From Multirotor Pilot Issue: 4
by Joe Cannavo
Photos Joe Papa
One of the most commonly overlooked challenges in the use of multirotors is vibration. I am not talking about large amounts of vibration that are obvious even to the layman, such as when a prop is severely out of balance. I hope that you would naturally identify and rectify prop vibration as part of your normal preflight checks. What I am talking about I consider minor vibrations. Most, if not all, multirotors have some level of vibration that many don’t pay attention to; however, it is important to remember that vibration, no matter how small, can be bad for your multicopter.
WHY IS VIBRATION BAD?
Vibration is not good on any RC model and the more mechanical an aircraft is, the worse it can be. Multirotors have a lot of machined parts as well as a lot of screws and nuts and even though you should be using thread lock, serious or long-term vibrations can cause these components to loosen over time and ultimately fail.
Vibration will also affect your video quality. Even though many gimbals on the market today are designed to stabilize video for the best possible quality, small vibrations can cause harmonic noise in the video, which we oft en refer to as “Jello”. This will not be noticeable from an FPV monitor, but once you look at the HD video in postproduction, you will see that even a small vibration can render your footage virtually useless.
One thing that vibration can affect with catastrophic results is the gyros on your flight controller. Prolonged vibration can cause the flight controller to respond erratically and in some very bad cases, it can flip the copter upsidedown or cause it to bank uncontrollably. If you are not prepared or the copter is in a bad location, you can even crash your machine, potentially taking a lot of expensive camera equipment with it.
WHAT DO I CHECK ON MY MACHINE?
The first and most obvious areas to check are the props. You should always balance your props using a device like the Du-Bro Prop Balancer. Balance your props so they remain perfectly level on the balancer and then add weight to either side of the prop hub so the prop will also remain vertical. You want them to maintain any position without a heavier end falling. To accomplish this, I usually apply thin CA glue to the lighter blade in small layers until I get it level.
The other thing you need to balance, and most people don’t, is your motor. Yes, even those highly machined precision motors from T-Motor and other popular brands can have a very slight amount of vibration in them. Again, I am not talking about the obvious vibration you can easily feel with your fingers. In fact, if you have so much vibration from just a motor that you can feel it, then I suggest finding a different motor.
To check the vibration and dynamically balance a motor, I use an app for my iPhone called Vibration Tester by Dmitry Eliuseev, which can be found in the App Store for $1.99. The accelerometers in the iPhone are so sensitive that with the use of this app and a special mounting clamp that I got from Aerial Drone Worx, I can now measure and minimize the vibration from each arm of my copter.
Aerial Drone Worx has a variety of clips available for the iPhone series of smartphones and Android models are coming soon. They have them for 15mm, 20mm, 25mm and 30mm booms. All you do is clip your iPhone into the mount and then snap it on the frame. I understand that Aerial Drone Worx will have their own vibration app designed specifi cally for multirotor applications by the time you read this. Visit their site at www.AerialDroneWrox.com/ vibration
To get the best results you need to work with one motor at a time, so disconnect all the ESCs from your flight controller except for the one you are testing. In fact, it is not even necessary to use the flight controller. You can simply connect the ESC for the motor you are working on to the throttle channel of your receiver. The first tests are with the props removed. Advance the throttle to full and measure the amount of vibration and write that number down. The next step is to cut a small piece of tape (I use clear packing tape) about a half-inch square and place it on the side of the motor’s can. Then spool up again and monitor the vibration number. Did it go up or down? If it went up then move the tape to another position on the motor. I usually work in 90-degree increments or if you are looking at a clock, 12, 3, 6 and 9 o’clock positions. Each time, monitor the vibration until you find a spot where the number decreases. Once the number decreases, add another piece of tape to the first piece and try again. The object is to get the vibration number as low as possible. It is important to note that you will probably never get it perfect. But in many cases, I am able to reduce the amount of vibration by over 50 percent by doing this. Repeat this process for each arm/motor.
The next step is to attach the props and perform the tests again. Please note that when you attach a prop, even to just one arm, you have to be very careful. On many copters that one prop alone can be enough to flip the copter over. For this step, I strongly suggest that you use two people and be very careful. Since your props are already balanced, you would not expect a lot of additional vibration. However, we are talking about very minor vibrations here and even something as small as a prop washer or slightly elongated hole on the prop hub can make a difference.
Again using a small piece of tape, I place it on the fatt est part of the leading edge of the prop and again monitor the vibration. If it goes up then place the tape on the other blade in the same position. If it goes down then try adding another piece. I will typically try three different positions on each side of the blade; six total per prop, and monitor the results. In some cases, the best result is with no tape while others are better with.
DJI S800, S900 and S1000 PROPS
DJI uses two-piece foldable props with their copters and they are a lot more difficult to balance on their own. I will typically use a see-saw heli blade balancer to make sure they are both the same weight, but that does not mean that they are balanced along their span. The above method is the only way to really balance each prop set. We just got a DJI S900 for review and I was blown away at the amount of vibration that we were getting at each motor. Measurements from the Vibration Tester app were upwards of 30 on each arm. Aft er balancing the motors and then adding tape to the props, I was able to get the vibration down to about 11; almost one-third of what it was stock out of the box.
THE FINAL WORD
I get real peace of mind from knowing all my copters are set up for the minimum possible vibration. I know that I am not going to have any problems with video Jello, my hardware is going to remain intact (even though I still preflight check my machines every time I fly) and most of all, I am setting up the best possible environment for my flight controller with the least amount of vibration. I am not going to lie; this is not a quick process. The S900 took over an hour to get the vibration to an absolute minimum. But trust me when I tell you that the time invested in the beginning with a new copter will save you countless hours and dollars in the future.