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The Dubai Drone is an Ehang passenger drone planned to start commercial services in July 2017.

If there is a failure, can such an electric multi-copter auto-rotate a landing?

I'm wondering, if an electric motor would even allow auto-rotation, and if the autopilot could handle it? Are there regulations for devices like that?

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    $\begingroup$ It looks like you could attach a parachute to that, at least. $\endgroup$ – JAB Apr 24 '17 at 14:37
  • $\begingroup$ It is presumably possible to build a "drone" rotorcraft with an autopilot capable of flying an auto rotation of a suitable rotor system (though the make the best of a bad situation logic in choosing a forced landing site in unsuitable terrain may be challenging) however the rotor systems of that particular aircraft is not suitable. You'd use independent power systems and/or a ballistic parachute as JAB mentioned. $\endgroup$ – Chris Stratton Apr 24 '17 at 18:59
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    $\begingroup$ Why are people referring to these aircraft as "drones"? A drone, by definition, is robotic and lacking intelligence for rational self-interest (so are generally autonomous for our benefit or are remotely piloted) - it seems people are now generally referring to all quadcopter or multi-rotor aircraft as "drones". $\endgroup$ – Dai Apr 25 '17 at 8:18
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    $\begingroup$ This device is a drone, because it is remotely piloted. $\endgroup$ – mike Apr 25 '17 at 8:21

If a single motor fails, it's not unsafe (supposedly).

The 184 is technically an X-8 multi-rotor, meaning that it has four points of thrust, each consisting of two motor-propellers, coaxially aligned—one “tractor” propeller and one “pusher.” This means that if any one of the motors dies, or a propeller disintegrates, the aircraft won’t flip and crash to the ground, although it would need to land rather quickly. The concept has never been tried in an operational passenger service, however, and some feel that without further safety enhancements the drone taxi idea is a disaster waiting to happen.


The article also discusses how Dubai's hot climate would limit the performance.

As for regulations, so far I can't dig up any, but in the US taxi drones are not allowed.

FAA regulations prevented it from being legally tested in the US. In Dubai, however, the Civil Aviation Authority partnered with EHang in testing the device.


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    $\begingroup$ I agree, though it's hard to find tests that prove it, if 1 of 8 props fail, you could still execute a powered landing. It might be a bit rough, and you really might not have a large range of "where" you were going to land, but you should be able to hit the "next flat spot". Of course all bets are off if the passenger starts freaking out.To correct for the missing prop the passenger would need to sit very still. $\endgroup$ – coteyr Apr 24 '17 at 13:57
  • $\begingroup$ Also it's important to remember "down is easy". 100% of the remaining power and "effort" can go in to simply slowing down the "drop" and making sure you "land" slow enough. $\endgroup$ – coteyr Apr 24 '17 at 13:59
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    $\begingroup$ @coteyr: Actually, "100% of the remaining power" would generally be a bad idea, since you do want the thrust to be approximately symmetrical or the thing will flip over. If you lose one rotor out of eight, you'll probably need to shut down another one (or, more likely, slow down both of the two) on the opposite side. $\endgroup$ – Ilmari Karonen Apr 24 '17 at 17:24
  • $\begingroup$ I didn't mean that you turn the props on 100% I mean that you don't need "save" power for going 100 miles you just have to go down. $\endgroup$ – coteyr Apr 24 '17 at 21:07
  • $\begingroup$ @IlmariKaronen: Assuming the rotors are all mounted with the diagonals intersecting in the vertical axis through the centre of gravity, even if three rotors on the same diagonal fail, you would still have one rotor left to prevent the thing from rolling around the other diagonal and four rotors for slowing the fall (and stabilizing over the other diagonal), I believe. $\endgroup$ – Jan Nash Apr 24 '17 at 22:01

That drone has fixed-pitch rotors, and that pitch is optimized for thrust, not for autorotation. In the absence of power, those rotors won't autorotate. They will stop rotating 'in the right way' and then start windmilling in the opposite sense. That windmilling will cause drag and some deceleration of the fall, but not of the same magnitude as an autorotation, that is a different condition.

  • $\begingroup$ So it will drop quickly? $\endgroup$ – mike Apr 24 '17 at 7:20
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    $\begingroup$ @mike And out of control. The only control of the rotors in that type of drone is by modulating the power of the motors, since the rotors themselves are fixed-pitch and not fully steerable. In the absence of power, there will be little control, if any... $\endgroup$ – xxavier Apr 24 '17 at 7:28
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    $\begingroup$ Also, the inertia of the small rotors looks too small to store enough energy for deceleration. Even if id did have variable pitch props, autorotation will only reduce the sink speed a little. $\endgroup$ – Peter Kämpf Apr 24 '17 at 7:29
  • $\begingroup$ @Peter Kämpf I'm not sure... The total disk area is relatively large... It's true that the energy of the rotors is very small, making a final landing flare impossible, but the sink speed will be considerably reduced... (in true autorotation, achieved with the right pitch setting...) $\endgroup$ – xxavier Apr 24 '17 at 7:40
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    $\begingroup$ The way you would make something like this safe would be to have sufficiently independent power systems for each motor that they wouldn't all go down at once. $\endgroup$ – Chris Stratton Apr 24 '17 at 18:55

Simple answer. No.

A drone with fixed wing propellers such as the one you describe can not autorotate. For autorotation to work the pitch of the blades needs to be variable.

At the instant of engine failure, the main rotor blades are producing lift and thrust from their angle of attack and velocity. By immediately lowering collective pitch, which must be done in case of an engine failure, the pilot reduces lift and drag and the helicopter begins an immediate descent, producing an upward flow of air through the rotor system. This upward flow of air through the rotor provides sufficient thrust to maintain rotor rotational speed throughout the descent. Since the tail rotor is driven by the main rotor transmission during autorotation, heading control is maintained as in normal flight. - https://en.wikipedia.org/wiki/Autorotation

A drone or multicopter with 6 or more propellers can continue flying as long as the thrust provided by the remaining propellers and engines is sufficient to keep it from falling.

The aicraft may however begin to rotate around its own axis. In normal flight there are an equal number of clock wise and counter clockwise rotating propellers. If a propeller/motor stops spinning the torque of these engines will no longer cancel each other out so in order to not descend the helicopter may be forced to accept a rotation around the Z axis. This same mechanism is used for turning a multirotor, lower the RPM of the propellers spinning in one direction and you will turn in the other.

I fly RC drones myself and have tested how my hexacopter behaves during a motor failure. The conclusion I came to is that if it was close to its carrying capacity it would rotate since it needs all the power it has and can't worry about rotation. But if it only carried a light load it woulden't rotate as it has power left over to compensate for it.

There are however a few attempts at making drones with variable pitched propellers Variable pitch drone

This however negates the pure simplicity that a drone provides as one of it's main advantages is that there are only as many moving parts as there are propellers.

In my opinion and others (http://diydrones.com/forum/topics/why-are-we-not-seeing-more-variable-pitch-quadcopters) the only way to provide a reliable drone is to have 6 or more propellers and lots of power to spare.

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    $\begingroup$ "A drone or multicopter with 6 or more propellers" You mean 4. One could design a drone that had 4 rotors that could handle a motor out condition. $\endgroup$ – Sam Apr 24 '17 at 14:12
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    $\begingroup$ @Sam true, although I have yet to see one. Also, with only 4 props loosing one would mean loosing a substantial amount of the total thrust which would mean that you would have to create something very overpowered or be forced into a descent upon failure. In addition the CG of the craft would be much more crucial and even a small mistake or misloaded cargo would cause a crash. $\endgroup$ – JensB Apr 24 '17 at 14:27
  • $\begingroup$ I think an odd number of rotors is the best. Like for multiblades propellers or fans, if you lose one blade, desiquilibrium is less dramatic with an odd number of blades than an even one. For instance, heptarotors are safer than octorotors, since they can both lose a maximum of 4 rotors. 4/7 is better than 4/8 ratio. $\endgroup$ – qq jkztd Apr 24 '17 at 15:14
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    $\begingroup$ @qqjkztd What? "heptarotors are safer than octorotors" they may be more "safety efficient" but are not generally safer. JensB is correct the torque of rotation is a design problem with odd rotor numbers. Also when you get beyond 1 lost rotor, you have to account for where and how the symmetry works out. $\endgroup$ – Sam Apr 24 '17 at 16:09
  • $\begingroup$ @Sam I'm talking about contrarotative propellers driven by one engine, like seen in related question about Dubai drone. There is no yaw stability issue when losing one or more engines, since each brushless engine dirves two propellers (1 fixed axle - stator 1 prop - rotor 1prop) $\endgroup$ – qq jkztd Apr 24 '17 at 17:10

I am a career helicopter pilot who enjoys watching the development of drone technology. I can tell you for certain that this aircraft is incapable of autorotation. A parachute would be it's best option for pure simplicity. This would not guarantee a safe landing, just avoid a 'splat'. It is very apparent to me that the number one challenge of the drone industry as it relates to increased payload (passengers, cargo, luggage) is power. Drone advancement is highly reliant on battery advancement. If you have a lightweight, high powered battery system, then the sky's the limit for drones, safety and otherwise. Short of that, I'm still waiting for the big breakthrough. For now, I'd just like to see y'all drone drivers get an anti-collision light. Oh wait...more power is going to be needed for that :)


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