This is really painful to admit, but anyone could have done it: I put new props on "my baby", and spun them down until they reached bottom, then didn't touch the bird for a few days. I certainly MEANT to tighten the props... BUT I TOOK OFF WITH THEM AS LOOSE AS IT SOUNDS AS THEY WOULD BE ABOVE. GO DJI! I flew for 10 minutes. High to low, a couple hundred yards away. THEN, got a RED WARNING about SEVERE VIBRATION, something worse like a prop strike, THEN: ENGINE AT IDLE. WTF? I was looking at the P3 at this point. It looked OK, I was afraid to barely move the controls. I settled into some grass. Uneventful -- except for the start of the "Drone Justice..." drama. Which wouldn't have happened except for this. :-(
I got in my condo, and had it idling. No binding, I was just about to hover when A BIG VIBRATION STARTED. AN UNBALANCED CONDITION TURNED VIOLENT BECAUSE I THINK IT WAS ASSUMING A "RESONANT" FREQUENCY. AMAZINGLY VIOLENT. I GRABBED A LANDING GEAR LEG -- NOT EVEN HARD -- AND IT ABRUPTLY QUIT (Damped out resonance)! Was I puzzled! Then I checked the props tightness, and they weren't! BUT, one HELL of a lot tighter than when I took off! ZERO tightness then L(cry)OL. DJI's self tightening props saved me from myself. USE CHECKLIST.
SO OBVIOUSLY THIS HAS BEEN ASKED, BUT I THOUGHT I'D THROW IN AN (EMBARRASSING) PERSONAL EXPERIENCE: WERE YOUR PROPS FIRMLY HAND SNUGGED DOWN? I'M ASSUMING THAT THE GIMBAL VIBRATION WAS AN EFFECT, AND NOT A CAUSE. MY EXPERIENCE WITH NOT DOING SO WAS A SEVERE VIBRATION, OBVIOUSLY AMPLIFIED BY A RESONANT CONDITION, THAT I THINK DID DAMP OUT IN FLIGHT -- I DON'T THINK MY ENGINE WAS IDLING, BUT THE RED WARNING DIDN'T GO AWAY. AND THE SEVERE VIBRATION GOT RECREATED ON A COFFEE TABLE IN MY LIVING ROOM. A RESONANT SITUATION, BECAUSE JUST PRESSURE ON A GEAR LEG STOPPED IT. NO PROBLEMS SINCE. AND I HAVE A CHECKLIST, AND "INSURE PROPS ARE SNUG IS ON THE TOP"!
PS I'm assuming my loose props were mimicking a severe out of balance condition at times. So an out of balance prop would be a potential underlying cause but to get as severe as you talk about, and what I saw close up, it seems like it would have to be WAY out of balance. I hope something here was useful information for you (or others).