Monday, August 12, 2013
GPS tube quad problems with changes from 3S to 4S batts
Fun day at the Pattaya Stadium field today with both the “gps tube quad” and a brand new APM ardupilot quad that “Paul the toy man” just received this day from Kun Ooh. We got to the field early before Kun Ooh arrived so we played with the “gps tube quad” that I brought with me on my bike. The main change on the “gps quad” was that we modified the settings in the naza M flight controller to allow it to run off 4S (4 cell) lipo batts instead of 3S (3 cell). The current draw on the 3S batteries was destroying all my 3S batts in my collection. With the increased 4S cell voltage the batteries could handle the current and also provided the needed additional watts to allow for longer 13 minuet flights instead of 3 – 6 minuets with 3S batts. We had already tested the 4S battery configuration at the beach the day before so we knew that was working OK. The first flight started out very good but ended in disaster. I started out flying very conservative and then I started pushing the sticks to the max positions in both thrust and roll and yaw to see how it performed. It seemed to fly fantastic with just a bit more power in thrust from the higher voltage batts that were providing a bit more max current to the motors. I was experimenting with methods of high speed decent first strait down that puts it into osculation and a second where I was moving forward at the same time as the decent. The later seemed much smoother until I approached the ground and started to slow the decent too quickly. As I was only about 3 meters above the ground it seemed the vertical gain thrust started to pulse without my input on the throttle. I felt I was loosing control as it seemed the flight controller had too much authority as I wanted to give less throttle it would provide more. And when I wanted more throttle it wanted to provide less. It was like osculation between me and the flight controller as to who wanted what. At that point I just wanted to put it on the ground and figure it out but that ended putting it down when it still had lateral movement so when it hit the ground the quad flipped over upside down and broke two props. The tube frame was undamaged but some of the motor mounts had also been rotated a few degrees. After replacement or the props and realignment of the motor mounts I took it for another flight. But now it flies with osculating vertical without input to the throttle even when I'm just hovering it will suddenly pulse and gain 3 feet then suddenly drop back down 2 feet. I recall noticing this action when we had a small crash on the beach the day before when Paul crashed it. At that time I found the motor mounts miss aligned and that seemed to partly fix the problem. Also note that the same prop that was slightly chipped at the tip (out of balance?) on Pauls crash, was still installed on the “gps quad” as we didn't have a replacement for ccw prop at the time. After a few more tests and one more minor crash (this time by Paul with an attempted manual flight mode flight) we ended up retiring the gps quad for the day. I also learned later it was not manual mode but atti mode that we played with on the gps quad when it crashed. That disables the gps as input but I think it still has the barometric pressure used to stabilize altitude. So that could partly be what's causing the problem with the gps quad. Over sensitive barometric gain settings. When I got home I did a gyro calibration check and it all looked good but I calibrated the gyros anyway just to be sure and will replace the chipped prop in hopes that will fix it. I also dropped the vertical gain from 90% to 80% as I read too much vertical gain can also cause this type of osculation. So it is now ready for another test flight with the new settings in hopes that we have isolated the problems.