I posted this in the trad heli user group but there doesn't seem to be a lot of activity there. Here is the original post:
I am hoping someone has some insight into a few questions I have. I am flying a Blade 450 with apm 2.5 mounted aft of the main shaft where the tail gyro would be. I am running 3.0.1 firmware. So far I have successfully setup the heli and have about a dozen successful flights (no crashes). Following the PID tuning procedure resulted in satisfactory roll and yaw axis control and stabilization, but the pitch axis is giving me problems.
Basically the heli will take off, hover, and fly forward at slow to moderate speed just fine. When I start flying forward with more speed the heli develops a wobble on the pitch axis. I can also make the wobble appear in hover by giving a sharp pitch input on the tx. The wobble seems to eventually fade away on its own and it hasn't caused me to crash, but there is definitely something wrong here. I have tried changing the rate and stab PID tuning to no avail. Lowering RATE_PIT_I seems to have the most effect but I have to basically make this term zero and even then the wobble is still there. I have a few theories about what the problem is.
1. Because the apm is mounted relatively far behind the main shaft (and cg) there is something weird going on with pitch rotation affecting the z axis accelerometer as well as the pitch gyro and that is what is causing the wobble.
2. I have the flybar weights all the way out and perhaps the apm and flybar are fighting each other because of the large flybar damping.
Does anyone have any ideas here? I am thinking about trying to turn THR_RATE_P down and maybe that would help decouple the pitch rotation and z accel (if that is indeed the problem). Or maybe I could move the flybar weights in or remove them all together.
Any ideas are greatly appreciated.
Replies
Hi Colin, sorry I missed all this I was on vacation last week.
Do you, or did you, have the Flybar Mode set to flybarless or flybarred?
The wobble, do I understand that it was only during very sharp and heavy pitch inputs in a hover (ie: back and forth to full pitch?) and/or were you doing very high speed forward flight in Alt Hold?
I just completed the poor man's flybarless modification on the b450 flybar head. It hovered in the backyard perfectly after quickly re tuning pitch and roll rate. The values are similar to what they were with no flybar weights. I ended up with 0.033 on rate roll/pitch P, 0.264 on rate roll/pitch I, 0.003 on rate roll/pitch D, and 0.02 on rate roll/pitch FF. PID tuned using the Ziegler-Nichols method for no overshoot. Stab roll/pitch P are stock 4.5.
I flew at the park today and it is definitely much better without the fly bar weights. I didnt notice the wobble through six batteries. I can still make it wobble if I try though. The tail doesn't hold either but that's another problem all together.
I am in the process of converting to flybarless so I will update after that.
Update: I removed the flybar weights and re-tuned the board. I hovered in the back yard a little and it seems slightly better, but I can tell there is still a strange wobble on the pitch axis with a sharp pitch input. I will update again after I fly in an open area.