Is balancing all that necessary?

[Erfolg]

I woke up this morning about limits and fits. Where I was debating if prop adapters were shaft based or hole based relative to "limits and fits". I was a little concerned that drilling the hole in a propeller or reaming the hole would produce the optimum fit.

 

I came to the conclusion that I was wrong and the adapter and probably IC crankshafts were mostly if not all hole based. I had been wrong with respect to them being shaft based.

 

Thank God I woke up, and told my wife I had, had, a nightmare, she was sympathetic, saying go and make some coffee for me, it will take your mind of it.

 

Whilst making coffee, there was a bit on the news about some steam engine excursion. Whilst looking at the wheels and pistons in motion, I was drawn to the balance weights on the wheels.

 

My mind went back some +50 years to my farther, telling me that the balance weight was a compromise (he was working on the design of BR Standard Classes of steam locos, before going onto the EM class of electric locos). It was a balance of trying to equalize out the forces imposed on the wheels by the connecting rods, due varying forces, whilst starting and at optimum running speed. The whole system was never truly balanced. It made the difference between a rough riding loco, to one that just bucked and snaked.

 

By now I was heading back upstairs, thinking about IC model engines, particularly 4 strokes. These devices are inherently unbalanced. Hence the much higher vibration level when compared to an electric motor. Other than using something such as a Lanchester shaft and the crank web, there seems little that can be done to reduce this vibration.

 

I then thought of the almost mystical adherence to the concept to balancing propellers. For electric motors, propeller unbalancing will mostly be the largest source of vibration. But is that so for IC engines. I then thought although accepting balancing should be beneficial particularly for electrics, does it make such a considerable improvement to IC models. Can you actually measure the improvement, by simple means?

 

I also started again to think of the real need to ensure that any enlargement of the prop bore, really needs to be in the centre of the propeller arc, that is truly concentric with bore as moulded. As if off centre, it is very possible, even probable that static balancing will not result in a dynamically balanced propeller. Again I thought, how much does it really matter?

 

I have noted, that although I do balance my electric propellers, they do not seem noticeably any smoother running than before balancing.

[Former Member]

[This posting has been removed]

[Ultymate]

Tom the part of your post reference chainsaw engines being deliberately out of balance to impart a hammer action to the chain is absolute nonsense. Single cylinder engine by nature of their design will tend to vibrate more than multi cylinder engines. One thing that is true is that if an engine is not perfectly balanced then fitting the heavy blade of an out of balance propeller in a different relative position to TDC may result in smoother overall running of the unit. As for electric motors I would say balancing props would be advantageous as obviously these motors are not subject to any reciprocating forces and are intrinsically well balanced therefore any prop unbalance would be more noticeable. Personally I always check and balance all new props.

Edited By Ultymate on 14/09/2011 13:32:40

[Lindsay Todd]

If chain saws are made out of balance to impart a hammer action? then surely this answers your question anyway, yes it is worth balancing your prop. anything that can have dynamic forces load and unload will impart force to weakest points causing stress and heat generation which directly relates to life expectancy of the engine. Engines leave manufacturers within a manufacturing and operational specification, it would seem therefore logical to impart as little additonal stress to the engine.

anyway thats my thoughts, I sense a lengthy debate - where is BEB.

Edited By Lindsay Todd on 14/09/2011 13:51:36

[Ultymate]

As further aside on the chainsaw reference most of the main manufacturers have spent millions on balance / vibration issues as these can result in white knuckle syndrome for long term users and there is serious legislation relating to this issue in most civilised countries these days.

[Ed Darter]

sorry - I think I am missing somethign here. Surely model engines are balanced? Remove the back plate on a 2 stroke and there is a counterweight built into the crank shaft? Never yet opened a 4 stroke so can't comment on them.

 

I for one have personally witnessed a big difference between fitting a balanced and unbalanced prop on a model, set to the same position as each other so I can finger start the engine. So I think it does make a difference.

[Erfolg]

Afraid not Ed.

 

The intense vibration gives the game away, at the operational level.

 

At the theoretical level, it is easy to see why. The crankshaft receives a force via the connecting rod, where the force changes dependent on its angular position (and of course the fuel air mix etc). With a 4 stroke, the force is not the same each revolution, for the same angular position.

 

The counterweight aims to address the issue, and is a partial solution.

 

Although i have never tried to quantify the vibration or the effect of an unbalanced crankshaft, I do believe that these forces etc. are far higher than most slightly out of balance propellers produce by a substantial margin.

 

I am not suggesting that balancing a propeller on an IC motor should not be done. I do question the mystical effect that some modellers seem to believe that balancing achieves. As I think you would be hard pressed to notice the difference, with the average well balanced propeller as it comes from the manufacturer. I would accept that a few hundred revs may be liberated in some instances, due to improved efficiency.

 

On the other hand it was found that planer crankshafts on racing car engines are more efficient at producing power than the crankshafts arranged for regular even transmission of torque. This is primarily due to better breathing of the engine, with the planer arrangement.

[Brian Parker]

It doesn’t matter where you position an out -of -balance prop in relation to TDC, it’s still an out of balance prop and will have some slight detrimental effect on the rotational balance of the engine.
Vibration in single cylinder engines is caused by the inertia of the accelerating reciprocating parts (piston ,rings, gudgeon pin and the top half of the connecting rod). The only way to truly balance a single cylinder engine is to neutralise by introducing equal and opposite reciprocating forces.

This is not practical, so a balance factor is used (as a compromise) when calculating the counter balance weight to achieve smoothness at the most appropriate RPM.

[Richard Sharman]

".....these forces etc. are far higher than most slightly out of balance propellers produce by a substantial margin......".

So why do I always notice an increase in vibration if I clip a prop on the ground and therefore unbalance it ?

 

Whether its 2-stroke, 4-stroke or electric, in practice you always notice a difference in engine performance when a prop gets damaged, and therefore out of balance - time to fit a new prop !

 

One thing that does help is a flywheel - I.C helis have a heavy clutch which acts as a flywheel and smoothes the tickover. I recently balanced the CG on my biplane by adding a prop-weight to the shaft of the OS90FS - not only did I have to add less weight to get the CG right, but the tickover vibration was dramatically improved (lessened).

[Erfolg]

Of course you are correct Brian in that the reciprocating parts contribute to the vibration, as does the impulses from the input forces to the crankshaft.

 

I was not really debating where the vibration absolutely has their origins, even the nature of the frequencies and pattern of the vibrations relative ti time. It is the scale of them relative to the forces from a slightly out of balance propeller.

 

I am not sure you are correct with respect to the propeller .that is out of balance being capable of reducing vibration, however little, or the practicality of actually doing it. As is the idea not similar to the counter weight or Lanchester shaft?

 

 

[Erfolg]

Richard

 

I was referring to relative vibration. The other issue is that clipping a chunk of your propeller is not a small discrepancy in weight distribution.

 

Remember I did not say there was no benefit, it is relative levels of vibration from a non balanced, well manufactured product and you an engineer, tut, tut.

 

Again being serious, it would be possible to rebalanced your chipped propeller, by removing weight uniformly from the opposite blade and achieve static balance, yet would be very unbalanced dynamically. Of course you would not do this, for a range of reasons. Although if you are not aware of the difference and why, you could. I know of course you would not.

 

I feel my tail being tweaked,

[Brian Parker]

Efrolg,
I actually said that an out of balance prop will have a detrimental effect on the engine. We are in agreement.

[Simon Chaddock]

I think you will find that a single cylinder 2 stroke and 4 stroke have exactly the same primary and secondary balance characteristics, after all, the crank, con rod and piston have the same motion. Where they differ of course is in their torsional vibration.

 

Only if you have 6 cylinders either in a line or horizontally opposed (unless you include a true rotary) can the first and second order vibrations be made to disappear completely.

 

 

As smooth as an electric motor but a great deal more expensive!

[Biggles' Elder Brother - Moderator]

Erfolg's compromise that he refers to comes about because there are (at least) two types of vibration forces we are trying to deal with.

 

One arises due to largely static effects and is linked to the engine's firing cycle - just like Erfolg's dad's steam engine crankshaft transfering the impulse load from the cylinder.

 

The second type are the dynamic vibrartion forces - these occur because of the rotating masses in the system and are dirived from the inertial forces those masses generate. Note these would be present even if there was no firing cycle!

 

At engine speeds above 3-4,000rpm for a single cylinder engine the dynamic forces become dominant. At lower revs the firing cycle vibration is dominant (that's why, paradoxically, 4 strokes in particular actually seem to vibrate more (at a higher amplitude) at tick over than they do at high revs.).

 

Our engines spend most of their operating lifes above 3-4,000rpm I would suggest - therefore they operate mainly in the regime where dynamic, or inertia related, vibration is dominant.

 

How much inertially induced vibration you get is a function of two paramemeters:

 

1. How fast the object is spinning

 

2. The object's moment of inertia.

 

The moment of inertia is a geometric property of a shape that, broading speaking, is related to its radius to the power of 4. So if I have an object of a given size and a copy of that object say twice the size, then the moment of inertia of the copy will be not twice as big, but 2x2x2x2 or a whopping 16 times bigger. As a generalisation the inertial forces (inclkuding those causing vibration) will as a result of this be 16 times greater on the copy as well.

 

Now we come to the nub of the issue! A propeller has a much bigger radius than a crankshaft - so (mass for mass) it can generate much larger vibration forces for the same speed of rotation. For example an imbalance of 0.01g on a cranshaft with a small radius will have far less impact than the same 0.01g imbalance near the tip of a prop with a radius of perhaps 6 or 7 inches (forgive the mixed units!)

 

QED - balancing props does matter because their much higher moments of inertia means that they are major contributors to inertially induced vibration.Indeed it is possible that given the radius to the fourth power effect they may well be the dominant source of inertial vibration in a prop/engine set up despite their lower weight.

 

BEB

[Erfolg]

BEB

 

I totally accept that the vibration is determined by the radius of gyration from the propeller. I am to be convinced that the forces generated by the propeller are as a significant as those which are generated by an IC motor. That is where the propeller has been produced by normal manufacturing methods. This is based on the observation, that I do not experience anything like the levels of vibration with electric motor and propeller arrangements, than is the norm for an IC combination. Normally a very small fraction.

 

Where I am really coming from, as a result of some difficulties I had with a electric model recently, I have been comparing, by observation, with no measuring equipment, the differences, between a small number of propellers before balancing and after.

 

I could not detect any differences between either. Which may not be that surprising, as the difference in weight is miniscule. Probably less than the 0.01g you mention.

 

I note that you say near the tip. Which is along the lines I was thinking, that the very small differences in weight are probably not from material at one point, conveniently located at the tip. But the material is probably distributed along the blade.

 

Perhaps it is worth noting that the vibration is imperceptible in either case.

 

It is this apparent nil effect, which caused me to think of the efforts to balance gas and steam turbine rotor assemblies, that reminded me that static and dynamic balance are not the same thing. We are back to the relative radius of gyration of the blades. Was this why I saw little change. It was still not actually balanced?

 

I balanced the propellers by removing material from the back or flat side, near the tip. Rather than equally along the length of the blade.

 

I was then thinking, the biggest potential factor in unbalancing the propeller was when opening up the bore. If the enlarged bore was not concentric with the original bore, I would be introducing an imbalance potentially that was far greater than that, that originates from the as manufactured item.

 

What has surprised me is that it is possible with at least one motor assembly to generate vibration forces far greater than I would associate an IC engine. To date I have not identified the root cause of the issue, other than suspect it is related to the geometry of the motor components, rather than the propeller. As I have now tried a wide range of diameter of propellers and the vibration sets in at differing speeds. The vibration is so sever that I remain fearful and have undertaken the testing in my garage, rathere than my modelling room. In the garage I can and do sepearte myself from the equipment

 

 

[Myron Beaumont]

A factor that comes to mind with electric motors is may be the protruding length of prop shafts distance from the main source of energy input from windings ? Although I'm no expert on motors,they do seem to have "spindly" shafts in general .Any dynamic unbalance would (may I suggest) bring about various resonances at varying frequencies/rpm/prop size that Erfolg has noticed.

Myro YO13 ( retired think tank-engineers dept.)

Edited By Myron Beaumont on 15/09/2011 08:26:25

[Erfolg]

I have been thinking about the issue of why vibration from the propeller on an electric model is very low, without being balanced, particularly when compared to a IC set up (balanced or not).

 

I know we are in danger of getting into more and more detail as the specific models are refined. I also was taught if something is not happening as you expect, something is wrong with your understanding, you may have stumbled on something exciting, more likely your assumptions have a flaw.

 

I think considering why BEB thinks that the propeller imparts more forces than my experience indicate

 

The first is related to the densities of the materials steel is 8 times denser than High Density Polypropylene.

 

The radius of gyration (or the centre where the mass can be considered to act) of a crank counterbalance web is deliberately located as far from the crank centre as is practical. Whereas the radius of gyration of a propeller tends to be located near to the hub, partly due to the management of stress. Plus the crank web mass is not uniformly arranged as with a propeller requiring the use of Parallel axis theorem, which will probably produce a higher value than immediately assumed.

 

I guess when we think of the centripetal force, that the motor shaft sees, it is a constant arrangement for most electrics(pusher zagi types being the probable exception). In the case of the IC motor, the shaft sees a number of cyclic forces from the acceleration and deceleration of the shaft, of a cyclic nature, which introduces additional cyclic forces onto the propeller. I am also assuming (now after some thought) that the piston velocity differs on the power stroke, relative to the induction or exhaust stroke, at most equivalent points. Though I guess we all knew this.

 

I guess it is the difference with a simplistic model of a plumb bob on a string, which is swung round in your hand, when horizontal, you feel a constant force at your wrist/hand that rotates with the bob. If you then move the plane to the vertical, the force is still felt, no longer constant, due to gravity, causing the force vectors to change. Where as a constant speed spinning disc, whilst held in one plane imparts very few apparent forces of a cyclic nature, as the system can be viewed as in equilibrium, with respect to the centripetal forces being balanced as far as the axis of rotation is concerned. As with most situations, I have conveniently assumed that the system is static, rather than how real life is.

 

I guess alot of what we have considered re-enforces why IC propellers are need to be more robust than electric.

 

I am speculating it is the combination of the physical arrangements of the masses, the geometry of the IC motor which is the source of the high ( on a relative basis) vibration, which improves liberated power, by an increase in revs. Whereas with an electric motor little benefit seems to be gained from balancing.

 

Now having a tachometer I can say with a little confidence that the tachometer is showing that there is little, if any apparent increase in revs after balancing. The max revs reading is a boon here. All my outrunners seem to rotate essentially at there rated load at about 10,000-12,00 revs.

Please do not get me wrong, I am not advocating, no balancing. it is that benefits are less than I assumed they would be, at least for electrics.

 

I am still undecided that it is my method of static balancing that is faulty, and the propeller is dynamically unbalanced after my efforts, or that the uniformity of the propellers as manufactured, leaves little room for substantial improvement. I am also suspecting that the electric motor and propeller model, is far less complex than with IC to adequately describe what is happening.

 

Although I have some way to go with my rogue motor.

[Myron Beaumont]

Erfolg

There is a simple way of resolving this quandry . Run engines and motors in a vacuum fitted with strain gauges with time synchronisation variables attached to an oscillascope.Then you will resolve this non- existant serious problem that has awakened "our little grey cells "

Myron YO13 .(grey cell time- expired rejuvenation dept)

[Erfolg]

Oh Myron it is to simple, isn't this sort of thing that makes modelling and life interesting?

 

Consider this other quandary, if hand tapping most people are taught to advance the tap about a half turn, then back off the tap. A machine tap is just run straight into the work, until the ratchet slips and or the depth setting is reached. The only possible reason for backing the hand tap of, is to break the cutting and clear the flutes. So why not just keep on turning and back of when it is seen or felt it is necessary. I have tried it, it causes no problems or issues.

 

Life's little quandaries

[Myron Beaumont]

Erfolg

You have just solved "your"problem and don't know it .A tapered first tap !!.Once "near enough" screw the prop onto the shaft .If it doesn't go all the way ,either force it on (the wrong threads will give ),or take a little more off .(proper job way ) You'll have about 20 thou' give in the thread so it will centralise itself .Yes--No ??

Doesn't matter what thread either !! Long as it ends up on the non threaded portion of the prop shaft reasonably easy ie You can get it off again !

Myron YO13 (overstretched brain dead rejuvenation think-tank dept.)

Addendum

What a funny day .Just herded a load of sheep into our yard for H&S checks before market.A now one -eyed cock bird chicken after a fight has adopted us in the house (frightened) .The cats keep bringing in shrews plus one baby lapwing when they're supposed to be ratting .The local cowboy topped up the tractor engine sump last year to overfull with diesel fuel (lots of blue smoke pouring out the exhaust on its start up ) By the way ,reffering to your comment about buying the Mills 75 & what SHE spends on footwear -I can tell you---A new pair of Wellies & I bought her a cheap Chinese designer copy of a huge handbag for carrying all her credit cards in. Sorry,drifting off topic

Just realised,it was BEB on the subject of blackmail  to get a Boddo Mills 75 not your good self

Edited By Myron Beaumont on 15/09/2011 14:48:05

[Erfolg]

Both cases illustrate to me that common held absolute truths, are often, not quite true in all circumstances.

 

I am a little more than surprised how accurate it appears the Chinese propellers are, I guess in weight distribution, I cannot say with respect to pitch.

 

I have thought how they make the items. I assume that the tool makers rough out the cavity, in the mould insert.

 

I am also assuming that they first make two electrodes for the top and bottom tool. With turbine blades in the past, these were milled on a profile milling machine, then hand finished using template gauges for each station. Then hand finished. I know that this process was less than absolutely accurate. Smaller ones were also cast, to rough shape before hand finishing. The billets for the bigger blades had a lot of processing to achieve improved structures, but that is another matter.

 

With the resulting electrodes, I have assumed that they either spark erode or ultra sonic machine. Both methods needing some hand finishing of the cavities.

 

So I am guessing that some other method is being used, as the process is prone to errors from the absolute values sought and it is costly.

 

Given the apparent accuracy of the propellers purchased and the low cost, how are the moulds made today?

 

How did Rolls Royce make their blades.

 

Is it possible that other methods are used today, which are both very accurate and low cost?

 

I am hoping that your quality of life has improved at your new location. Seems so.

 

[Tim Mackey]

Can we keep on topic please

[Erfolg]

Timbo

 

Surely the manufacturing process and how it impacts on the finished product is relevant to the balancing of the propeller blades.

 

I am increasingly finding difficult to believe that it is possible to make a propeller by moulding accurately enough, not to need balancing.

 

Yet it does appear that my propellers to date seem pretty good in that respect.

 

Am I lucky, or am I misunderstanding how the whole process is structured.

 

Perhaps I heed even more propellers.

 

I am even more surprised that they can be made as cheaply as they are from some sources.

 

Ah, while I am at it, rather than remove material, perhaps coating the light blade with varnish or paint could be better?

 

I also now relies I am missing out the process of ensuring that the radius of each blade is identical.

[Lazygit]

I coat my blades with varnish on the light side. That way I can get them balanced accurately. Then I paint the tips white so they are more visible when running and check the balance is still ok. Then I put them on an IC engine and the airframe vibrates as usual (caused by the crank web whacking back and forth along the line perpendicular to the motion of the piston). Then I fly it, and the prop hits the grass now and again, wipes the paint and varnish off, and hey presto!!, I don't notice any difference

[Richard Sharman]

Erfolg wrote:

.....I have been comparing, by observation, with no measuring equipment, the differences,....

 

Surely not ? This is art, not science !

 

Before too much additional theorising I would respectfully suggest that some attention should be paid to questions such as:

* is the propellor symmetrical ? or if not, what tolerance do we allow ?

* is the material of the propellor uniform ? Plastic propellors even look uneven, wood certainly, metal illegal,

* is the shaft hole central ? In my observation of commercial products this is the most common error, closely followed by the modeller opening out the hole to fit a larger shaft and in so doing making the original hole off-centre.

* what vibration is observed at specific rpm settings ? the sort of machine that checks wheel balancing on your new car tyres, comes to mind.

* ... and so on....

 

On the question of engine imbalances as opposed to propellor imbalances I am more inclined to accept the product of the engine manufacturer than that of the propellor manufacturer - but is this justified?