Dynamic balance on props

[Graeme Evans]

Just read the article "Question of balance" in the Autum 2012 special... and was confused by the desciption of dynamic balance, Brian describes it as "it must be in balance over their length, i.e. half way along the blade (for eg) they must be equal in weight" and goes on to describe balancing them in sections along the blade.

This is not my understanding of dynamic balance at all, infact the relative weight along the blade is a self canceling compnent of any balanceing system.

Car wheels are dynamically balanced but at no point are weights added at different distances from the centre, rather on the inside/outside as needed at various calculated positions.

It was my understanding that dynamic balance referred to the difference in weight of parts of the rotating object along the axis of rotation, not across it, easier to think of if you picture an axel... if you put a weight on the side of an axel and another identical weight opposite but further along the axel it would still be in balance laying across a simple prop balancer but if spun sufficiently would vibrate.

additionally in the same article suggests drilling many holes in the hub to remove weight... i really woudn't like to be near that at 14,000 rpm.

Is anyone else botherd by this ot is it just me?

[Shaunie]

Not got the special yet but I think your visualisation of dynamic balance is spot on.

We could get dynamic balance issues with a perfect statically balanced prop if the blades aren't tracking correctly i.e. the arc of one blade is in front of the other. As for drilling the prop hub, no way!

I think the best way to balance the prop would be to move the centre of the hole but have no idea how it could be achieved in practise. I read something recently that APC props take a spacer in the recess to centre the prop, strange that you can get the props everywhere but I have yet to see the spacers on sale.

[PatMc]

Some folk used to use single bladed props for free flight competition models - both ic & rubber. The ic ones usually had the balance weight contained within the spinner. They had to be perfectly balanced as they ran at very high rpm.

[Ian Jones]

Shaunie, aren't those spacers intended for electeric props?

[Biggles' Elder Brother - Moderator]

I have to be honest I haven't read the article yet so I'm only going on your explaination of what Brian said! But I think I can see what he's saying. Let me try to explain - not easy!

Static Balance - lieterally means "balanced whilst not moving" (hence the static bit!) This is what we do on a prop balancer.The problem is - as some have indicated - something can be statically in balance - but still not be in balance when you actually spin it up. Why?

Dynamic Balance - lieterally "balanced whilst moving" is fully balanced when actually rotating.

So how could a prop be in balance staically, but not in balance dynamically?

Well static balance just tells us that overall one blade is the same total weight as the other - so if positioned on the prop balancer, with the blades horizontal, it will stay there.

But let's suppose the density of the material from which this prop is made is not constant, and lets say one blade has some denser material near the hub whilst the other blade has the denser material near the tip. Now we could statically balance this prop so that the overall weight of one blade is the same as the other. But when we spin it up - we would have problems! The disrtibution of weight along the blades is not the same - the total weight is the same but it is arranged differently!

This would mean that the inertia forces (which depend only on the weight) would be different at matching points (equal from the hub) along the length of each blade. Result inertia (dynamic) inbalance forces would be set up and the prop would vibrate.

Are you likely to experience this as a problem? In my personal view - no, very unlikely.

If you did have it this effect can you fix it? In my view not practically - you don't have a dynamic balancing system! Although I suppose you could in principle fix it by removing different amounts of material along the length of each blade in order to restore the same weight distribution along each blade. But as I say, I doubt that is practical without highly specialist kit.

What should you do if you do have this problem then? Again in my view - throw the prop away and use another one!

BEB

Edited By Biggles' Elder Brother - Moderator on 18/09/2012 22:38:14

[PatMc]

So how can a single bladed prop run at high rpm without any problem & how are car wheels dynamically balanced simply by adding weight(s) to the rim ?

[Bob Cotsford]

WOO confused me too with his dynamic balancing, which seemed to be more to do with squaring up the geometry of the prop than anything, and as for drilling 4mm holes around the hub? Not likely.

Incidentaly BEB, don't you think the spiral airflow from the prop impacting on the left side of the fin comes into the initial roll direction changes in the take-off? I know torque and procession have some say in the matter, especially with large props on electric power - my Strutter will almost turn 90 degrees if I whack the throttle open - but I do think slipstream plays a part too.

[Biggles' Elder Brother - Moderator]

Pat - the tyre machine is a very pragmatic solution. There is an encoder on the shaft so the machine knows the position of the wheel in its rotation. There is also a load cell on the shaft - so it can generate a sort of "imbalance force versus angular position" graph. It then also knows the spin speed and the diameter of the rim - so it can work out how much lead just where on the rim you'd need to exactly counter the imbalance force.

Bob - I actually believe the air on the side of the fin argument is very over played - yes it may have an effect in some arrangements - but in others its minimal. P-effect and torque however are always with us! Happy to discuss this further - but maybe in another thread so as to not go off topic!

BEB

[PatMc]

It seems to me that Brian Winch is advising a solution where there's no problem.

[Bob Cotsford]

maybe tomorrow!

[Tony K]

Posted by Biggles' Elder Brother - Moderator on 18/09/2012 22:31:22:

 

Static Balance - lieterally means "balanced whilst not moving" (hence the static bit!) This is what we do on a prop balancer.The problem is - as some have indicated - something can be statically in balance - but still not be in balance when you actually spin it up. Why?

 

 

That is the literal meaning of static balance however revolving parts can be, and often are, statically balanced by spinning them on a balancing machine.

For example, a motor car crankshaft and flywheel. The crank would be dynamically balanced first, then the flywheel would be fitted and the assembly spun up on the same dynamic machine. The flywheel, however, is statically balanced.

I'm with Graeme Evans and Shaunie on this.

As for "inertia forces", surely they balance out?

Edited By Tony K on 19/09/2012 06:32:49

[John Olsen 1]

Well...static balance only takes into account the masses and their moments either side of the axis. Dynamic balance considers also their disposition along the axis. That is probably a bit pedantic to be easily understood, so let us consider a real world example.

Suppose we look at a bicycle crank. It has two cranks, at 180 degrees to each other, and both of identical construction. So apart from minor errors in construction, it will be statically balanced. Each crank is balancing the other. However, they are located at the opposite ends of the shaft, so when the crank is spun at high speed, the unbalanced force from each crank is not in line with the balancing force from the other one. So if you hold the frame up and spin the cranks (with the chain etc left off) you will be able to feel that the crank is trying to wobble the frame. Because the forces are not in line they are creating a rocking couple. This is dynamic imbalance.

It will be apparent that dynamic balance is not likely to be much of an issue with an object like a propellor where both blades are pretty much in the same plane, provided the moulding is up to scratch. To correct a dynamic imbalance condition, it is necessary to apply balance weights to create an equal and opposite rocking couple, for instance we could balance each bicycle crank individually.

It should be noted that it does not matter if an out of balance condition is corrected with a large mass close to the axis or a small mass further out. We learned this early when using seesaws with mixed sizes of other children. So for a propellor the distribution of mass radially along the blade does not matter, so long as the other blade is the same, or else so long as we provide an appropriate correcting mass.

Of course, I too have not yet seen the article, which is not likely to arrive here in the antipodes until somewhat closer to Christmas.

John

[Dave Bran]

Posted by PatMc on 18/09/2012 22:58:06:

It seems to me that Brian Winch is advising a solution where there's no problem.

+1

[Myron Beaumont]

Couldn't agree more ! In fact when I saw the hub being drilled etc ,I moved onto the next article & maybe will read it later now that you've all pointed out the obvious misconceptions involved regarding a very thin light profile prop compared with all the upy-downy happenings a couple of inches behind it .I'm surprised H&S hasn't been brought on board whilst making up non -existant problems .Now ,if you were talking gas turbine compressors or turbines or even EDFs it might warrant consideration (and it does of course) I'll read the article now I've had my early morning whinge

[Biggles' Elder Brother - Moderator]

Tony,

the inertial forces will not cancel out because the axis about which the prop is being forced to rotate by the shaft does not align with the principal axes of its moment of inertia (because of the non symetrical mass distribution) - the result we have an imbalance which only manifests itself in the presence of acceleration - of course anything rotating is in a constant state of acceleration.

If inertial forces did cancel out then, by definition, dynamic imbalance couldn't exist as that is exactly what dynamic inbalance is - ie inertially (acceleration) generated out of balance forces! If acceleration wasn't needed to create them then they wouldn't be dynamic force but merely kinematic forces.

I agree you can statically balance by rotation but I was just trying to make the distinction. When I said dynamic balancing involves movement I was simplifying things slightly. The key is it involves forces created because of acceleration, and hence inertial in origin. In practice of course for a rotating body - as I say above - all motion involves acceleration even when the rotational speed is constant. So I was guilty of using a "shorthand" there!

This stuff is murder to try to explain without the maths!

Regarding the a general point I agree with others. I don't think there is a problem here - as I implied in my original post. It is present as a problem in principle - but not I believe in any practical sense. Its very unlikely to occur and if it did there's nothing we could really do about it other than disgard the prop!

BEB

Edited By Biggles' Elder Brother - Moderator on 19/09/2012 09:25:23

[Greybeard]

I was also surprised to see the drilling of holes in the prop advocated, it sort of sent shivers down my spine. I’m with BEB of this one, if vibration is such an issue as to require this sort of treatment then chuck the prop and try another.

[Peter Beeney]

I also have not read Brains’s article, so this is only likewise based on what has been said so far, and therefore any comment might be premature, but it does sound quite remarkable. The prop balancing bit sounds a bit like he might have just got a bit confused, static and dynamic balance has long been known about with regard to vehicle wheels, from the time when cars started travelling faster than about forty miles an hour, anyone working in the motor trade from 50 or 60 years ago will know all about wheel balancing. I believe that static balancing is actually equalling the balance evenly across the diameter of the wheel, dynamic balancing is correcting the balance across the width of the tyre, from side to side, with regard to the centre line of the wheel, the balance weight goes on the inside or the outside rim? A bit like adding weight to a wingtip, maybe? Perhaps this is just two different ways of describing the same thing? I’m sure it’s quite possible that full sizes prop are balanced when they are made, but as far as propellers for models are concerned I’d personally not put a lot of emphasis on this.

But advocating drilling holes in the hub must be in a different league! In my lowly opinion this has to be a exceedingly dangerous piece of advice, and coming from someone paid to write specific advice on engine care and safety makes it doubly so. I wonder what any propeller manufacturer would say if I told them that I was doing this? I think I would need earplugs! The advice has always been that if a prop is chipped, cracked or damaged in any way it should not be used; and I would agree with that. If someone turned up with a propeller so maltreated and we knew about it we would do our utmost to persuade them to change it. Should this fail, then I think we would advise everyone that we had a model that had a prop that could potentially break in half at any moment, probably when it was going at high revs. If a prop is thrown off complete, such as when a four stroke detonates, it tends to go more or less straight ahead, but in two halves I’m not sure where they would go. I’d tend to think that propellers generally have a good safety record, I’ve heard little adverse comment over the years, but if this is done on in a big way that could change. But I’m sure that’s most unlikely, I guess most folks will think this is just a little too dangerous.

I remember reading once when someone was doing an engine test, not Brian Winch, and when using wooden props they had trouble with the power stroke gradually breaking the propeller boss out. So the severe impulses definitely are there. They resorted to using plastic types. It’s quite possible our props might survive this sort of treatment too, but I’d say that’s much more down to the integrity of the plastic prop, rather than any calculations that Brian may have done to establish if this is possible so to do. All in all I would say this is definitely not good advise, but unfortunately I’ve found in the past that if you take the argument back to Brian, like some others, he soon tends not to reply.

Maybe a little case for a bit more experimentation, perhaps……. But very carefully…

PB

[Erfolg]

I would suggest that when using a IC engine, the propeller statically balanced is good enough, as the principle out of balance forces are coming from the motor. If the prop is massively out of static balance it then matters, in a practical sense.

I have had only one prop out of balance, which occurred after crashing, not sure where the mass was lost, as no visible damage was to be seen. Although it was felt with electric power.

At AEI they had a little model which demonstrated how bad dynamic out of balance could be.

something like this 4" arm one side 1kg mass on end, the other 1" arm with a 4 kg mass

so 4 *1 = 1* 4

boy did it vibrate.

My own view is that if you loose some material from one tip, the other tip needs material needs removing to achieve static balance. If the problem appears to be a heavier blade, then the whole should be lightened.

Although pooh hoed by others I use a taper reamer to get a good, sliding fit, using the rear of the fixed collet, to ensure that the prop fits square. By this method, my props so far are statically balanced when new.

[Myron Beaumont]

Erfolg

Surely you are talking about static balancing ie moment arms .Not dynamic side to side from the centre of rotation -so to speak.

Myron -wobbly dept

[Erfolg]

It was an illustration of an extreme example of static balance, when rotated is out of balance due to unbalance polar moments of inertia. Perhaps I should have mentioned the arm assembly was then rotated?

On rotating machines these can matter a lot on turbines etc. In the Turbine development department there were rigs where a rotor could be deliberately excited by out of balance mass and various methods of damping the initiated events.

The revs in some instances were surprisingly low, some 6,000, the out of balance mass a matter of a few grams, on significant rotor mass. Get up to a 20, 000, well that had me worried, not withstanding the armoured container and concrete bunker.

[Graeme Evans]

Wow... what a response.

While I do still think the dynamic balance as desribed in the article is not as others would decribe I am increasingly worried about the drilling of the the hub for balancing. I can well imagine some people getting carried away and having very little hub left.

PS: Sorry to Brian Winch if you read this I don't mean to be ungreatful for the effort surely taken writing quite a long an important article but I am seriously worried about drilling the prop hub directly as illustrated.

Edited By Graeme Evans on 19/09/2012 21:11:50

[Braddock, VC]

There are two issues here.

The first is do you need to balance props?

I hadn't bothered to balance props until a couple of years ago when I bought one of those magnetic balancers which showed me right away that I'd wasted my money. Stick with master and graupner and they don't need balancing.

( I won't talk about apc, the fact that the hubs are partially hollow on the drive faces puts me right off buying them, I tried filling the holes with epoxy etc but found it to be more beneficial to buy either of the two makes I mention above, also I can recommend jxf as supplied by giantshark.)

Edited By John Gibbs on 19/09/2012 21:37:50

Edited By John Gibbs on 19/09/2012 21:38:29

Edited By Biggles' Elder Brother - Moderator on 19/09/2012 22:16:52

[Graeme Evans]

John

I know some props are pretty well balanced and for IC I would largely agree, though I do usually check the balance anyway. Afterall a 2stroke 90 engine shakes like hell anyway!

For electric howerver you usually end up spinning the same prop at similar RPM on half the size shaft in a model half the mass, then I find that even the brands you mention can be tweaked with noticible improvment in flight.

[SDF]

I haven't seen the article either but it sounds like snake oil - show me the maths.

Edited By SDF on 19/09/2012 23:16:18

[Braddock, VC]

Posted by Graeme Evans on 19/09/2012 21:48:17:

John

I know some props are pretty well balanced and for IC I would largely agree, though I do usually check the balance anyway. Afterall a 2stroke 90 engine shakes like hell anyway!

For electric howerver you usually end up spinning the same prop at similar RPM on half the size shaft in a model half the mass, then I find that even the brands you mention can be tweaked with noticible improvment in flight.