My AC2 is assembled, and I was able to do a couple indoor tethered test flights to make sure things are connected and setup properly.  Here is the first test:


This was somewhat of a success for the first flight, in that all of the electronics were basically working correctly, including wireless telemetry.  After re-checking all of the connections to determine why it was immediately pitching over, I found that the 4-wire connector from the APM to the PWB was plugged in backwards.  I switched it around, and re-ran the pre-flight test, paying closer attention to actually which motors were speeding up and slowing down at what time, and tried it again:


It's not immediately trying to pitch over, so I think the stabilization is basically working

A few other things are also going on here:
- There is probably a lot of weird air re-circulation going on in this small area that is causing it drift around, as well as dynamics from reaching the end of the tethers that are making it difficult to stay level.
- Toward the end, it seems to pitch to the back to the right. At this point, even after landing, the attitude showing on the laptop was far from level, I'm sure this is because the magnetometer was not getting a good reading inside, and also because the cords artificially held it at a nonzero pitch attitude without accelerating in that direction.
- I'm using a 20C 2200mah battery, which is too small for this since there are obviously 4 20amp speed controllers.


The next step is to take it outside to eliminate any re-circulation effect, make sure the magnetometer is getting a good reading, and get a GPS lock.  I will probably test it tethered again, maybe with longer tethers -  I think if I position the anchors further apart, the tendency might be to pitch back toward center when it reaches the end instead of pitch away.

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Comment by Aurelio R. Ramos on October 23, 2011 at 8:28pm

Just some warning: I tried doing some tests tethered, but the tether loads cause instability of their own. One type of test you should consider is the test while holding in your arms (very carefully!) to check if the quad "resists" your attempts at moving it in the various axes.


A low altitude (say 1 meter)  flight over grass is also a relatively safe way to test it during the early flights. If you learn to do hovers at such low altitude, once you start to fly at higher altitude it gets easier because you avoid the "ground effects" which is what you describe as air circulating in a confined space.

Comment by Aurelio R. Ramos on October 23, 2011 at 8:31pm

Oh, another type of test is to "crawl" the quad on the floor. Use the pitch / roll and yaw controls to get the quad to slide on the ground (without actually taking off). You just have to watch the throttle and keep it low and steady. If your radio allows it, you can also put a lower endpoint on the throttle. If you do that make sure to run the calibration without the endpoint engaged, otherwise it will calibrate to the shorter range and the endpoint will have no effect at all during use.

Comment by Randy on October 23, 2011 at 8:32pm

I wouldn't blame the mag for the level being off.  the mag is only used for yaw correction so it's unlikely.


I think you should check out what the artificial horizon is showing when the quad is sitting on the ground with both engines running and not running.  So even before trying to get it off the ground, see if the orientation gets messed up because of the engines running.


How have you mounted the apm to the quad's frame?  any rubber or foam padding between them?

3D Robotics
Comment by Sam Kelly on October 23, 2011 at 8:42pm

That battery should be good for about ~10 or ~15 minutes of flight time (depending on how aggressive you fly). The ESCs are rated at 20A but that's not constant, we've measured the current consumption on the stock 3DR quad kit at the 3D Robotics shop and at full throttle we saw ~40A, so that's around 10A per ESC plus the APM electronics and the radio receiver.

Comment by AndrewF on October 23, 2011 at 9:18pm

Thanks for the advice - I will try running the motors for a longer time on the ground to see if that results in drift on its own.  I do have some padding between the APM and the frame, but I have not yet soldered the bridges to enable the hardware filters. 

Comment by Don Cramer on October 24, 2011 at 12:25am

Tethering IMO is a useless practice for setting up any rotor craft. They just do not like it and its really hard to determine even if there is really a problem or the way it behaves is from being tethered.


It is more useful to hold on to the bottom of the quad (get a firm grip), hold it over your head and power it up to determine if all is well. You do not need hovering power just enough to determine if the motors and props are spinning the right way and manually tilt it around to determine if stabilization is working. Once this is done and all is ok it is now ok to begin flight testing.


If any of you seen the copter control video for PID tuning where the quad is mounted so it rotates along one axis, this here is really the only real useful use of tethering, IMO (i been wrong before so who knows, just my opinion from experience).

Comment by giu on October 24, 2011 at 1:06am

I had similar problems, and little by little, with the help of emile, we discovered many things you can check

1) First the mounting of propellers of AC2 kit sometimes is not good, in the sense that although the props seems to be tied to the motor arm, they actually do not. Tre to strength the props or change prop mounting

2) Check the level many times

3) Try ESC calibration, both auto and manual.

Comment by Dany Thivierge on October 24, 2011 at 9:40am

poor thing... let it fly louse! :) 

Comment by estebanflyer on October 24, 2011 at 9:55am

your P and D values seems to be fine, you might be ready for an untethered test. go little by little. (step by step in Argentinian haha)

Comment by Ellison Chan on October 24, 2011 at 6:34pm

Looks like a lot of nervous fingers, and the tethers don't help.  

The quad is bound to drift a bit, you need to get used to doing tiny corrections.  Get familiar with how the sticks control the quad.

The battery is a problem, not because of the capacity, but 20C will only support a discharge of 44A for this battery. Your motors/escs will max out at 80A.  So you're probably not getting the power the quad needs to do smooth stabilization at the top end. How far do you have to throttle up to get lift off?  Also drawing 80A and from a 20C battery that maxes out at 44A will cause the battery to overheat causing battery damage.  If you want to stay with a 2.2A battery, you will need at least a 40C battery, maybe get by with a 30-35C if you don't go high throttle all the time.


Also, calibrating the ESCs will correct some stabilization issues, since it sync up motor speeds. 


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