With all due respect I believe the vast majority of folks in this thread are confused about how self-tightening propellers work -- they will NEVER unscrew themselves. They ONLY tighten.
Thus I don't believe this is the cause of the crash we're discussing here.
Personally, when I'm setting up my drone I place the propeller on top of the screw/motor and give it a single "flick" with my fingernail to screw them on -- I do ZERO hand tightening beyond that. As soon as the drone is powered on, they're getting tightened. As as you take off they tighten even more.
There is literally NO case in which the motor turns in a way that unscrews the propeller.
I'd love to hear this explained more because it makes no sense, sorry folks. If the drone is staying in the air, there's a force downward on the air under it -- Even when you're descending, the propellers aren't actually switching direction of spin, they're simply spinning a bit slower so there's less force. But again there's ALWAYS a downward force so the propellers are ALWAYS TIGHTENING THEMSELVES, NEVER LOOSENING. If there WERE zero force pushing down, it'd be in free fall. To unscrew themselves there'd somehow have to be a negative downward force (i.e. Props reversing the direction of spin) so the aircraft in that case would literally be descending faster than a free fall. Or you'd have to be flying it upside down. Neither of these cases happen in normal flight.
Simple.....and it makes perfect sense if you are able to consider ALL the factors.
(and btw, all caps doesn't make a point valid)
No matter how much time you spend not seeing how this makes sense, it still doesn't change physics or aerodynamics. There's also always a downward force on a car...does that mean the tires never come off the ground?
Vortex Ring State is just one time when the downward force is virtually eliminated. So much for the theory that there is ALWAYS a downward force. At certain times that downward forces changes from positive (thrust) to less thrust than needed to sustain flight (descending) and is at a maximum when ascending.
Read this thread over at DJIForums...
DJI Forum|All 4 props come off mid flight
It takes approximately 4/100's of one second for a prop to completely unscrew.
You're thinking minutes instead of milliseconds.
If the prop is loose (for whatever reason)...and the motor suddenly slows down what will happen to the prop if there is insufficient resistance (ie wind / air) and sufficient inertia in the propeller?
Correct (it WILL spin off the hub). This has nothing to do with switching direction.
The props spins off in the direction OF ROTATION.
You are forgetting inertia. You are neglecting to consider momentum of the aircraft. You are forgetting the whole vortex ring state discussion. You are assuming errors are never thrown to the motors that could cause hesitations or anything other than smooth running....ever.
Think about that for a moment. Really? A glitch from one ESC motor for 4/100's of a second could completely unscrew a prop. How can you be certain that's not what happens sometimes when a Phantom crashes and a prop is found way over there?
If you are ascending rapidly then pull back on the throttle suddenly, the aircraft will continue to rise for just a few seconds on momentum. What's happening to the air around the props at that moment? Is the pressure the same as when it was climbing under full throttle?
Fortunately this is not a common occurrence since not only do the props have to be loose enough to freely unscrew, but there also has to be almost no resistance to the props at the same time and this has to coincide with a significant enough drop in motor rpm while this is happening for a prop to actually spin off.
But....CAN it happen.....sure. It happens..
To convince yourself otherwise is at your own risk. You may never have a problem.
All this is saying is why risk it with loose props?
And that isn't to say you should jack down on the props until they strip or break. I like mine to be just a bit beyond finger tight, And yes, I use the wrench and slight finger pressure.