Turn props off in mid-air for fast descent (and on again :-)

[MAvic_South_Oz]

It's been suggested in this thread, that this 'shut-down' manouvre could be used as a way to out-descend birds that are hassling your drone. I'm wondering if the drone comes down any faster, if you 'forward-stick' to get max speed, hold that speed, and then push down-stick along with left-rudder (or right) - so that you are in a powered spiral. It would be interesting to see if that gets any more negative metres/sec over just holding down-stick???

I can see the bird escape thing if say an aggressive eagle is probably going to take it out anyway.
Can also imagine if you are in a tough return to home situation fighting with wind at height etc, and you realised you weren’t going to make it, you could do this manoeuvre over a safe place and get it down fastest and land controlled relatively safer than a total power loss eventually.

I do usually just use forward or backward (alternating) full stick along with full descent and it does come down as fast as possible in a controlled way.
Don’t think it would matter if it was also done in a spiral manner, the important thing is getting out of the prop wash.

 

[msinger]

This thread has a bizarre alternative reality flavor to it. Even though I posted the data from a flight log of exactly this maneuver early on in the thread, several posters described having successfully tried it, and there are links to videos of the maneuver, we still have multiple posters continuing to speculate that it's not possible.

Welcome to the Internet. This happens in lots of forum threads.

 

[ff22]

This thread has a bizarre alternative reality flavor to it. Even though I posted the data from a flight log of exactly this maneuver early on in the thread, several posters described having successfully tried it, and there are links to videos of the maneuver, we still have multiple posters continuing to speculate that it's not possible.

Okay, I accept that it is POSSIBLE. But what about the possibility it will not work in a few, some or many attempts?

 

[kilomikebravo]

I'd guess it would be a successful attempt less than fifty percent of the time and if that's close to being true and, one does not want to risk their expensive toy, then it really becomes a moot point, doesn't it? OTOH, if you don't care about losing your toy, then it's also a moot point.

 

[FormerMember07122019_2]

Some people will push the boundaries of everything. This is actually a good thing. Innovation is typically a result of a lot failed experiments before arriving at a solution. I say, let them experiment and we will all eventually benefit.

 

[Monument.AL]

If you descend straight down, your bird will be flying into unstable air (vortex) created by the props and this can lead to a loss of control. We get taught this in the Australian CASA RePL certification course and I would suspect other countries would be doing the same. Apparently helicopter pilots have to be aware of this as it can bring them unstuck in very bad ways. My theory (and I must say I don’t have any evidence to support this) is DJI must understand this and therefore reduce the maximum potential decent rate to keep your drone within safe operating parameters.

Knowing the above, when I need to decent quickly I put the left stick in the 7:30 (hour hand) position to decent and turn left, and the right stick in the 10:30 position to fly forward with some left lateral movement. This seems to result in a very quick spiralled decent. I’m guessing, again no evidence (yet), that DJI will be allowing a faster decent as you are now moving forward and keeping out of that dreaded vortex. You can also achieve the same by turning right with a 4:30 left stick and 1:30 right stick combination.

Next time I’m out flying I will need to check the decent rate of both of these (straight & spiralled) to see if there is indeed any difference, and to see if my theory holds up. Give it a go anyway as it will definitely result in a safer decent.

 

[sar104]

Okay, I accept that it is POSSIBLE. But what about the possibility it will not work in a few, some or many attempts?

That's certainly possible although, in all the accounts of this being tried with DJI aircraft, I don't recall any that failed.

I'd guess it would be a successful attempt less than fifty percent of the time and if that's close to being true and, one does not want to risk their expensive toy, then it really becomes a moot point, doesn't it? OTOH, if you don't care about losing your toy, then it's also a moot point.

That's not supported by the data so far. The data that I posted previously was from a log that included three attempts, all successful. I linked that discussion in another thread. Out of interest - why would you guess a less than 50% success rate? Do you have any examples of this failing?

 

[kilomikebravo]

SAR: The experiment you mentioned was controlled and my guess was based on what I think might happen in an uncontrolled situation, i.e., someone is at max altitude and wants to get down quickly and doesn't prepare for the restart in time. IMO, not everyone would be successful and even one drone loss isn't worth a rapid descent. To me it is "teasing the animals."

 

[sar104]

SAR: The experiment you mentioned was controlled and my guess was based on what I think might happen in an uncontrolled situation, i.e., someone is at max altitude and wants to get down quickly and doesn't prepare for the restart in time. IMO, not everyone would be successful and even one drone loss isn't worth a rapid descent. To me it is "teasing the animals."

That's a valid concern - for example if the maneuver is started too low then it will end badly. We should probably distinguish that from the question I thought we were considering, which is whether stabilization after a restart, with sufficient height (i.e. >> 50 m) is a robust and repeatable process.

 

[kilomikebravo]

SAR: I'm sure it is repeatable, your example clearly showed that. But as you said, even if the operator acted precisely, if the initial altitude were low enough, then disaster is almost a certainty. According to my limited knowledge, at 400' AGL, a free falling object has about five seconds until impact. I don't recall at what height the experiment restarted the motors but five seconds is pretty short and it would be easy to make a mistake and start them just one second too late. That is why I wagged it at a 50 percent probability of success.

So again, repeatable but probably not by every operator under every set of conditions and obviously, the minimum motor start altitude is critical.

 

[FormerMember07122019_2]

SAR: The experiment you mentioned was controlled and my guess was based on what I think might happen in an uncontrolled situation, i.e., someone is at max altitude and wants to get down quickly and doesn't prepare for the restart in time. IMO, not everyone would be successful and even one drone loss isn't worth a rapid descent. To me it is "teasing the animals."

To add to your point, I’m not sure I would trust myself not to panic and fail to use the proper control inputs as I see my expensive aircraft hurdling toward earth. I believe SAR104 has analyzed a few recent crashes where there were erratic control inputs in emergency situations. Granted, I’m a newbie and maybe a more experienced pilot might not have this issue.

 

[sar104]

To add to your point, I’m not sure I would trust myself not to panic and introduce some erratic control inputs as I see my expensive aircraft hurdling toward earth. I believe SAR104 has analyzed a few recent crashes where this has happened. Granted, I’m a newbie and maybe a more experienced pilot might not have this issue.

Yes - it's clear that if the pilot cannot competently execute the maneuver then this will fail, but that's true of flight in general.

 

[FormerMember07122019_2]

I would classify emergency maneuvers as a very small subset of “flight in general” that is probably not part of normal practice routines.

 

[Drgnfli]

I don't care to "experiment" with a $$$1,000 toy!! I'll just keep on keeping it a few feet above the trees...

 

[CaptBill]

This demonstrably did work, multiple times.

Just curios, you keep pointing out to people that the graphs show this maneuver would work, but, you also claimed numerous times that you wouldn't do it. So if you won't do it, can't you understand the fears of those who suggest not doing it?

 

[sar104]

Just curios, you keep pointing out to people that the graphs show this maneuver would work, but, you also claimed numerous times that you wouldn't do it. So if you won't do it, can't you understand the fears of those who suggest not doing it?

Absolutely - I would not recommend this to anyone except in an emergency. I'm not criticizing anyone for not wanting to try it or worrying that it might not always work - that's not the issue. The issue is multiple posters still asserting that it will never work, or will only work a small percentage of the time.

 

[Diablol]

Can you actually confirm (again) that we talk from a Mavic 2?
Obviously it works with the MP but I see no hint for a MP2.
Besides this I did not manage to start my motors under simulated situations like a fall. Everything not standing still or flat no way to start the motors.
Hard to believe it will in a free fall...
But I wish to...living on the edge..

 

[sar104]

Can you actually confirm (again) that we talk from a Mavic 2?
Obviously it works with the MP but I see no hint for a MP2.
Besides this I did not manage to start my motors under simulated situations like a fall. Everything not standing still or flat no way to start the motors.
Hard to believe it will in a free fall...
But I wish to...living on the edge..

The data above are from a Mavic Pro - hence my earlier comments about aircraft versions and firmware. If the firmware prevents motor restart while in the air due to attitude - which is not the same thing as preventing motor start or causing motor shut off on the ground (a different aircraft state) due to attitude - then this will fail. As a relevant data point, note that the motors on the Mavic 2 do not shut off in mid-air when the the aircraft inverts - that only happens when the aircraft state is "Ground" as opposed to "Sky".

 

[zxcflyer]

I'd be worried about that. The examples that I was looking at showed no issues, but I'm not sure if all the firmware versions behave similarly.

That test was conducted with some parameters set to allow the motors to start in any attitude.

 

[sar104]

That test was conducted with some parameters set to allow the motors to start in any attitude.

Right, but it's never been established (as far as I'm aware) whether the attitude restrictions on motor start apply when the aircraft is in the air. The auto shutoff mechanism certainly doesn't work in the air.