I just finished building my first Pixhawk drone and I’m having some stability issues. Whenever I try to increase the throttle it seems like the rear motors are lifting more than the front motors and so the drone sort of scrapes forwards along the ground for a while before it manages to hover a few feet off the ground but it’s incredibly unstable and moves randomly while doing so and usually crashes shortly after. Furthermore, it seems like increasing the throttle mostly amplifies the instability and doesn’t really change the altitude past a few feet. I’ve checked the motor orientation/directions(top right is 1, bottom left is 2, top left is 3, and bottom right is 4), with 1 and 2 ccw and 3 and 4 cw. I’ve also checked the props as well. I did all the mandatory hardware calibrations in mission planner including my escs (I really thought that was gonna be the problem but it persisted even after I calibrated them). Here are some log files from a small flight I tried to do. 1 12-31-1999 7-04-00 PM.bin.log (608.6 KB) 6 12-31-1999 7-07-06 PM.bin.log (716.4 KB) Does anyone have any ideas why it’s being so unstable? Also if you guys need any pictures or any other info about my setup, please ask.
Is the copter itself nicely balanced (Center of gravity in the middle of the copter)?
When the copter is lying flat on the ground, does the Artifical Horizon / do the Roll/Pitch values show the correct attitude (0°/0°)? If not, try re-calibrating the IMU and then leveling the IMU when the copter is level.
Are you using the stock PID values?
It’s sort of hard for me to tell accurately but it seems to be well balanced to me. I can balance it on a finger from right about the half way point so I think it is but if there’s a better way to tell please let me know. Also I calibrated the IMU a few times and when I set the drone flat mission planner shows the artificial horizon to be just about level (it isn’t perfect but it’s incredibly close). Also to be honest I don’t really know what PID values are other than that they have something to do with tuning so I assume mine are stock. Thanks for your reply!
Your PID values look stock to me. That’s probably not helping but I don’t think it’s the cause of the problems. There is a spreadsheet with some initial value calculations that you can enter to get you started. I don’t have it handy but if I come across it I’ll link it here for you (if somebody doesn’t beat me too it)
I think there is a problem with your battery/motor/prop combination. Your battery voltage is low to start with (I’m assuming you’re using a 3S LiPo). You start 11.2 and as soon as you lift off you’re down to 10.5. If the battery is good, it needs to be charged. Those lower voltages aren’t helping.
Also a couple of your motors are going to max RPM. For just a quick initial hover check it shouldn’t need to get to those values. Perhaps these motors are undersized or the total weight of the copter is too high.
It’s a new battery that I assume came fully charged so I hadn’t done it yet but I’ll try that to see if it helps. I have these motors and this frame and this battery. Also the props are the ones that came with the motors. As far as the max RPM goes I did try to max out the throttle because the drone didn’t seem to be climbing and the motors sounded like they were spinning up but the drone didn’t climb any higher, it just sort of moved around from side to side more quickly until it eventually crashed. Also, thanks for the really informative reply!
LiPo batteries are always shipped in a storage charge. ~3.8v/cell. You need to charge it before you use it.
I have a similar F450. Those motors should do the trick but with a 3S battery you’re going to be on the back side of the power curve. You should consider bumping up to a 4S. It’s not about creating a rocket ship. You want to have enough power to get out of a jam. You should be able to hover around 50% throttle. Granted the sagging battery voltage didn’t help, but you’re at 70%+ and not getting far.
I just charged the battery and it’s doing about the same thing. Do you think upgrading to a 4s would fix the issue or do you think it’s something else aside from the battery?
That would help because your motors are at full power just to get off the ground. It won’t have the power to make corrections. Either a smaller lighter 3S, or a 4S would be a good start. I’m not saying that’s going to make everything better but it should help.
Personally, for a first time build, the 3S should suffice but I think you need to calibrate your ESCs directly with the receiver. Check out this video on the process.