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Comment by advisedwang | Hacker News Reader

I'm very skeptical of their claims of "unparalleled safety". Thanks to "autorotation"[1] regular helicopters can safely land in most situations even with complete power failure. The claims of safety of this craft seems to derive from N+1 battery redundancy which isn't that impressive. They say they have 8 independent motors, but don't say how many are needed to stay aloft.

[1] https://en.wikipedia.org/wiki/Autorotation

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Small "personal" helicopters like the Robinson R22 are quite difficult to autorotate safely in a real emergency, due to low rotor inertia. [0]

For the SureFly, Hypothetically if loss of several motors caused the aircraft to be unable to maintain altitude, the aircraft would not suddenly drop like a brick, but drift down to a controlled landing.

[0] http://philip.greenspun.com/flying/robinson-r22

No, worse. That thing would turn into the tumble dryer from hell with partial power. It has no blade pitch control (like a helicopter does), requiring power on the motors in all four corners to maintain stability.

The real emergency situation to worry about would be a total power failure. Failure of the individual electric motors will likely be extremely rare but a battery fire or controller failure could send this thing falling like a rock due to the lack of auto-rotation. It has a BRS chute but that won't do anything in the critical ~500ft band above ground level, too low to deploy normally. Additionally, I hope they've figured out how to guarantee safe chute deployment while inverted because of the tendency to tumble with no/partial power.

BRS parachutes have been tested as low as 260' AGL at 187 knots (several times faster than this craft would ever go) and individuals have reported successful deployments below 100' AGL[0] The question of course is how quickly it can be deployed in this craft as the vertical speed will likely be much faster than a Cessna or Cirrus in the event of total power failure.

[0] https://www.flyingmag.com/g00/how-it-works-brs-aircraft-para...

I can't imagine an inverted deployment at 100' would end well...

For a drogue-deployed parachute, horizontal speed helps, as the required travel through the air is accomplished faster, and therefore with less drop. BRS parachutes use a rocket to fully extend the lines, but there is still the matter of inflating the canopy.

> For the SureFly, Hypothetically if loss of several motors caused the aircraft to be unable to maintain altitude, the aircraft would not suddenly drop like a brick, but drift down to a controlled landing.

Source? I'd think loss of two stacked rotors would lead to loss of control, tumble, and dropping like a brick (until BRS deployed), not controlled descent and landing.

there's a parachute

that's what I mean by BRS deployment.

https://en.wikipedia.org/wiki/Ballistic_Recovery_Systems

my mistake. thanks for the link

Robinsons in general have this problem (among others, such as their tendency to disassemble their own rotor systems in flight, and their incredibly awkward t-bar cyclic controls), but I'd still rather take my chances with a Robinson autorotation than rely on a ballistic parachute system.

That said, I would never willingly fly this thing or a Robinson, so I guess I'll never have to deal with either situation.

Electric motors are less complicated and more reliable than piston engines (as in small helicopters), I'd say (not sure about turbines). And then there's the ballistic parachute rescue system, though of course there's a minimum altitude you need for that.

The Volocopter (a competing concept that actually exists and has had manned flights since 2016) has 18 rotors (six arms with 3 rotors each in a single plane), opposite rotors driven by the same battery/drivetrain ensemble, from what I gather, and remains controllable with up to 3 rotors (at least) faulty.

I'd hope well engineered aircraft of this sort will surpass safety of traditional helicopters, but let's see.

Yet even with autorotation, helicopters are somewhere between 27 and 85 times more dangerous than cars (depending on your methodology) [1].

[1] http://www.slate.com/articles/news_and_politics/explainer/20...

They got a parachute, and with a rotor layout like this, attaching a parachute safely is pretty simple.

Going down on a parachute is not exactly comfortable, but at least you're not going to crash at the earth like a falling rock. It turns a catastrophic crash into an unpleasant but likely quite safe collision.

Could it be because autorotation requires a skillful, vigilant pilot, and the redundant systems of this machine help avoid urgent need for it?

Autorotation is often taught after only 8 hours of training. It is very feasible to have an AFCS that will do an autorotation, but it is harder to find a place to land.

There’s also a parachute.

Should be four or less considering each arm has two props. (Unless the motors aren’t powerful enough)

A lift/weight ratio of 2:1 would be quite remarkable, and the ceiling of a mere 4000' suggests that it does not have that sort of reserve, even at sea level and when being powered by the gasoline motor. Being anywhere somewhat hot or high would quickly erode that margin.

On the other hand, the chances of four motors failing in the time it takes to set down is probably not high. Back to the first hand, if you do have multiple failures, it is not likely to be an optimal grouping.

If all four on one side went out, I assume you'd be in big trouble.

I'd imagine if two stacked ones go out, you'd be in trouble (as the centre of gravity will not be exactly in the intersection of the two diagonals).

Actually that's a benefit of the quad-copter design. It can sacrifice an axis of control in that situation (a "minor" one, like rotation about the gravity vector). In this case it would likely make the passengers sick, but it would still be able to fly in a controllable fashion.

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

with the counter-set props probably less spin even

Yes, you go from 8 df in the control to 7 df (instead of from 4 to 3). So, you could basically "hang" the weight off the remaining diagonal, and then use the working off-diagonal rotors to keep roll/yaw/pitch in line. But then the rotors on the diagonal have to generate twice the force. I'd rather not be a pax in this when two rotors fail.

And if one on one side went out, you'd immediately descend.

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