Bought me a used Phantom 4 the other day. Still learning the different functions. Something I am not going to try is emergency stop in air, but what if you from quite high altitude use it, and after 2 or 3 seconds of free fall try to arm the motors. Would it be possible to arm the motors and recover?

The question has been asked before, many, many times. And one of the most common replies is to say, why not try it out and come back and tell us how it went. With pictures.

Geebax Posted at 2018-8-11 15:24
The question has been asked before, many, many times. And one of the most common replies is to say, why not try it out and come back and tell us how it went. With pictures.

hahahahahaha wicked man....

GaryNash Posted at 2018-8-11 15:32
hahahahahaha wicked man....

Sorry, I could not resist that. The simple answer is this, the aircraft may or may not re-initialise in time before it hits the ground, so you have to ask yourself 'Do I feel lucky?'

Well?

Maybe from 1000 feet or so...

Mark The Droner Posted at 2018-8-11 16:53
Maybe from 1000 feet or so...

https://www.youtube.com/watch?v=P20jEzQ5eU0

Aloha Mark,

     First time I saw it work!  Mahalo!

Aloha and Drone On!

Mark The Droner Posted at 2018-8-11 16:53
Maybe from 1000 feet or so...

https://www.youtube.com/watch?v=P20jEzQ5eU0

WOW, didnt think that would work.... shut down and restart .... id give it a 1 in 50 chance or recovering...  i dont have the money to try it myself ...

Mark The Droner Posted at 2018-8-11 16:53
Maybe from 1000 feet or so...

https://www.youtube.com/watch?v=P20jEzQ5eU0

Great. Exactly what I was looking for. If the actual fall is normal speed, this is what I see
1.33 - 300 m - stop command. Rotating descend as left stick is bottom right.
1.37 - 290 m - motors stop, Start to tumble and speed quickly increase to about 15 m/s
1.43 - 190 m - arm command. Vertical speed close to 18 m/s
1.47 - 138 m - recovered

Notably terminal velocity seems to be below 20 m/s

Gabriel_swe Posted at 2018-8-12 00:05
Great. Exactly what I was looking for. If the actual fall is normal speed, this is what I see
1.33 - 300 m - stop command. Rotating descend as left stick is bottom right.
1.37 - 290 m - motors stop, Start to tumble and speed quickly increase to about 15 m/s

Note that after shutting off the power, the fall is unstable until the propellers have spun up in reverse and are providing lift again and the drone itself is also rotating to provide gyroscopic stabilisation, after that the aircraft will remain upright and reasonably stable with a fairly stable fall speed, restarting the motors at this stage is likely to be successful. Restarting them earlier is much more of a gamble and the time taken to regain control will be very variable.

The main risk seems to be that the motors may not restart properly.  If you start them while spinning backwards at the wrong speed then the position and speed sensing may be very confused.

There are not enough examples of people doing this to be able to judge how reliable, or unreliable it is.  It is not something that the aircraft is designed to do.

Actually it appears I misspoke... He started at 300 meters and recovered at 135 meters, so that's a drop of only 165 meters which is only a little over 500 feet!  

Gabriel_swe Posted at 2018-8-12 00:05
Great. Exactly what I was looking for. If the actual fall is normal speed, this is what I see
1.33 - 300 m - stop command. Rotating descend as left stick is bottom right.
1.37 - 290 m - motors stop, Start to tumble and speed quickly increase to about 15 m/s

You can try it in the flight simulator in DJI Go 4, tap on ME at bottom of screen then go into 'Academy' then tap on flight Sim. Note that the aircraft needs to be powered up to run the flight sim, but motors are never run, still good practice to remove propellers.

In flight SIM you will not be able to pull off a recovery much below 500ft.

I've done it with a P3P/A.
The props provide the drag, not lift, and keep the drone upright though it wobbles a fair bit ( from the drone's DAT ).
The motors restart BUT AT IDLE so you have to give it full throttle.
From memory it spun the whole way down until the motors were either restarted or throttled up, I don't remember which.