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.