I have been doing some testing (outside the US) that involve very high altitudes - 1800 meters or more. The climb takes no more power than coming down. Even though going up takes more power per minute, coming down is slower, so the total power consumed when coming down is easily equal to the power going up.

I was thinking that it would be more energy-efficient to shut off the motors at the highest point, and letting the quad free-fall and then turning the motors on again at 30 meters or so to right itself and slow down for landing.  Has anyone tried this?  I have been told that dji can do this.

I'm using APM and PixHawk controllers.

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Well, comparing to toys (DJI) does not help.  - nor is it safe to use it on toys - unless said toys uses sensored motors. (like RC Cars)

The problem with switching off motors in hover, is that many airframes will fall with rather stable due to payload beneath, and windmilling propellers higher up.  And during the fall, the windmilling propellers will be spinning backwards, something that any ESC and sensorless motor will struggle with.

to dive fast, you can switch to acro mode, and just pitch 90degree down ,motors running almost idle,  spiraling down (a little roll input).  just increase throttle and reduce pitch when you have descended enough.

or, of course, you could also design the airframe it so it would stabilize upside down when falling, (then the propellers won't spin backwards - having a reliable spin-up.)  

I've never pulled the entire throttle down, but instead, I lowered it enough so that, the copter could come down much faster.

While the copter was coming down, I tried flying it in a downward slope, rather than in a straight line.

I find letting it come down, in a straight line, makes it a bit more challenging to control it, because of the prop wash.

But letting it come down in a downward slope, and increasing the throttle gradually, as it nears the ground works better.

The trouble is - I wasn't flying FPV, and it was so high I couldn't see it (it disappeared at about 1500').  On top of that, I wasn't in the US and didn't know the area.  I wanted it to go straight up - and straight down, so that if something broke, I would know where it would land, and it wouldn't hurt anyone or any thing.

I did have telemetry.  I suppose I could have flown using that.  The 3DR 915 Mhz telemetry signal was still at 90% one mile up!

My controller was still working as well. I have a HAM license, and a 2 Watt booster on my controller.

Hi Charles,

I suppose you already know this, but flying that far out of sight isn't a good idea anywhere.

Although it is statistically unlikely you would collide with an airplane, at that height, one could easily show up and you would have no idea how to respond to avoid it.

Even if it is not illegal where you are, it would be a really good idea to have at least a good wide angle FPV system with a boosted transmitter and to be spinning slowly to observe for threats.

If you did succeed at being one of the first to have a midair collision with an airplane you would be instantly very famous, but in a very, very bad way.

And I suspect you would find yourself on the wrong side of the bars regardless of what current law is.

It is very much up to you to ensure that this ABSOLUTELY does not happen.

And the point about spinning backward when falling is very true, flight brushless motors do not use hall device sensors and so have very limited ability to sense whether they are going forwards or backwards and if you apply power and get it wrong, 3 out of 4 motors, truly won't cut it. 

of course, I assume the altitude is reached within local regulations, that you got permission/established danger zone/NOTAM , or fly in controlled airspace, whatever needed

It was a one-time event, not to be repeated. 

Hello, while I am not sure this is the kind of talk our governments is fond of seeing us have, I think about it as a great thing, would enjoy hearing more while your project unfolds!

Lowering throttle and tilting forward so that the copter slides into a controlled dive, will bring you down much faster. Going straight down fast will always be problematic since you are flying into your own turbulence.

Completely stopping the propellers is problematic because they might start to counter spin, and this might cause the motor controllers to fail (motor sync) when they try to restart.

Dive example: https://vimeo.com/22205147#t=145

I set up one of my radio spring switches for that, it can be useful for more than just descending quickly. I was practicing maneuvering around trees, and getting very close to them, and of course my blades started getting into the leaves and then getting non-horizontal. I switched the motors off, my Iris+ descended out of the leaves and righted itself, then I released the switch, motors came back on out of danger and Iris+ stabilized.

It's also good after a crash when it seems you never can get the motors to stop fast enough, or when you're landing and a little kid starts running up to it.

estas en lo correcto pues es lo mejor seria bajar al mínimo el gas y bajara rápidamente 

I do this with my racing quad.  I send it up to around 100 meters, put it into a crazy spin, power off and let it free-fall for about 6 seconds (disarm is set to 10s plus) then power up 10 meters from the ground, it's a lot of fun.  I don't know how the larger ones would handle it, I'd be concerned about the props reversing though.  I don't know if escs can startup a motor spinning in reverse without overheating something.

Even if the motors didn't start counter-rotating, a lot of power would be needed to bring it back to a hover.  I certainly notice this with my racing quad, If I punch the throttle after a long free-fall, the voltage alarm sounds for a few seconds even only half way into a flight which  reveals the heavier than normal current draw on the battery.

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