From Multirotor Pilot Issue: 8
by Peter Hejl
It has probably happened to all of us; coming home excited after a great flight, just to discover that the footage we brought back is nice, but full of vibration-induced “Jell-o” effect. The Jell-o effect is caused by the rolling shutter used in the CMOS camera sensors, where the photodiodes (pixels) do not collect light at the same time. All pixels in one row of the image sensor collect the light at the same time, but the collection time for each following row is slightly delayed. If the vibrations move the camera slightly between the times the light is collected for different rows, it will show up as a distortion (or Jell-o) in the video. Unlike the rolling shutter sensors, the global shutter sensors (CCD) start and end the light collection for all the pixels at the same time. On those cameras, the vibrations may not present themselves as a Jell-o, but rather “shakes”, or a slight movement of the whole image, mostly noticeable around the edges or on objects in distance (horizon).
Ruined footage is not the only concern with propeller induced vibrations. More importantly, it can compromise the IMU (inertia measurement unit) sensors to the point where the flight controller can lose its level reference and abruptly tilt, flip, or fly away. Some early DJI S-800 users may remember the “flip of death” issue, when the copter abruptly flipped upside down and obliterated itself. The manufacturer has traced this to the vibration affecting the Wookong IMU, and rectified this by stiffening the frame and introducing more robust IMU. Vibration can also loosen the frame screws, causing the frame to lose integrity. All these are really good reasons to try and eliminate as much of the propeller induced vibrations as possible.
Virtually every (even the high end manufacturers’) propeller comes with slight imperfections in its balance. Considering we can have four 12-inch fast spinning props on a multirotor frame, that is a lot of vibration for the copter to deal with. While the Jell-o effect in the footage can be helped by using the right dampening mount and an ND filter (the neutral density filter tricks the camera into lowering the exposure, which causes the GoPro and similar action cams with fixed aperture to lower their shutter speed), the best way of dealing with vibrations is to make sure that our propellers are well balanced.
As an RC pilot, I have been through a few different balancers before I sett led on one that I think does the best job getting the props perfectly balanced. Illinois-based Du-Bro company’s Tru-Spin prop balancer has been a part of my toolkit for a while now, and it has helped me tremendously with any vibration issues on my multirotors. It sports virtually friction-free aluminum wheels that hold a precision hardened balancing shaft . The prop is held on the shaft by two aluminum cones, one of which is pushed onto the prop center with a spring. Offering a few different configuration options, this balancer can accommodate any classic props used on the multirotors.
With the recent addition of special balancing shaft s with 5, 6, and 8mm thread, it is now possible to use it to balance the threaded (screw-on) props used on DJI Phantom, early Inspire, Yuneec Q500, etc. Du-Bro is currently working on a shaft that would accommodate the latest DJI Inspire “push&lock” props.
HOW TO USE
The balancer assembles easily in under five minutes (instructions are included), and it’s simple to use.
Choose a balancing shaft depending on your prop type and attach the prop to it either by using the included aluminum cones, or one of the threaded rods for threaded props (be sure to choose the correct side – one side has clockwise thread, the other is counter-clockwise). Place the shaft in between the balancer’s wheels and make sure that the shaft is level. Then move the prop around and try stopping it in different positions. A well balanced prop will stay in any position that you leave it in. Unbalanced props will tend to turn and eventually stop with their heaviest part facing down. Keep in mind that you’re looking to balance both the blades and the hub, and in some situations the prop stops only at horizontal, but tries to come back to that from any other position. This means that the part of the hub facing down is heavier, and the hub will also need to be balanced. Once you determine the heavier part of the prop, there are a few ways to adjust the prop balance, depending on personal preference. You can either use sand paper to sand down some material from the bottom of the heavier blade or the heavier side of the hub. Sand only in small increments and re-check the balance oft en. As an alternative, you can add small squares of tape to the lighter blade, or add a little epoxy or hot glue to the lighter side of the prop hub. I prefer using the sandpaper, as the tape may come off without me noticing it and reverse any of my balancing efforts. Be careful when sanding carbon fiber props as inhaling the fine CF dust is not the healthiest thing to do. Take your time and make sure that every prop stops in any position on the balancer before moving to the next one. Once all the props are balanced, mount them back and you’re done.
BALANCING THE MOTORS
Balancing the props usually does a sufficient job in helping mitigate the vibrations, but if you’d like to take it one step further, there is also a way to balance the motors.
Balancing the motors is a little bit more complex process and is not always necessary, but it does help.
You will need a smartphone app that allows you to sense and record vibration (i.e. the free DroneVibes app in iTunes app store), attach your phone to the arm either using a Drone Vibes mount or by simply taping it to the arm. Make sure all the other props have been removed or the motors disconnected and spool up the motor without the prop attached. To make describing the process easier, please, look down at the motor, and visualize it as a clock dial. Take a reading of the vibration, then stick a small square of tape to the side of the motor (this will be your 12 o’clock position on the motor, looking from above), and spool it up again. If the vibration is higher, move the piece of tape to another position (3 o’clock position on the motor, looking from above) and try again. If the vibration is still not lower than without the tape, move to 6 and 9 o’clock positions until you find one that gives you the lowest amount of vibrations. Once you do, experiment with the amount of tape in the same position to find the one that again gives you the least vibrations. When this is done, add a small piece of tape 45 degrees away from the first piece of tape – if the first piece is at 12 or 6 o’clock, then add another piece at 3 or 9 o’clock, and spin it again to see if the vibrations have changed. If they’re higher, place the tape to the opposite position. If lower, experiment with the amount of tape in that position until you find a point with least vibration.
THE FINAL WORD
Properly balanced propellers help greatly to mitigate vibration related issues in the multi-rotor frames that may lead to electronic and mechanical failures, or render your footage useless by vibration induced Jell-o effect. A small investment in a proper balancer and taking a little time to balance your props may save you a lot of money and disappointment later