Cut motors to drop altitude fast

[PropPilot]

It's physics. To tighten itself it would have to have a design with a centre of gravity that would tighten it.

Think of a ship in the water. It requires a buoyancy to bring it upright so weight at the bottom and flotation at the top. Keeps the wet side down.

The Mavic has zero inherent or built in stability and relies totally on the rotors keeping it upright when starting upright.

If you had a fixed pitch helicopter and stopped the bladed and the aircraft rolled which it would for the same reasons, starting the blades would not bring it upright.

I welcome comments to my theory....

 

[maikm]

It's physics. To tighten itself it would have to have a design with a centre of gravity that would tighten it.

That would only be needed to ensure an upright position while falling with stopped motors. With the four motors though, the drone can rotate itself in any direction. If it is upside down, it could spin just two adjacent rotors fast and the others slowly, and it would rotate until it is upright again. At which point it would then engage all four motors equally to start braking. Even if the downward speed exceeded the maximum thrust, the fast spinning rotors would still generate less air resistance than the other two, which also would lead to the drone rotating.

The examples you mentioned, like the ship in the water, only apply to passive devices that have to rely on gravity. A quadcopter doesn't.

That said, there is probably no easy way for us to know whether or not the flight control software in the Mavic would behave as I described. Even if the physics would allow it, the software could always do something else.

 

[PropPilot]

I agree. If the flight control software knew what was happening then the propellers could likely be modulated to 'right it' up. My guess would be that it doesn't and that the Mavic has no inherent stability or aerodynamics that would right it from the effect of falling. I suspect we will know soon as someone is definitely going to CSC their drone in a hover to see what happens.....

 

[niclariv]

Hey everyone, maybe the situation described in this post isn't a good one to be in because it's so avoidable by watching the battery levels, but I do wish the Mavic had a faster descent option (in sport mode maybe). There are situations where it would be useful (like if there's a low flying aircraft suddenly approaching and you need to get out of the way). I'm guessing they don't let it descend faster than 3 m/s because of either safety or stability. I notice with mine that if I descend straight down even in no wind the Mavic looks a little wobbly (though the video looks stable). I haven't worried about it because I found another post on the official DJI forum where someone asked if that wobbliness was an indication of a problem and the answer they got was it's normal because it's descending through the prop wash when it goes straight down. So maybe if it was allowed to descend faster it would be so wobbly that the gimbal couldn't compensate and stabilize the video (or so wobbly that it wouldn't be able to stay generally upright).

Anyway, the upshot is, for emergency situations it would be great if there was a faster safe descent.

For those of you doubting the ability to recover from a mid air CSC, I've seen a couple of videos of Phantoms successfully doing it. One was an accidental CSC that was recovered from just in time over the ocean, and here's an on purpose CSC recovery test. Not saying that because it works on a Phantom it will work on a Mavic, or is the best solution (better would be the Mavic attempting to descend faster under power but not free fall).

DJI Phantom 4 Pro CSC test！Stop the motors in the air - YouTube


iPhone 6S

 

[niclariv]

Hmm it didn't seem to like that link - At least on my phone. [YOUTUBE]P20jEzQ5eU0[/YOUTUBE]


iPhone 6

 

[F6Rider]

The trouble with a fast decent is you will be descending into your own prop wash, descend fast enough and you will lose lift as the air mass below your props is being blown down at speed. We had this issue with the first P2's, they descended so fast that they would start to wobble from air turbulence. You have to maintain a forward or lateral speed to avoid the turbulence and lose of control.

 

[PropPilot]

He did the CSC at what ? 200 meters ( 600ish feet up ) and it looked like it recovered just before going into the dirt. In aviation, you crash if you run out of airspeed, altitude and ideas all at the same time. My comment would be that this recovery took far more altitude than our legal max altitude in Canada and USA so.....

 

[niclariv]

The trouble with a fast decent is you will be descending into your own prop wash, descend fast enough and you will lose lift as the air mass below your props is being blown down at speed. We had this issue with the first P2's, they descended so fast that they would start to wobble from air turbulence. You have to maintain a forward or lateral speed to avoid the turbulence and lose of control.

So maybe (in sport mode or something like that) you could have the option of a faster descent while travelling forward at the same time as full stick down?


iPhone 6S

 

[anim@l]

Here's one for you... that seems to follow the theory that the MaVIC can't recover if it's tilted:

On a recent flight, I got snagged by an invisible tree branch. It stopped one or two of the propellers, and sent it tumbling to the ground. Even though I had applied Full power when I saw this fall... the motors refused to respond... ultimately resulting in a crash into the ground from a large height (100 feet?). It resulted in two broken propellers and a cracked camera lens (Cover lens?).

All the way down I remember thinking - Why isn't this thing responding? If Applied full power I could get it to FLY before it crashed into the ground.

At first I thought, maybe I didn't respond FAST enough or apply full/Enough Power soon enough?

But now I see that I couldn't overcome the programming if it was programmed to refuse Power up if tilted.

I realized this was the case when I tried to take off from a tilted Car roof and it refused to power up.

That's like a modern car refusing to turn over if it's parked on a HILL or tilted street.

 

[niclariv]

That could also be because the motors fully stopped when it got snagged, so 'if' the Mavic allows you to start it again when it's titled, pushing up full isn't enough, you need to start them again with a CSC.


iPhone 6S

 

[F6Rider]

Helicopters can auto rotate due to the inertia that can be stored in the spinning blades. As they start auto rotation the pilot lowers the pitch of the blades and the airflow caused by the descent keeps the blades spinning. As the helicopter nears the ground the pilot will increase the pitch on the blades to decrease the vertical speed for landing. It's got to be timed right, if the pilot increases pitch to early and runs out of the momentum stored in the spinning blades the helicopter will drop out of the sky. So our quads have a couple of problems. The blades are light and can store very little momentum. The pitch of the blades can not be changed, so it won't lower pitch during the descent and then increase it to cushion the landing. In addition I am not sure what the hinged blades of the Mavic would do without the centrifical force of powered flight. I suspect the terminal velocity of an unpowered Mavic is high enough to be fatal.

A Mavic will never auto rotate due to a fixed pitch and folding blades. And you are correct, by lowering pitch and applying full forward cyclic you increase the rotor and forward speed as well as decent rate, pulling pitch and cyclic, in effect 'flaring out', slows forward speed as well as providing lift. You have to repeat this not allowing the A/C to drop below critical air speed for the current altitude, or you drop like a rock. You have to time the last flair so the forward air speed is near zero at ground level. This almost never happens, I have been in copters doing auto rotations many times and sometimes we slide 40' down the runway, and sometimes we have a 'hard landing' where its possible to spread the skid arms, at the least we always have to replace the skid shoes. I have only seen a few 'perfect' AR landings, and these by a CW4 test pilot.

 

[RallySoob]

There has been a couple of people over on the Phantom board who did an accidental CSC in midair, every one ended in a freefall to the ground. As the Mavic will not even start the motors if the IMU detects it is tilted to far common sense dictates that it will not restart while tumbling to the ground. This is not a 3D racing drone.

Common sense dictates? So ur assuming its common sense to know a Mavic can only start on the ground? Hmm not sure that is how common sense works bro you might want to look up the definition. Common knowledge are the ideas most commonly known to all. Some of us actually just got into this hobby believe it or not

But if that information is true, i now know it isn't possible to restart a Mavic

... i lost a syma x5 last night by putting it up 150ft too high and it drifted off climbing elevation whole time til seemingly went mile away. I cut throttle but it had no effect. I really could have used that technique last night had my x5 not took off on stray signal or whatever cuz i wanted to get it down asap

 

[F6Rider]

Common sense dictates? So ur assuming its common sense to know a Mavic can only start on the ground? Hmm not sure that is how common sense works bro you might want to look up the definition. Common knowledge are the ideas most commonly known to all. Some of us actually just got into this hobby believe it or not

But if that information is true, i now know it isn't possible to restart a Mavic

... i lost a syma x5 last night by putting it up 150ft too high and it drifted off climbing elevation whole time til seemingly went mile away. I cut throttle but it had no effect. I really could have used that technique last night had my x5 not took off on stray signal or whatever cuz i wanted to get it down asap

If the Mavic will not start while sitting at to much of a tilt while sitting on the ground, then common sense says it won't start while tumbling out of the air. If you want common knowledge on this then by all means give it a try and report back so the rest of us can have this knowledge first hand. The Mavic is not a 3D stunt or racing drone, it is a flying camera for aerial photography. Sorry you lost your Syma X5, that sucks.

 

[RallySoob]

Really? Its not a 3D stunt race quad? I had no idea it even had a camera on it wow who would of guessed its made for aerial photography weird

Lol Now all that was implied common sense

 

[tuzzman]

Say your mavic is way high in altitude & for whatever reason you only have a percent or two of battery power to get it landed or else its due to lose power before auto land would be able to get it to the ground.

Would u consider cutting the motors to allow it to free fall and try to restart/recover the craft at low altitude in attempt to save the craft?

Anyone ever tried this?

Could the mav do that?

Just curious... i got stuck in menu & started to panic the otherday..this scenario possibility came to mind

Ok so I found this on YouTube. They used a P4P and had success with this question. Check it out.

 

[ClaudioNC]

This guy has a valid question. Too many people are too quick to take up their pitchforks. Put down your righteousness for a moment ffs.

I agree with you, there are also too many people living with blinders on, limited horizon, too little imagination and a lot of obviousness in their own narrow home enclosure, even between drone pilots, common people, after all.

 

[erkme73]

I remember way back before I got my P1, I bought a bunch of the UDI micro quad copters with orange and black blades. They were a great way to get the muscle memory for controlling w/o GPS or other sensors. What as interesting is on identical units, some would self-right on power up, others would simply drive into the ground. There was no difference is version or model number between them - even on the PCBs. Yet, on throttle up, if held upside down, it would immediately snap right-side up before hitting the ground. Others, if started upside down, would just pull straight down.

There was no way to test for it other than to see if only two props spin up (when started upside down), or if all four did. I may have to try that. Though, with me being at Disney, I'm thinking I've got other variables at play.

 

[ccpyle]

I had a friend do this with a Phantom2 - multiple times in the same descent. He would cut motors, let it fall for a while, start them back up, get an altitude reading, calculate time vs. altitude delta, and then do it again. Same situation - he was really high up and the battery would have died before he got down.

 

[F6Rider]

Really? Its not a 3D stunt race quad? I had no idea it even had a camera on it wow who would of guessed its made for aerial photography weird

Lol Now all that was implied common sense

So.. have you tried it yet? all of this is speculation until you do, just make sure you have care refresh, please report back on the results. (include video please)

 

[tommyrich]

I have done this with a toy drone messing around that weighs about .40 pounds from about 150 feet took almost 60 feet to get the motors back on and it still hit the ground. Do that with the Mavic it's exploding on impact lol

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