Nothing detracts from a beautiful FPV video quite like jello. No, not that kind. I'm talking about vibration-induced screen wobble on GoPro and other CMOS cameras, caused my the camera's rolling shutter. Dedicated AV multirotor and heli pilots often go to great lengths to isolate their GoPro cameras from the airframe, but on small and medium-sized foam fixed-wing platforms or a small fpv multirotor, where space is often at a premium, there's not a lot of room to come up with a camera isolating system that works well; most HD cameras on FPV planes are just attached to the airframe with rubber bands or hook and loop tape and that's it. Sometimes we can get away with not balancing our props; the soft foam can assist in dampening the vibrations. But on most fixed-wing airframes and especially multirotors, the airframe material is rigid enough to transmit the vibrations straight to the camera.
There's a few steps we can take to substantially reduce vibrations on our rigs. The first step is balancing. Yes, we all balance our props (you're at least doing this, right?), but how well do you actually balance them? Are you taking into account the balance of the hub as well as the blades? What about blade tracking? Just like a heli, if the blades don't pass through the same plane (i.e. one blade is higher or lower than the other when spinning), then substantial vibrations will occur at most rpm's. Are you taking the time to balance your brushless outrunner? An out-of-balance motor-can will output some noticeable vibrations that only adds to the problem.
Ideally, you'll have a decent blade balancer on hand, such as the Dubro Tru-Spin. With the prop on the balancer, hold it horizontally and see which blade drops. You can either remove material from that blade with a fine sandpaper, or use a very thin tape and add it to the lighter blade until they balance. Now, turn the prop so its vertical and see which way it falls. Chances are, the hub will be out of balance and the high blade will fall about 90-degrees either way. Note the direction; if it falls to the right, then the hub is heavy on that side. Just like balancing the blades, we can either remove material from the heavy side of the hub or add material to the light side.
My personal preferred method for balancing is to add thin tape to the light blade for quick and effective blade balancing. The tape is thin enough that it doesn't upset the airflow over the blade. For the hub, I prefer to sand the light side with a drum sander attachment on my rotary tool. Yes, technically removing material from the hub could weaken it. But sanding down the side of the hub a few millimeters isn't going to cause it to self destruct (unless it's a Funjet on 6s!). If you aren't into removing material from the hub, try building up CA glue on the light side. Add a few drops, then spray with accelerator and repeat until you've amassed a little bump of hardened CA; if the prop hub isn't too far out of balance, this method will work well.
As I stated in the opening paragraph, blade tracking on a multirotor or airplane is just as important as on a helicopter. If the blades don't occupy the same arc as they spin (i.e. one blade is higher or lower than the other), you're going to introduce some serious vibrations. Place the prop on a balancer and watch the blades as they spin; there should be no deviation in the arc path. If there is a difference in the arc, it's likely because the mounting surface of hub isn't true. Plastic and composite props are more susceptible to this than wood props, because there is sometimes a plastic flashing, or lip, left on the bottom of the hub after the injection process. To remedy this, I use a Great Planes sanding bar and gently run the hub across the bar to flatten out any raised spots. I've successfully trued many hubs using this method, and the reduction in vibrations is substantial.
An out of balance brushless-motor can contribute to a wavy GoPro video quite noticeably. Unfortunately, it's not easy to put the motor can on a balancer, so we use a different method that's very effective. If you have a "smart phone" such as an Android or iPhone, download a free app called Seismograph. This app is a very sensitive vibration detection program that takes advantage of the phones internal accelerometers. It's easiest if you don't have the motor mounted on the aircraft; just bolt it to a piece of lite ply, position the phone next to the motor and run the app. With the motor hooked up and powered, advance the throttle and see how much the seismograph registers the vibes. Now, experiment with a small square of tape and stick it to the motor can and observe the vibrations. Did they increase or decrease? It takes some time to find the heavy side of the can, but once you do, add additional pieces of tape until the vibes subside. You can also find the heavy side quickly by using a zip tie, then switching over to tape. Heres a youtube video that explains the process:
|HOW TO BALANCE A BRUSHLESS MOTOR WITH YOUR IPHONE / ITOUCH (7 min 46 sec)|
|How to use an iPhone app to balance your motors (1 min 46 sec)|
The settings you choose on the GoPro play a huge part in the amount of noticeable vibrations you see. High frame rates will reduce the shaking considerably. That means shooting in 720p 60fps rather than 1080p 30fps on the GoPro2, and either 1080p or 720p at 60fps on the GoPro3. Wide angle views tend to mask the shaking a bit as well So experiment if you have the option between wide, medium, and narrow field of view. Additionally, higher shudder speeds (not to be confused with frame rate) will pronounce the jello effect. The brighter the light coming in, the higher the shutter speed and the more vibes are seen. Neutral-density (ND) filters are available that drop the f-stop by almost 2 stops, and really help in removing the wobbly effect of the CMOS sensor in bright light by dropping the shutter speed. Heres a link to some ND filters available for the GoPro
There's a number of ways to get the shake out of your GoPro video. Dedicated aerial-video rigs can pad and isolate the camera quite well, but small fpv ships usually don't have that luxury as we Velcro and rubber band our cameras to rigid pan/tilt units, or cut out foam noses. But taking the time to balance the prop, true the hub, balance the motor, and run the GoPro at the best settings can eliminate vibrations and the dreaded jello effect once and for all.Last edited by Matt Gunn; Jul 10, 2013 at 02:58 PM..
|Jul 11, 2013, 07:02 AM|
Sweet article here Dr. The smartphone app for the motor balancing is cool as heck! I never knew a motor could be that out of balance. One thing I like to do is balance the motor with the motor mount on to detect any wobble in the mount too.
|Jul 11, 2013, 07:52 AM|
thanks. And if you really have some time and patience, you can dynamically balance the motor and prop together, then mark their position on the hub and motor so you always put them back on at the same spot.
|Jul 11, 2013, 01:12 PM|
Joined Oct 2012
I strapped my phone into the wing rubberbands. ran it that way.
Oddly, try as I may, I could not stumble upon any application of tape that improved what I had from just a simple prop balancer off the motor.
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