Propeller and Motor Balancing

One of the biggest hindrances to the success of any multi-rotor system is unwanted vibration.  Vibration interferes with the delicate sensors on your controller board and can lead to unpredictable and possibly dangerous behavior from your multi-rotor.  In addition it makes for bad video and photos; this is especially the case when using cameras such as the gopro or any other camera which uses a CMOS sensor.  CMOS sensors write image data one pixel at a time starting from the top left of the sensor and continuing across in rows until the entire image is captured.  This is done on average about 30 times per second and hence is not visible to the naked eye.  Under standard conditions CMOS sensors can achieve beautiful and fluid video and photos.  But when vibration is introduced to the cameras it leads to a “Jello” effect on the video that makes the video unusable.

Below is an example of “Jello” video.

Vibrations also cause unnecessary wear and tear on your equipment and can loosen components.  So it is very important to minimize the causes and effects of vibration on your multi-rotor.

There are a couple factors that lead to excessive vibration and they are mostly motor and propeller related.  The first of these factors we will be looking into is motor vibration.  Most beginners in multi-rotor assume that the motors that they buy from their local retailer come pre-balanced.  I have in my experience found that more often than not that is not the case.  This has even been true with motors that are advertised as balanced and were more expensive than similar models.  So our first step in removing vibration from our multi-rotor will be balancing our motors.

The most efficient way I have found to do this is with the motors already mounted on the multi-rotor frame with the ESCs connected either to a controller board or to a servo tester that you can use to control the motors speed.  You will also need some way to detect and visualize the vibrations.  This is where your smart phone comes in handy.  Because most modern cell phones have three access accelerometers built in to them there are free software solutions that will help you quantify the vibrations much like a seismograph.  I have an android base phone and I use Seismos which is available in the Google Play store.  If you have an Iphone there are many solutions available to you as well.

Step 1 The Rig

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As you can see, we have assembled all of the aforementioned components into a rig that makes it easy to run the motor at varying speeds and determine how much vibration is being produced.

The first step is to use the servo controller to slowly run the motor up to full speed while running Seismos on your smartphone.  You will notice that there is not a direct linear relationship between the speed of the motor and the vibration produced.  In fact it varies by RPM and every motor will be different.  What is important is running the motor up and down several times until you have a good idea, on average, of how much vibration are being produced by watching Seismos and keeping an idea in your head.

Step 2

Look at the motor from the top down and create a cross section in your mind.  I usually split the motor into quadrants.

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The next steps are trial and error.  You need to cut a small square of electrical tape about the height of the bell of the motor.  Then choose one of your quadrants and apply the tape to that side.  In the photo above that would be where one of the lines intersects the motor bell.  Also as shown below.

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Once you have your tape applied, run the motor up to speed again and see if the vibration has decreased.  If it has not, move the tape to another quadrant and test it again.  Continue in this manner until you have found a location for the tape that decreases the vibrations the most.  When you find the best spot, you have basically found a place on the motor bell that is lighter than the rest.  You might also want to try adding another piece of tape on top of the one you already have applied just to see if the added weight decreases vibration even further.

Once you have done this for all of your motors you are ready to move on to the next task which is balancing your propellers.  There are different sorts of do-it-yourself prop balancing methods out there but to get the best results you need a proper propeller balancing tool such as the Top Flight Propeller Balancer.

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This type of propeller balancer gives you the best results.  This is because the shaft that you mount the propeller to is suspended between two magnets.  This allows the propeller to move back and forth with almost no friction.

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Once you have placed the propeller in the balancer, wait until the propeller has settled and stopped moving.  If the propeller is vertical that means that the blade that is at the bottom is heavier than the blade at the top.  In order to fix this we need to either remove material from the bottom blade or add material to the top.  I use the subtractive method and will remove the propeller from the balancer and lightly sand the back side of the bottom blade.

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I will then place the propeller back in the balancer and I will continue this process until the propeller is balanced.  You will know your propeller is balanced when no matter what position you place the propeller in the balancer the propeller will stay in that position and not move.

Very often it is possible to get the blades balanced but the propeller will still not be.  This is because the hub of the prop may also be unbalanced.  If your propeller will rest in the horizontal position parallel to the table surface then your blades are most likely balanced.  To determine which side of the hub is heavier you can place the propeller vertically in the balancer and then let it go.  Generally which ever side of the hub comes to rest at the bottom is the hub that needs some material removed.

This is a process which can be quite tedious and requires patience and practice to get good at it.  It is, however, of paramount importance as you build and fly your multi-rotor – this cannot be overstated.  Spend a lot of time balancing your motors and props.

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