[Tricopter] weird torque issue

[ayomeer]

Hello Flitetest community

After a minor crash with my tricopter it seems to generate way less CW torque than the FC expects, making the tri spin CCW. To compensate, I have to bring the tail rotor in a position where it's sligtly tilted to the left rather than being tilted significantly to the right. I have checked the obvious things like:

-Motor mounts being level
-Motor spin direction
-Props being mounted propperly
-The front booms extending to equal length

Seems to me that the cause of such a drastic discrepancy should be fairly obvious but everything looks to be perfectly normal. Has anyone here expirienced something similar?

 

[andybenton]

Is your fc still firmly attached and square?

 

[SkinnyDave]

Not sure what FC you are using, but on both of my tri build/re-builds when it spun in any direction it was because my radio/yaw input were being reversed. Double reversed actually (if that makes sense). With the tail pointing towards you the prop should tilt the opposite way your yaw stick goes. If it does that, make sure your radio is registering correctly (if you can...depends on FC) in the FC.

At first I had my FC registering my yaw input correctly, but prop tilting the wrong way...so I simply reversed my radio thinking that was all it needed (since the prop moved in the correct direction)....but then it would still spin in some sort of ninja-lawn-mower-from-hell fashion. I had to reverse the servo response in the FC, then on my radio as well....(I know, sounds confusing) to get the yaw input to report correctly on the FC AND have the prop tilt the correct way.

 

[ayomeer]

I didn't have to rebuild anything. The crash was just me clipping the ground with one of the props, chipping them. I have not undone any connections or changed anything on the radio, so the data handling or whatever you want to call it "should" still be the same. However I will check that. Thank you for the input. I'll also upload a video demonstrating it's behavior when I get home.

I am using a KK2, the receiver test shows the expected values. Also to clarify: the tricopter does respond correctly to yaw input and is very controllable, which is why I think there's something off about the geometry. I just have to hold the stick to the left (flying with inverted yaw, don't ask me why) to compensate.

 

[stay-fun]

Check your trims, and do a recalibration of the gyros. Also, check if any motor is crooked, as in, do they all generate thrust directly downward, or under an angle?

 

[andybenton]

If your yaw is inverted it could be the problem...

 

[ayomeer]

Check your trims, and do a recalibration of the gyros. Also, check if any motor is crooked, as in, do they all generate thrust directly downward, or under an angle?

Checked all of those things. Appreciate you throwing ideas at me though

 

[Craftydan]

hmmm . . . a tri that appears to be defying physics . . . that must have been a heck of a crash

are all three props the same or is one counter-rotating?

The bizzare part of all this, is it hardly matters -- you can't be loosing thrust/generating less torque from one of the opposing motors -- you wouldn't be able to keep that boom level wiht t he other two. You have to be vectoring thrust on the other booms to need to change your adjustable thrust vector from left to right.

I know you say the motors are level (level to what?). . . but are any of the booms warped? are any cracked? a cracked boom might not be obvious and can warp under load, then twist back to level once back on the ground.

 

[ayomeer]

I'm about to leave work, going to make pictures and videos to give you guys a chance to understand the situation very soon!

 

[cranialrectosis]

Welcome to FliteTest!

Video will help a ton.

Is the prop you clipped on a CW motor?

 

[ayomeer]

Alright, I got some media to help me out here. First of all the video showing the tri's behavior in flight:

And here's me lining up the little gap in the motor with the top plate of the frame body to make them (approximately) level to each other and a straight shot from the front for good measure (sorry for crappy quality):



Of course this is by no means perfect but this technique has worked well for me in the year I've been flying multirotors and the small offset from perfection should be well within what the FC can correct. I also measured the distance from the ground to the rotor tips and put a water level on the motor mounts. The results of all of those things where ok.

Thank you for taking time out of your day to help me! I'm sure once I [stumble upon]/[get told] the source of the problem I will be calling myself an idiot

 

[ayomeer]

Is the prop you clipped on a CW motor?

Yes the two CW props met the ground. Are you on to something?

 

[cranialrectosis]

Make sure this is not a subtrim or trim. Before you look at the hardware, check these items.

Set all trims on the transmitter to center. In the receiver test on the KK2, ensure that your values are all 0 using subtrims.

Re-calibrate ACC.

Re-Synch the ESCs and ensure that all three motors start at the same time.

Re-test flight. If these didn't fix it, look for hardware glitches.

Do either of the front motors get hotter than the tail motor or when you shut down, does one motor stop much faster than the other two?

If not, check the tail servo linkages. I release these and re-center the servo and then re-connect these linkages after a crash. Horizontal shifting on the tail linkages can create stresses that push the tail motor left or right and make insidious yaw issues. Any way you slice it with a tricopter you have two spinning one way and one spinning the other. The balance will be made up by a touch of yaw on the tail motor. Shifting these linkages may cause that balance to shift as well. Re-centering allows you to run with 0 trim during flight.

If so, check to see that you don't have a motor full of sand and grit causing your motor to spin slower and need more power (hence the heat).

 

[ayomeer]

So i was going under the assumption that the tail rotor should be pointing to the right to counteract the torque from the 2CW motors but of course the CW motors make the tri spin CCW so the tail motor must point to the left to counteract... Ok that makes the issue much simpler. Was not able to resolve it yet though. I did re-calibrate the ESCs and ACC and non of the motors get hot at all.

 

[ayomeer]

I now put some pressure on the servo linkages which, together with quite a bit of trim, resolved the issue. I did not know about that technique, thank you cranialrectosis!

Off to crash again