The prop balancing myth

[sturgm]

I am still surprised at the amount of miss information there is about prop balancing so I got an old posting of mine
and edited for clarity and posted here where it should belong.

In one of my last posts I had been trying to reduce my vibration as I intend to use the quad for some FPV. I am also aiming for about 50% throttle hover as this seems to be a requirement if I choose to try Mega Pirates for position/altitude hold as when exiting it will use your throttle position and this can lead to some surprises, either roaring of up or down if the position does not correspond to your hover point.  I speak from experiences on this point.
It is a shame as in my heli flying days 75% hover was recommend with idle up and inverted flight so 50% for me does not really feel that comfortable at the moment.
Anyway I viewed and read once again all the topics I could find on the subject and ordered some more props.  So I ended up with six sets of props from various suppliers with a quite large difference in prices ranges from about 10$ a set to about 40$.
I also went back to basics.  What am I trying to achieve?  The answer is three 0';s on my gyro with default LFP.
So I clamped my quad to a bench but with a pair of arms over the side to cut down any prop wash which effect the results.
Uploaded a version with default LFP.
Connected the quad to the MultiGui.
Selected only the gyro trace to be displayed.
Selected Acro mode on the tx
Removed all the props and ran the quad up to 50% then 75% throttle to check the balance of the motors.
The result was all 0';s.  So the motors were perfect.
I stuck a piece of tape onto one motor just to see the effect and it showed up as a couple of 1';s and 2';s here and there
so it proved that everything was working as expected.
Now the important bit
I connected motor 1 directly to the throttle channel on the rx so there was no input from the FC into the motor as I had found that the FC will try and react to the vibration and make matters worse causing more resonance resulting in greater vibration and so the results are of no use.
I also attached a new prop that had not been statically balanced.
Another important bit
Static balancing on a prob balancer is of little or no use.  Yes you are reading right here.  I discovered over many days of balancing and testing that
this is true.  As an example  I can balance a prop to perfection on a prop balancer fit it to a motor then balance it by running it up and studying the gyro trace and carefully applying tape till I obtain my 3 0';s and a silky smooth spin up then take that off of the motor put it on the prop balancer and it is clearly ';out of balance';.  So in short I miss out completely the static balancing!



I bought two middle of the road prop balancers to check as I had read some articles about quality issues.  Both are of the magnetic type. 
This type is a must with these very light props.  Firstly I checked if the spindles and fittings were balanced.  They were both out.
So how could an out of balance prop balancer, balance props correctly ?

The shafts were straight and true but one had been machined at the ends off center. 
So I machined these up which made the shaft shorter so I added another magnet to one end to compensate.
Next both of the knurled prop grippers had not been drilled true.  This could be seen by fitting both to the shaft with no prop and checking if they were out of balance. 
So I gave up with those and made two new ones.
I now had a prop balancer that would balance.
I compared the results of the two balancers on a single prop.  Both gave different results.
On the stock balancer if the prop was balanced true then moved around by 180 deg it was now NOT balanced!
Also if the prop was taken off and then refitted it was also not balanced!
Try this on your balancer and see if you get the same results.







Back to the Quad
Another issue to look out for is the prop shaft adapter.  The only ones that I have found to be correct are the ones that come with
Graupner Elektro Props ( well they have to be purchased separately ) all the others are at best questionable, at worst useless.
I discovered how bad these are after I had balanced a prop on a motor took it off, then refitted it only for it to now produce
vibration even though I had marked the prop and motor so it was put back on the same position.
Further investigation revealed some small slop in the fit either on the shaft or on the prop on all of the prop sets except the Graupner.

The main problem is how the adapters are made.  Being injection moulded the moulds have a slight camber to enable the parts to be released in production. 
This produces a slight taper in the adapter which can cause it to tilt slightly when fitted to the prop
so if the prop and adapter are now pushed onto the shaft slightly out of true the adapter will allow the prop to sit slightly off center. 
After being taken on and off a few times it is unlikely that the prop will  sit in the same position twice in a row. 
Also the adapter should only be used as a guide for the prop for centering, the machined bottom of the prop is used for leveling only. 
This must also be checked for pimples etc from the moulding process and should be carefully filed away with no disturbance to the mating surface.
Even after all this work the odds are still against getting a perfect fit that can be repeated.

[attach=1]

This picture graphically shows what static balancing actually does.  It makes sure that the center of gravity (cog) is straight through
the axle.  The problem here is that once fixed to a motor the cog has nothing to do with things anymore.  It is centrifugal force that now takes
center stage. 
(Centrifugal pulls away from the axis where as Centripetal pulls towards the axis)
So if the mass distribution along the props blades is not equal on both sides vibration will occur as the rotational velocity increases.
Think of a conker on a string the longer the string the greater the pull the shorter the string the smaller the pull. 
So even though the conker has the same mass in both examples
it excerpts a different quantifiable force in each case.  This is the same on a prop.
If on one side there is more of the mass closest to the axle and on the other side it is closer
to the tip it will balance OK statically but will cause vibration dynamically. 
(Getting your car tyres balanced is a good example of dynamic balancing).

This imbalance can be quite dramatic when the prop is spun up to 1000 rpm. 
A simple calculation of the centifugal force involved would show that a 250mm (10") prop with just a 1 gram offset at the tip
will produce a centrifugal force of 0.139Kg on the shaft.



The balancing is quite straight forward.  Apply small pieces of tape on the props, spin up and check. 
Keep doing this until it shows 0';s at your hover point. 
This is a time consuming and tedious job but it can be done by careful observation and and small adjustments. 
You can also check on You Tube for some vids.

After each prop has been balanced all four should be replaced and the rx connected as usual. 
Bring the props up to speed and look at the gyro trace.
It is unlikely that you will get 0';s so recompile with a lower LPF until the 0';s return.


You will now have the smoothest quad around

Lastly I now only use Graupner Elektro Props.  Three of the four I bought produced three 0';s straight out of the packet. 
The last one just needed a small amount of tape. 
Also the adapters are machine finished and do a brilliant job.

Any questions?  Feel free to ask


Mike

[sturgm]

One thing I forgot to comment on also is the desire by many for carbon fibre props.
The question is what do people think that they will get from these props.
Well for most it is just less money in their pocket.
All rotary powered vehicles rely on the gyroscopic action of the rotor itself to contribute to stability.
A great example of this effect can be seen with the child';s toy gyro.  Which when spun up by the cord
can sit at 90 deg from the vertical on the top of a bottle, defying gravity and if pushed will ';fight'; back
to keep its position.
If the mass of the rotating part were to be decreased then the rotor was loose some of its effect and
thus would become less stable and produce less ';fight';.
If this knowledge is now applied to props it should be noticed that heavier props induce stability and lighter
one decrease stability.
The only upside is that the quad would become more nimble.  Which would be an advantage to competition pilots
but to your humble quad pilot it would appear as a more twitchy, slightly harder to control machine.

So in short.  If you want a nice stable quad, say for FPV then good quality heavy props are for you.
If you are the boy racer type and like a challenge, then maybe light carbon fibre.

[teslahed]

All rotary powered vehicles rely on the gyroscopic action of the rotor itself to contribute to stability.
A great example of this effect can be seen with the child';s toy gyro.  Which when spun up by the cord
can sit at 90 deg from the vertical on the top of a bottle, defying gravity and if pushed will ';fight'; back
to keep its position.
If the mass of the rotating part were to be decreased then the rotor was loose some of its effect and
thus would become less stable and produce less ';fight';.
If this knowledge is now applied to props it should be noticed that heavier props induce stability and lighter
one decrease stability.
The only upside is that the quad would become more nimble.  Which would be an advantage to competition pilots
but to your humble quad pilot it would appear as a more twitchy, slightly harder to control machine.

I think there may be another factor as well. Multirotors gain some stability from the gyroscopic action of the props but most of it comes from the gyroscopes and accelerometers that the flight controller itself uses to stabalise the multirotor by varying the speed of the motors and props many times per second.

Lighter props means less stability from the gyroscopic action of each prop itself but more responsiveness to the stability that the gyros and accelerometers and other sensors produce via the flight controller.

Stiffness is another factor. Carbon fibre props are much stiffer than the plastic ones i';ve used before, which means that when pulling high G manouvers the prop will deform less and hold it';s shape better. Since it';s shape is what produces lift and stability, you may get better stability with stiffer props, at least when they are under strain. This is particularly noticeable when the props are traveling backwards - i.e during a flip when the quadcopter is temporarily upside down. I presume this is when the props are under most pressure.

I haven';t done much aerial photography work though - mostly fun and stunt flying - which may account for our different experiences.

[sturgm]

I think that we pretty much agree on the principle.  However, the auto Stabilization will somewhat negate the effect being discussed and should not be considered in this context.  That said, I did mention that Carbon fibre, which is stiffer and will not deform as easily would be more suitable for more acrobatic maneuvers, which it would seem you do.  My main experience has come from helicopters with no Stabilization systems and in these cases it is more than obvious what the differences is.  But, with multi prop machines the difference is somewhat more clouded by the use of electronics for stability.

Anyway thanks the input.  Much appreciated.

[teslahed]

the auto Stabilization will somewhat negate the effect being discussed and should not be considered in this context

In terms of ';auto-stabalisation'; I';m including gryo only (acro flight mode) as well as self leveling mode. Both produce stability by varying the speed and thrust of the props -which will be enhanced by using lighter props that change speed more rapidly. 

With regular helicopters i believe most of the stabalisation does come from the weight and gyroscopic action of the props - so i would definitely expect less stability with lighter props. I haven';t flown them myself though.

With quadcopters i would be interested to know the relative strength of these two opposing effects. It';s quite possible that in some circumstances heavier props would produce better stability through the gyroscopic action of the props themselves and in other circumstances lighter props would be the better option.

If i had to guess i would say that lighter props would produce more stability in most situations with multirotors. I think multirotors get a lot more stability from the action of the sensors and flight controller on the motor speeds than they do from the gyroscopic action of individual props. I wonder if anyone has tried flying a quadcopter with really heavy gyroscopically stabalised propellers and no electronic stabalisation (just thrust mixing based on control inputs). I doubt it would fly.

Interesting discussion though

[sturgm]

Have a look here
http://en.wikipedia.org/wiki/Quadrotor

Mike

[teslahed]

Have a look here
http://en.wikipedia.org/wiki/Quadrotor

Interesting stuff. I presume you were directing me towards;

However, early prototypes suffered from poor performance,[1] and latter prototypes required too much pilot work load, due to poor stability augmentation[2] and limited control authority.

Which suggests i am wrong, you can fly a quadcopter without electronic stabalisation, only it';s not worth the effort (stability is so poor).

Although that does kind of suggest that the electronic stabalisation is a very important factor - meaning lighter props should make a positive difference to stability in most situations.

[sturgm]

Hi,
Not really interested in the right/wrong issue .  As you said earlier ';interesting discussion';
I really thought the prop balancing issue was of greater import and a bit controversial.  There you go

Once again thanks for the interest

Mike

[teslahed]

I really thought the prop balancing issue was of greater import and a bit controversial.  There you go

The prop balancing info was very worthwhile, thanks. I haven';t got anything to add there and it sounds like you have a lot more experience than me.

Hi, Not really interested in the right/wrong issue .  As you said earlier ';interesting discussion';

I';m only interested because you raised an interesting point If anyone else has anything to add regarding stability issues and lighter / heavier props i would be keen to read about it as i am still not certain which factor is most important in what situations.

[sturgm]

Having had a bit more time to think about your requirements have a look here

http://www.rcgroups.com/forums/showthread.php?t=1594904
and here
http://www.rctoys.com/pr/2012/05/07/rc-propeller-thrust-curve-and-efficiency-test-data-251-unique-propellers-tested-including-quad-copter-counter-rotating-propellers/

I discovered these some time ago while researching quads

This last one was interesting but somewhat dated and they missed the point of dynamic balancing.  You may fine some of the maths a bit heavy but you should still be able to glean a few gems of interest

http://www.wpi.edu/Pubs/E-project/Available/E-project-042308-215545/unrestricted/Report_Main.pdf

Lastly.  It is possible to prove empirically the stability issue once a frame of reference has been established in a steady state system but this is more of an academic proof and as such has little impact in the real world of quads.
My angle on this has been more on the marketing of Carbon fibre blades to your average park flyer or novice.  Also not all carbon fibre blades are the same.  Some of the budget end types are more resin than fibre!
Happy flying