Recently I attempted to take off a 25" quadcopter from high grass. When the props spun up to lift off, one of them fouled on the grass and the quad flipped over. Three of the four props continued to spin while the fourth was stuck on the ground, unable to spin.
For some reason the vehicle did not disarm right away, and the three motors continued to run for about 15 seconds until I was able to carefully (very carefully) disconnect the battery. The fourth prop and motor were stuck in the grass during this time.
After a quick check out, I decided to try again. In retrospect this may not have been too smart, but I was testing new camera settings and wanted to see the results. Also, I was in a remote, safe and open location with no people around. This time I placed the quad on a square of cardboard to give a better take-off platform (I should have used the platform the first time), and I started the mission again.
The quad took off and went on its way. The machine flew, but wobbled and made chirping sounds. But but it completed the short mission, which was to climb to 100 feet high, travel one hundred feet out, then RTL and soft landing.
Later on the bench I found that one of the ESCs, likely the one that got stalled by the high grass, was damaged. It worked, but now had only off and full speeds - nothing in between,
Arducopter must have worked really hard to keep the machine flying. I reckon that the chirping sounds were the bad ESC/motor being turned on and off by ArduCopter to approximate controlled thrust (kind if like a WW1 Nieuport fighter plane). And by the way, the quad RTL’d and landed right next to the takeoff cardboard - about one meter from the home location!
Amazing system. Nice work Randy Mackay, the development team and community of ArduCopter.