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Constantly Changing Trim

Loc8tor

Senior Member
#1
I am having an issue where the tricopter drifts in one direction. I trim that out and it flies ok for about 5 secs. Then it will start to drift again that direction. Again trim it out and the same happens. Until full trim is reached. Also, it will be one direction one time a different direction the next. Or it will actively change during flight. I had mentioned this before in a different thread. It was suggested to me to check my balance. With measuring I have the center of gravity about where the buttons are on the kk board on a batbone frame. I am for the most part balanced. I also have a non slip spot for my battery so my battery can not longer shift during flight. I also have a smaller battery. I have had a lot of flights without this issue. Then I had a crash that damaged my yaw mechanism that flitetest sells. I have a new yaw mechanism that works great. I don't know why I am having this drift problem. I have a friend that keeps saying it's my p and I gain. But like I said I had 10 successful flights and no drifting issue. What could have started this problem Can I solve it? I already have a new board coming.
 

Cyberdactyl

Misfit Multirotor Monkey
#2
Sounds like a problem I had a few months ago.

Look closely at how well you have balanced the props and motors. Also how the FC is mounted. If you have moderate to heavy vibration and the board is too loose (exple; too much foam, sticky tape coming loose) odd things will happen. If it doesn't have enough vibration isolation, (solid mount with screws, extra thin sticky tape, etc.) the same.
 

Loc8tor

Senior Member
#3
Sounds like a problem I had a few months ago.

Look closely at how well you have balanced the props and motors. Also how the FC is mounted. If you have moderate to heavy vibration and the board is too loose (exple; too much foam, sticky tape coming loose) odd things will happen. If it doesn't have enough vibration isolation, (solid mount with screws, extra thin sticky tape, etc.) the same.
I do have a slightly bent motor shaft. I have the board on the foam lid it came with.
 

xuzme720

Dedicated foam bender
Mentor
#6
Oh...
Well...I'd be tempted to say it was about the time you started having the trim problem!
I know, that was not funny. Just trying to cheer you up a little...
 

cranialrectosis

Faster than a speeding faceplant!
Mentor
#7
Ouch.

A bent shaft will play havoc with vibration on your copter. Said havoc often looks like your copter wobbling, losing level or falling out of the sky to land upside down in the mud in your wifes' rosegarden.

With a rotor shaft it is or isn't bent. Once it's bent you fix it (only way I know how is to replace it) or you have trouble. I think jhitesma replaced one with a nail in his ultimate scratch build.

In my case, I decided to try to fly on slightly bent. Next day it was very bent and I had to replace another boom.

I would consider that the most likely culprit for the problem and replace that motor or shaft.
 

DDSFlyer

Senior Member
#8
After replacing the yaw mechanism, did you retune your P and I gains?
My drift issues went away after I mounted my KK board better with some sticky foam mounting pad. And then retuning.
 

Loc8tor

Senior Member
#9
I ordered a new motor. I didn't retune my P and I gains. I don't really know how to tune them. It just doesn't click with me. i followed the pdf that says take your p gain up until it wobbles a lot and then take it down til it goes away. i have even watched youtube videos on it. For some reason the P and I gain doesn't compute for me.
 

xuzme720

Dedicated foam bender
Mentor
#10
This might help you see what each one does. The oscillations are very different but if you don't know what to look for, it's pretty hard to figure it out.
 

Loc8tor

Senior Member
#11
Ok guys. I changed my motor out. I flew and I didn't have to change trim constantly but I did have some extreme trim input to get it to sit still. I changed the board for new one. Never took the battery off or anything, just swapped the board. But forgot to center my trims first. Now I have to have my trim damn near opposite of what it was before. The center of gravity has changed zero but instead of drifting forward it drifts back. It drifts so much that I have the trim almost maxed out and when I give it forward it doesn't whip very hard because most of it is used up in trim if that makes any sense. Any thoughts? And I know you have shared with me before but just to verify what are you checking cg tips?
 

Craftydan

Hostage Taker of Quads
Moderator
Mentor
#14
How level are your *props* to the board? How level is level?

Two things to check:

Props level to ground -- on a flat surface, one prop at a time, measure the distance from each blade tip to the surface. Don't move the prop between measuring reach pair of tips, but it wouldn't hurt to measure this in a couple of prop positions. These measurements tell how much the prop disk is tilted relative to the flat surface. These numbers should be the same for any single prop. If a single prop disk isn't level, it will contribute to drift. Take a look at the mount and boom for any twisting or warp, and correct as best as you can. If all the prop disks seem to tilt the same direction by about the same amount, see if you can adjust your landing skids to get them the same -- fixing the skid's level wont' make the airframe fly any better, but it will allow you to calibrate the accelerometers to the same "level" the props are mechanically trimmed to.

Board level to ground -- When the airframe on a level surface, open the sensor test menu and read and remember the ACC #'s, then turn the airframe 180 degrees and set it down. If the numbers aren't close to the same, the surface isn't level. To get it level manually, Average the numbers you got from each orientation. This should be "level". Next, start shimming the landing skids until the numbers are close to the averages. A pad of paper works great -- keep removing sheets until it's dead-on.
 

Loc8tor

Senior Member
#15
Dan, once I have the pad of paper and have it level dead on is that just for calibration? or should I add to the feet so it is always level . after I typed that sentence I realized that it wouldn't make a difference if one foot is shorter than the other as long as it's calibrated level..... Right?
 

Craftydan

Hostage Taker of Quads
Moderator
Mentor
#16
The trick of this all is we don't really care if the props are level to the ground when it's on the ground, but they *do* need to be aligned with each other, and level with what the board thinks "level" is. If they're all calibrated at the same "level" (the ground) then we've guaranteed this . . .

. . . but yeah, as long as the props keep their alignment with each other and the board, that's all you need that for. You can set the front to "low rider" mode, and look cool swooping forward at takeoff . . . or not. If you've engaged the self level and are off the ground, the board will automatically level the frame, and the props with it.

If it still slowly drifts after that, then touch your trims. If they still keep changing, check your board and motor mounts to ensure they can't wiggle out of alignment.

BTW, keep in mind, these accelerometers *are* temperature sensitive. A shift of 5-10F degrees won't matter, but 20-40F, you'll notice. As we get into spring it's not that big of a deal, but if the temperature inside vs. outside is big (like in the dead of winter), don't fully trust your self level until the board cold soaks to ambient. As the outdoor air dramatically cools the board, level will change in mid flight. A quick cal should get you back in trim range again, but the outside temp will mess with your trims. having everything always level to the ground makes this adjustment easier.
 

Loc8tor

Senior Member
#17
Ok. I turned 3 cups upside down and then placed a dice in the center of each cup. I set the tricopter booms on the dice so it was only touching the wood of the booms. I checked my sensor test and wrote down the numbers. I then turned the tricopter around moved the cups a little and to get the dice under the booms and the numbers were the same. So I'm guessing that mean level. I then ran my acc calibration. I then measured the props tip to surface then the other side. One was the same the other was off by about .25 inch. but the motor mount is flush with the boom. So the boom must be slightly twisted. I can't twist it straight so idk what to do there. The rear was way off but it's on a pivoting servo so there is no way for me to have that perfect for measuring. But as far as I know the board is calibrated to perfectly level. That's where I'm at. No test fly yet. any thoughts, input?
 

Craftydan

Hostage Taker of Quads
Moderator
Mentor
#18
ouch. I think you've found your source of drift. Dunno how long your props are but an 1/8" each way on one motor makes for a pretty bad alignment.

When I've got a good hover, I tend to double check the alignment by looking down the side or front, and slowly rising so I can look down the prop disks -- if everything is good, they line up perfectly. If one is off it's visible, but usually I'll feel it first in how much effort I've got to put into flying it.

Double check the boom's mount to make sure it's not messing things up, and if the boom is still to blame, time to take a trip to the hardware store to buy a replacement. Bent really isn't as bad as broken, but only because you can still fly . . . but who wants a copter that flies crappy. replace it -- you'll be glad you did.

BTW, clever idea about the cup and die. I'll have to remember that.