing my prop. I then researched prop balancers. Found two of interest, Vibrotech (Australian) sell balancer for around $1500 AUD, the other thing I found was a little thing called a "Balance Master". I was sceptical, but after having a bit of a think about it, I realised that it worked on the same principle as the bifilar weights on a Blackhawk rotor head, and purchased one from the US to try. Have to say, I was very pleasantly surprised with just how well it works. Cost me a couple of $100, but it works, and it works well. I told Bob Keen of Avid Aviation in Dalby about it, he bought a couple to try, now he's the Australian distributor for them.They are made for Rotax 2 and 4 strokes, lycomings and much more.

I can put you in touch with Bob if that interests you.

 

HOME

Thanks I have to do a balance on my feathering prop anyone can rent be a balancer or offer some help

thanks

 

I'm very interested in the Balance Masters, concept, but the prospect of having a large quantity of mercury on my plane worries me.Here's a couple of things that I think need to be considered;

1) How robust is the containment system in the Balance Masters?

 

2) Would it withstand a crash scenario?

 

3) If it failed in a crash or just in normal usage, it would spray liquid mercury in all directions. Who would be responsible for the clean up?

 

4) How well do they work at lower temperatures (ie what is the freezing point of mercury)?

 

5) What is the Aus government's rules on these types of devices? They are banned in New York State already and there are moves to further ban or restrict them in the US and other places.

 

I believe after doing some reading that they actually do work, but I'm a bit concerned about the practicalities / legalities. I might contact the guy in Dalby and investigate more about this.

can I rent one of

 

Who's for it & why?Who's against it & why?

Give me all your glowing success stories and dirty little failures.

 

I have painstakingly static balanced my prop.

 

Made multiple pitch adjustments & test flights, to arrive at an acceptable (to me) propeller performance for my take off / landing environment.

 

In my obsessive pursuit of perfection I am now wonder about dynamic balancing as the next level of worthwhile/dodgy investment.

 

Anybody care to recommend a NSW dynamic prop balancing service provider?

 

Just thought I would throw this little hand grenade into the discussion pit & see what jumps out.

is there a contact number to rent this balancer

 

 

Link to post
Share on other sites
Guest nunans

As far as these fluid balancers goes, aren't they what has been used in spin dryer/washing machines for many years?

 

Dynamic balance offers the ability to balance more than one axis where static doesn't. This is the difference in my mind. So picture a long shaft with a big weight hanging off each end where one weight is out of phase with the other by 180 degrees on the shaft. Ie one weight at 3 o'clock position and the other at 9 o'clock. When its stationary it could be perfectly bslanced in any rotated position so a static check would say "spot on" but if you were to spin it then it would shake off its mounts. By dynamically balancing it with two axis ie both ends measured at the same time tjen it could be corrected with two counterweights and it would spin without vibration.

 

For a prop which is effectively one axis and there is only one measurement taken for prop balancing it's hard for me to see how, if the engine is already correctly balanced a dynamic balance can offer more than an "ideal" static balance.

 

 

Link to post
Share on other sites
  • 2 weeks later...

OKAY!!! here is my report on DYNAMIC PROP BALANCING -

 

Just had my 912 prop dynamically balanced by Superair @ Armidale Airport, NSW Ph (02) 67725055 - very friendly & professional service. Despite being a bit rushed due to approaching weather concerns, job done . Incredible difference at take off power and cruise RPM 4800-5200 - almost turbine smooth. I am very impressed, definite convert, wish I had done it years ago. Not only does the engine/prop feel better the whole aircraft seems to be just that much "sweeter". There should be flow on benefits for improved component life and possibly reduced fatigue for the pilot.

 

 

  • Like 1
Link to post
Share on other sites
And that is why I kept my dynamic balance weights on when I fitted the Balancemaster thingy.

Hi:

How much did the Balance Master weigh? Did you have to redo your W&B after fitting it?

 

Someone likened mercury to "liquid lead" and saying that, I'm wondering what effect it would have putting a weight like that at an extreme arm on the plane such as out on the prop.

 

Thanks in advance.

 

 

Link to post
Share on other sites
Hi:How much did the Balance Master weigh? Did you have to redo your W&B after fitting it?

Someone likened mercury to "liquid lead" and saying that, I'm wondering what effect it would have putting a weight like that at an extreme arm on the plane such as out on the prop.

 

Thanks in advance.

The Balance master thing only weighs 4oz (about 115 grams) according to their website (I haven't weighed mine, but it's not large). No I haven't redone my weight and balance after. The Drifter prop flange is not really at the most extreme moment arm.

 

 

  • Caution 1
Link to post
Share on other sites
The Balance master thing only weighs 4oz (about 115 grams) according to their website (I haven't weighed mine, but it's not large). No I haven't redone my weight and balance after. The Drifter prop flange is not really at the most extreme moment arm.

Thanks for that. 4Oz doesn't seem like it would be enough to counter the imbalance of a prop, but I guess we're not talking gross imbalances. For my setup though, the prop is a tractor style, at the very front of the plane so it could have been significant if there was significant weight in the thing. I'm thinking that 4 oz wouldn't require a re-W&B though,

Thanks again.

 

 

Link to post
Share on other sites
  • 6 months later...

Hi All,

 

I have just fitted another engine in the Sporty and I found this vibration around 3000 rpm when coming back to idle, you don't seem to feel it as you take off and all the other the other rev rages it is smooth, it is just around the 3000 rpm, the old engine never had this vibration that I have now which seems so strange to me, same prop.

 

Dieter the L2 has checked everything else and now it seems to be this balance issue, so to eliminate this and hopefully fix it who is the best person to get this prop balance done and how much does it cost approx?

 

David

 

 

Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now

×
×
  • Create New...