I just wanted to make a post to specifically address the issue of vibrations on helicopters, as I am seeing quite a few logs which exhibit excessive vibrations. This not only affects Loiter, but can also affect Stabilize as the AHRS system loses track of which way is “down”. This can result in you actually having to fight the machine to keep it level.
Now, I comment so often on this, I begin to wonder if people are starting to think “he just says that about everything”. So I want to provide an example of what I consider low vibration. The attached image is from my Trex 500. There’s really not a lot special about the machine. It’s just a good quality, solid build, with attention to detail and balancing. It started out as an HK500FBL TT, but now is almost entirely Align, as well as some other upgrades, but nothing fancy. The vibration damping system is… simply a Pixhawk, mounted sideways on the frame using the 5mm thick black foamy double-sided pads that 3DR supply with the Pixhawk. That’s it. I find this 3M mounting tape to be extremely effective, and dead-easy to use.
I’ve started using it in place of my other elaborate systems, since it pretty much just works.
I’m actually amazed when I look at this graph. It took me a long time to achieve something like this on my other helis. This one I got it right pretty early, using the same principles I learned on the others.
[attachment=1]GoodVibrationsHeli.png[/attachment]
Compare that to an example from a machine with high vibration:
[attachment=0]BadVibrationsHeli.png[/attachment]
These are the signals coming from the accelerometers on the APM or Pixhawk. As you can see, my log provides a clean signal which the AHRS can use to keep track of which way is “down”. There is a bit of vibration (the small lumpiness) but you can clearly see the motions of the airframe (the larger waves). I was flying aggressively in this log which is why you see big motions.
When comparing to the example with high vibration, it’s pretty hard to see where the airframe motions are because they are lost in the “noise” from the vibration. The system tries it’s best to extract a signal from this, but it’s not great.
Now, this is not to say that everybody needs to achieve what I have. But the X and Y axis need to be lower than +/-2, and the Z-axis should be lower than 5-15. In the example above, the Z axis is actually not bad at all.
So how do you reduce vibrations?
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Make sure all shafts are straight. Check them any time blades touch the ground. All “clone” shafts come pre-bent.
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Make sure all bearings run smooth. Feeling them by hand, they should be… smooth. If they are notchy, they cause vibration. Most clone bearings, and even some genuine Align bearings are notchy right from the package. Again, these need to be checked anytime blades touch the ground.
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I find 3 main-shaft bearing supports help a lot. Most machines don’t come like that anymore, I retrofit them into everything. All deep-groove radial bearings have radial free-play. Even the best quality ABEC-7 bearings do, they have to for assembly purposes. Therefore it’s impossible to have a stable main shaft that is only supported by 2 bearings. 3-bearing systems are usually “over-constrained”, meaning the shaft will be snug going in. This eliminates the radial free-play.
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Blades must be statically balanced in a balancer. Check them! Many blades do not come balanced. I also do a dynamic balance. What this means is that I use tape to make sure that the blades not only balance on a balancer, but are also exactly the same weight measured on a scale with 0.01 Gram accuracy. It happens often that the heavy blade goes up on a balancer, so you add weight to it. Now it’s even heavier. So it balances on a balancer, but soon as you spin it up, there’s an imbalance because it’s CG must be further inboard than the light blade.
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Blades must be tracked. I exclusively use turnbuckles for the pitch links, with Left-hand threads on one end, and Right-Hand on the other. Heli Option is a nice source for these. They allow infinitely precise adjustments.
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Main gears, make sure they run straight and true and don’t wobble excessively. Most clone gears come pre-bent.
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Make sure the frame is stiff. Obviously, make sure all your screws are good and tight. Use Loctite. But more than that, I find most frame designs lacking in stiffness. I try to use after-market frame sides that are thicker than normal. I also try to replace all the plastic pieces with CNC cut aluminum. So, the frame bottom, battery tray, bearing blocks, motor mount… everything, even the tail boom mount if I can find one. All of this increases the weight of the heli. But they vibrate so much less, it’s well worth the effort in a UAV application, particularly if trying to do any imaging.
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Head setup. The head should be well set up, the pitch links have to move freely. But with minimal slop. Any stickiness in the head will result in vibrations.
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Tail system. Again, this all needs to be well set up, good bearings, shafts straight, blades balanced. Tail drivetrains are particularly problematic. I always use at least two Torque-tube bearing supports. If the kit only uses 1, add another. Make sure the tail rotor shaft is straight!
I used to use a lot of clone kits, but it’s just gotten to the point where I spent more time trying to solve problems with them that I don’t bother anymore. You constantly have to check parts for straightness, binding, manufacturing mistakes, etc. I do use a lot of upgrade parts, such as from KDE, Heli Option, and Tarot. Some clones such as Tarot and ALZRC might be OK. But any no-name kits, and most of the stuff from Hobby King are just not acceptable quality.