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Comment by crazygringo | Hacker News Reader

Wow, that's shockingly low. The paper doesn't describe why -- do you have any idea what part(s) are failing so quickly? The rotors, the motors, the battery?

For commercial delivery service, it seems quite clear that we'll need either

1) "enterprise-grade" drones (like difference between consumer and enterprise SSD's),

2) reliable sensors to detect imminent failure before a flight,

and/or 3) easy replacement of failing parts.

I mean, for many hobbyists 100-200 hrs seems like a reasonable cost/benefit tradeoff... but not even close for commercial delivery services.

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> do you have any idea what part(s) are failing so quickly? The rotors, the motors, the battery?

It doesn't really matter what fails, what matters is that there is no way to do a controlled descent once anything goes wrong on a loaded drone because there is no margin for error. A drone this heavy relies on the whole chain of components functioning perfectly.

I'm surprised it is as good as it is given the number of parts. If it were a winged drone you'd at least have a chance to glide it to a safe spot, but a drone that is kept up by spinning props can only crash uncontrolled. There was a video of downing one in a more controlled way but the only fault it could cover was single rotor failure and even then the amount of control was very limited.

These things have absolutely no business over areas occupied by people.

Why not autorotation? https://wikipedia.org/wiki/Autorotation

Is it less mass and therefore momentum in the rotor/drivechain?

That needs forward flight to work. A quad copter does not have a glide ratio high enough that it would be able to start up the rotors to spin fast enough to get some lift out of them. A quad copter is best modeled as a brick with a magic anti gravity device attached, if the anti gravity device fails it is a suspended brick and gravity will soon assert itself.

A real helicopter (or an auto-gyro) is best modeled as a rotating wing, as long as the blades rotate you can control the craft to greater or lesser extent. It still won't be pretty in a proper helicopter but at least you have a chance.

Additionally, you need "collective pitch" control, i.e. the ability to angle the rotor blades for windmilling and then for hover flight right before touchdown. Multicopters just have fixed-pitch rotors and rely on differential torque/thrust for attitude control and ascend.

>A quad copter is best modeled as a brick with a magic anti gravity device attached, if the anti gravity device fails it is a suspended brick and gravity will soon assert itself.

Very nice definition.

There isn't AFAIK such a neat distincion in English, in Italian the iron (the device you use for ironing, maybe flatiron?) has a specific name "ferro da stiro" so it is common to use it as a term of comparison for such a behaviour.

They have parachutes installed. In the 2nd crash it deployed but the line hit a sharp part of the craft and was severed. Upgrades are being made (redundant cords and metal braiding) and the whole fleet was grounded in the meantime.

> what matters is that there is no way to do a controlled descent once anything goes wrong on a loaded drone because there is no margin for error.

Not entirely true. The drone had a parachute system to allow a controlled descent in situations like this and an alarm to alert nearby people to its descent. The problem here was that the parachute's single tether was severed shortly after deployment.

> The drone had a parachute system to allow a controlled descent

The parachute would allow the drone to avoid a rapid descent. Most parachute systems are not steerable/flyable so they won't make it a controlled descent.

Parachutes won't control the descent. Might slow it down a little, but that is of arguable value if all 10 kg landed on top of those kids mentioned in the article.

They don't control the descent, but they do control the descent. That is the descent (rate thereof) is controlled to a safe speed while the alarm alerts those below of its approach.

Is there any reason a drone couldn't autorotate like a helicopter does to slow its descent?

For autorotation you need to passively stay upright and a certain propeller area to weight ratio.

All of which are not present in a typical multirotor.

And you need pitch control ("collective").

Helo autorotation largely requires the momentum of the blades to keep going after engine failure (as well as the pitch and forward momentum). I don't think the blades on small drones are heaby enough to store enough momentum for autorotation.

More importantly, most drones cannot physically adjust the tilt of the rotor blades (which is called collective pitch control in helicopters). Helicopter autorotation requires rotors with a lot of angular momentum and usually an abrupt change in the collective pitch right before landing to turn that momentum into thrust and slow the helicopter down.

There are RC aircraft with collective pitch controls, like most RC helicopters and even some multirotors. These RC aircraft generally can autorotate, as well as do things that are probably not practical for full-scale aircraft to do, like fly upside down. :)

A collective pitch quadcopter flying upside down: https://youtu.be/MVL_Hf4ilLg?t=162

An RC helicopter doing, well, all sorts of acrobatics that hardly make sense: https://youtu.be/KmPchrGW1TQ?t=48

An RC helicopter autorotating: https://youtu.be/bxQ5kwqiPN4?t=136 (this one has an internal combustion engine, although that shouldn't make much of a difference for autorotation)

>that hardly make sense

Just a guess: stronger strength to weight, and power to weight ratios, compared to the real thing. The tail rotor in particular appears to be quite a bit more powerful for the weight of the model than would be expected in a full size aircraft. And then also, these kinds of movements with a person inside? Haha! No!

Yeah, I've been pretty deep in the RC community, but what doesn't make sense to me is the physical skills of pilots like that! Handling orientation changes like that is insanely difficult. I practiced flying RC helis for a while in a simulator and even the basic orientation exercises are pretty challenging.

A helicopter's rotor has a complex control system that has you changing the pitch of the rotor blades and the angle of the rotor disk, with the rotor behaving similarly to the wing of an airplane. Quadcopters usually rely purely on thrust for control. For example to bank, the motors on one side spin faster (more thrust) and the motors on the other side spin slower (less thrust). If you lose a motor, or lose the ability to control a motor, you lose the ability to control the quad.

In comparison, a helicopter will have a clutch that can disengage the rotor from the engine and let it turn freely. The pilot still has the ability to control the rotor, so she has some control during the autorotation. At the end, just before landing/impact, she will perform a flare, using the rotational energy in the rotor to slow the helicopter's descent.

  If it were a winged drone you'd at
  least have a chance to glide it to
  a safe spot, but a drone

The last time I looked at the retail quadcopter/RC plane market, when faced with a communications failure most of the quadcopters would return to the launch point and safely land. None of the RC planes had such a function - most seemed to just turn off the propeller and glide in whatever direction they were facing at the time.

Seemed to me the quadcopter response to communication failures was a much safer one. Is this still the case? It was quite some time ago.

>The last time I looked at the retail quadcopter/RC plane market, when faced with a communications failure most of the quadcopters would return to the launch point and safely land. None of the RC planes had such a function - most seemed to just turn off the propeller and glide in whatever direction they were facing at the time.

Once you have a quadcopter you've got enough digital hardware and software that adding "return to base" is basically free since it's just an additional software routeine.

RC planes don't already have the required hardware/software to implement a return to base feature because they don't need it and of course nobody is going to greatly increase the cost and complexity of the product just to add that one feature.

A handful of the quad controllers also support planes. I have multiple RC planes with that functionality - it adds a little complexity but not a ton.

> The last time I looked at the retail quadcopter/RC plane market, when faced with a communications failure most of the quadcopters would return to the launch point and safely land.

That isn't really accurate at all.

> https://hobbyking.com/en_us/turnigy-t1000fc-auto-pilot-syste...

>https://www.amazon.com/Parrot-Disco-FPV-minutes-goggles/dp/B...

Very few micro and mini quads have this functionality.

Return To Home functionality is common any medium and larger RC aircraft intended for FPV flying.

Often fixed wing RC with RTH functionality won't attempt to land, it return to home and enter a circular holding pattern until contact is re-established or the battery fails.

I tested some quadcopters and all had the behavior to land immediately on connection loss, no matter what is below them. Better not have something like that happening above a pond. So no automatic navigating to a starting point. It has been a few years, maybe that changed.

3D Robotics Solo has that feature (return to launch), but that was 5 years ago now. You could also set it so when the battery started to get too low it would return home as well. But that was one of the most advanced hobby drones in terms of software that existed at the time. DJI now uses it as well. It's all built into ardupilot. http://ardupilot.org/

maybe you could have started a new comment thread since your post doesn't remotely answer the question you quoted

Is it possible to design a self-destruct sequence so powerful that the entire thing is fragmented to dust over a wide area?

I'm sorry, did you just suggest we pack explosives in commercial cargo drones as a safety measure? Explosives enough to vaporise 10kg drone plus its cargo. Are we looking at mini nukes here?

I don't know, as you can tell I don't know much about explosives. But you wouldn't need to literally vaporise it, I guess, and I doubt it'd need a miniature nuke.

In my defence, a quick search shows this story:

https://www.digitaltrends.com/cool-tech/amazon-delivery-dron...

So apparently it's not a stupid suggestion and Amazon are thinking along the same lines.

I only ask because it seems making drones land safely is hard. I mean, a paracute with a whistle is a great idea until it drifts in front of a car moving at high speed, or lands on a deaf person or shorts out a high-voltage cable. They all have problems. If there was a way to convert the failed drone into widely dispersed very small objects, the harm may be much lower.

Sure, but it might explode near people

I didn't say it had to be explosive, but at any rate, you know anyone can buy fireworks right?

I think the combination of being airborne, remotely controlled and high explosives might get you attention of a kind you would like to avoid.

> Wow, that's shockingly low. The paper doesn't describe why --

It's because many drones, even "commercial" ones, are built using hobby parts with poor QC. If you use high end components then the reliability will be up by a two orders of magnitude, as will the price.

(Also as mentioned by others, multirotors and especially quadrotors have many single points of failure and so even with reliable parts the whole contraption isn't very reliable.)

Commercial drones can push to 10^3 hrs but that's still far cry from aviation standards of 10^5 hrs. According to this nav system and fuel+actuators are two big failing components: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6165073/pdf/sen...

That article seems to be referring to winged UAV drones (e.g. Predator) not the quadcopter-type drones used for deliveries.

Good catch. It is possible for quadcopter-type drones to come with four extra motors that sit opposite of the traditional four motors and props and face down with slightly different sized props and rotating the opposite way.

In the case of failure of a prop in this configuration (the most common failure in my own experience with quads and octos), it is possible to program the n-copter to automatically change to flying in 7 prop mode or 6 prop mode or 5 prop mode, and so on and so forth as a redundancy feature. This could help with a not insignificant amount of crashes but obviously comes with increased costs.

Airplanes get regular (and quite extensive) maintenance though.

> do you have any idea what part(s) are failing so quickly? The rotors, the motors, the battery?

My bet is on the power electronic semiconductors, mostly MOSFETs, in the motor controllers. Life time of these components decreases exponentially with the temperature difference they see in the semiconductor junction, i.e. the difference between hot (current is flowing, heating the junction with a power of I^2 * R_ds_on) and cold.

To make this deltaT_junction small, you need to choose big components, but at the same time you want to save weight, which is fundamentally at odds with overdimensioning everything.

Those are solid state components, though, and they exist (or can) in the downwash of the rotor, so it really shouldn't be that hard to keep them cool.

I'd think the motor and rotor itself would be much more likely to be the problem.

I don't know of any multi-rotor craft that incorporates the ESC (electronic speed controller) in such a way as to be cooled by the downwash of the props. Not saying there aren't any, but I haven't seen it.

Most hobbyist multirotors use smaller controllers (which are essentially a PWM'd set of mosfets) zip-tied or otherwise affixed to the arms of the machine, with short connections to the motors and to the controller/and power-supply distribution board on the body.

Consumer multi-rotor craft will generally have the ESCs integrated on a motherboard or daughterboard in the main body of the craft (cheaper to manufacture), well away from the prop wash and enclosed (not good for cooling).

I'm not sure what commercial (or large-scale) multi-rotor craft use, but it is probably similar to the others.

Again - I'm not saying it hasn't been done, or couldn't be done; in fact, it seems like a good idea. I can even envision how it could be integrated on the motor itself (board on the back with mosfets, with an extended shaft with fan for cooling the mosfets (inrunner motor), or with the fins of the mosfets in the wash of the prop, or something like that for an outrunner motor).

It wouldn't surprise me if such motors don't come on the market (or maybe they already exist - I haven't looked)...

Ok - well, apparently the idea isn't a new one:

https://www.rcgroups.com/forums/showthread.php?1149319-brush...

...and it has been tried and sold to some success. So I imagine such motors are still available somewhere. TIL.

A helicopter can autorotate when its single engine quits, or continue flying when one of its multiple engines quit. Quadrocopter requires all four motors to work to remain in the air. This is a reasonable tradeoff for light hobby drones, but not for heavier drones for commercial delivery.

This isn't quite true. Cheap drones may not have this capability, but controlled, safe landings of quadrotors with a single failed motor have been demonstrated, see this ETH Zurich video from 2014:

https://www.youtube.com/watch?v=ek0FrCaogcs

Obviously there are still other single points of failure such as the battery or controller, but at this point there's no excuse for crash landing a commercial drone just because a single motor (or rotor) has failed.

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