Quadcopter Still Not Stable

This is only the third real attempt to get a stable hover from my multirotor. Everything appears to be balanced — fore-aft CG, props, etc. — yaw seems stable, but I still require manual trim input on the Tx of about 20% right roll and 5% up pitch for it to behave as well as it does in the video. As an aside, I have Ch 6 set to adjust the 'I' value for Rate Pitch/Roll. I'd love for it to be rock steady but I'm not sure know how to get there.
Video is here: http://youtu.be/MKuDWPVwAaM and log file is here.

The other thing that concerns me is are two of the motors. After this video I checked them and motors 3 and 4 were too hot to hold tightly while the others two were barely above room temperature.

For safety, the skids are covered in foam pipe insulation and a prop protector stick was added to each arm and I'm wearing ear muffs (the sticks are from 2 carbon fiber arrows -- $3.50 each at Walmart =).
Specs
===================================
Arducopter: 3.1.2
Props: APC 12x4.5
Motors: NTM Prop Drive 28-30S 800KV
ESCs: Turnigy PLush 40A
Battery: Turnigy 40C 5A 4S
Flight controller: APM 2.6 w/uBlox GPS
Receiver: FrSky D8R-II PLUS (modified antennas)
Transmitter: ER9X with FrSky ACCST 8CH module
Weight as shown: 2.4kg


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  • Where do you need to put the stick before the quad starts to hover? If it takes quite a bit more than half stick then your motors may be underpowered. That would also explain the heat issue.

    I think the bouncy landings in your vid are somewhat normal. The shock of touchdown jarres the imu sensor initiating a compensatory throttle response. Touching down gently then lowering the throttle very quickly will help to stop that reaction.

    Your quad looks pretty awesome btw.
    • About 20% right roll and 5% up pitch. Power isn't an issue, I don't think the throttle ever got above 40%.  The overheating is only on 2 motors -- the other two are more or less at room temp.  The bearings on the hot motors are also louder than the others -- bit of clattering sound.  I expected better from motors with less than 15 minutes run time on them.

      Yeah, I really should be doing this outdoors where there's more room to maneuver but it's been raining here.

      Thanks!  It's been a rewarding build and it's not done yet.  =)

      • One more thing, make sure you do not use transmitter trim. Re run transmitter calibration after transmitter trims are zeroed. Also, it is important that you start with a good accel calibration I.e use a flat surface. Run the esc calibration to ensure each esc has the same throttle response. This should get you hovering with very little corrective stick input. Then use auto trim to fine tune from there. To help diagnose problems, use the mission planner's terminal mode to enable logging of the motors. This will log each motor's control signal so we can see if they are all balanced as they should be when in stable hover. 20% corrective stick is a lot.
        • Still no love.  At least not with the CW motors.  Re re re re re-calibrated everything. Again. Also re-balanced the props, correcting for some minor hub imbalance, and I disassembled the two hot motors, lubricated their bearings, and balanced their cans. The CW motors are still too hot to touch after 5 minutes while the CCW motors are cool.  Bizarre.

          It is more stable now, maybe adding 1% or 2% right roll and about 5% of up pitch.

          At one point during the last test I lost throttle response even though I was still getting 15.1 volts from the battery.

  • Try moving props around. If all of your props do not all provide the same amount of lift then the motors with the bad props will need to work harder and as a result they will get hotter. If motors 1 and 2 get hot after the swap then you know bad props are the problem. This could probably explain your stability issues too.
    • Looking at the log file is interesting.  The graph shows that motors 1 & 2 -- the CCW motors -- very nearly mirror each other in output power, while motors 3 & 4 -- the CW motors -- show they are drawing nearly twice the power!

      I've done everything I can think of, from re-calibrating absolutely everything to balancing the motors bells to lubricating the bears to re-balancing the props. 

      The copter still requires trim at the Tx and the CW motors are still overheating.

    • Thanks, hadn't considered that. I'll give it a shot tomorrow.

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