Noise In Your Quad? Hot Motors?

I was out flying with a buddy of mine and he was flying a different type of flight controller.  He was playing with some of the built in settings native to that system.  On one setting his motors came down after a quick hoover really hot.  Not quite smoke your motors hot but hot enough that had it been more than a quick hoover and fly by, it could have been bad.
 I had an issue with one of my builds giving me undesireable flight carachtoristics.  My yaw was really twitchy, and unpredictable.  Sometimes I would get random pitch and roll introduced while I was flying.  While it wasn't substantial, it did effect my flight where I didn't feel confident flying.   Also, my motors were coming back hot.

What's going on?

My buddy's quad:

He swapped some settings and his motors came back nice and cool.

My thoughts are he needed additional filtering for his quad.

My quad:

Turns out I had a bundle of wire bouncing on my gyro, giving it bad information.

Why does filters on one quad fix an issue and rerouting wires on another fix the problems?

Your flight controller picks up what we call 'Noise.'  Noise isn't something you actually hear, but vibrations your gyro picks up.  There are some examples I can provide that illustrate what I'm talking about:

You crash, hit a gate, limb, or even look at your prop wrong and it gets bent and you fly back and there's a weird buzzing sound radiating across your ears.  It sounds horrible.  That is audible noise, but a lot of noise your flight controller picks up is not in the range of human hearing.

When your introducing noise into a system, it increases the likely hood of error from all sorts of things, causing interesting things like motor timing errors for example.  There's more to it than just that, but lets keep things simple.

When your telling your quad to move, your introducing noise to your flight controller as well.  However, that's a desired noise.  So, the firmware has to decide, 'is this good or bad noise' and then what to do to correct it.  In my experience the flight controller uses your motors as the whipping boy for controlling noise.  Hence why, I think, most pilots talk about their motors getting hot all the time.

Minimizing Sources of Vibration:

Lets look at sources of noise we can control BEFORE we even leave the work bench.  We've established that all vibration is noise.  So think about your build and what might cause vibration. 

Some things I've noticed in my builds that have introduced unwanted noises to my flight controller's gyro:
1. Wires sandwiched between my FC and my ESC
2. Wires not secured to the frame in anyway (usually because my tape separated from my frame and wires in flight.
3. Zip ties that aren't clipped neatly
4. Zip ties that shifted off the item they are securing.
5. Wires that are just hanging out.

What I did to correct those things:
1. Pulled the wires from under my FC and then secured them in some way.
2. Have better taping technique so my wires quit bouncing on the frame of my quad
3. Clip my zip tie
4. Find that 'sweet spot' on something I'm using a zip tie to secure
5. Keep my wires from hanging out, by braiding, twisting or taping them so that they don't move as freely.

All of this sounds simple and it is.  However, the potential of having a poorly tuned quad or worse a quad that smokes motors because these simple things can get over looked is a reality.

Additional note on minimizing gyro noise: soft mounting your flight controller will help, from what I understand there is mixed opinion on soft mounting motors in current generation of flight controllers.  I personally do not, I think they provide any benefit over hard mounting. Also, (this maybe psudo-science as I can't verify any of it) I think the dampened motors may not respond as quickly and actually flex on the soft mount.  

In Your Flight Controller Settings:
There are several filter settings that will keep your motors running cool.  Using unfiltered gyro data, in my opinion is the easiest way to determine what and how much filtering you need.  I have done this with Betaflight, however other flight controller options I'm unsure the best way to gather that information.

In Betaflight if you have blackbox logging, use this command to get unfiltered data:  
set debug_mode = gyro_scaled 

To use blackbox information, you need to use a black box explorer and then determine what filter ranges your quad needs.  It is (in my opinion) the easiest way to determine what filters you need.  

However, be aware less filtering can lead to hotter motors.  This applies to ALL types of flight controller software.  Keep in mind, that the less filtering you have will make your quad more sensitive to bent, chipped and anything else you can think of that will happen to your props.  Finding that balance on your quad can be a real battle.

Pros to running the least amount of filtering:
Your quad will be really locked on.
Your quad will feel dynamic

Pros to running more filters:
You will have cooler motors and potentially longer lasting motors
You will be able to fly with damaged props without having to worry as much about props hurting your motors

I run default Betaflight filters, they seem to do well and handle a bent and the occasional broken prop without causing me any excessive worries.  Only with a broken or a bent prop bent over 30 degrees or greater at the hubs have they ever come back to me warm.

I will be experimenting with reducing filters in a controlled environment and will fly with a variety of damaged props and record the outcome as best I can.

Until then, enjoy.
 




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