Hacker News Reader

Comment by crocal | Hacker News Reader

Before talking about anti-safety culture, I observed that the real issue in a purely hardware/software-driven culture is that people focus on technology. For safety systems, this usually fails / overbudgets because there needs to be some systems thinking /before/ about why and how the proposed system can be declared safe. In the case of drones, I am afraid that the systems design to make these devices acceptably safe is yet to be formulated (Note: these are not planes, they fly close to the ground so they do not benefit from the safety of altitude to have time to react in case of failure)

View on HN

Safety, very much like security, needs to be built in during the design phase - or you're trading off the development cost for an expensive retrofit while gambling with the risk of killing the project (or even setting back that entire class of transportation) by killing people.

Can't emphasize that enough. I read a book in 80's on safety critical design and that was the first rule. Safety has to be a primary design goal from the start or you are hosed.

Helicopters are certifiable for carrying passengers, and yet take offs and landings have no altitude nor speed benefits.

In what way do multirotors / drones differ?

I know exactly nothing about aircraft safety, but since this is HN, I want to throw out a guess: in case of landing, could it be that helicopters make the blades spin very fast prior to beginning the landing maneuver, compensating for extra lift with pitch control, and then keep them spinning fast throughout the entire landing process, and brake them only after touchdown? This would, in my mind, let them do a soft touchdown via autorotation in case of sudden engine failure at any point of the landing maneuver (at least if the pilot has fast enough reflexes).

That does sound like a reasonable guess.

Can anyone with more knowledge chime in?

I’m off to bed, otherwise I’d try looking in to it.

I'm going to go on a limb here, you'd need to put enough energy into the rotors of: helicopter weight * recovery time (so distance * descent rate) * gravity². that is a big number to get rotor mass * rpm to equal.

Autorotation is already the standard emergency landing practice in case of loss of engine power mid-flight; I'm only guessing that landings could be performed in such a way as to maintain the capability for autorotation throughout the maneuver.

That's what I was trying to explain, that energy to maintain rotor speed in an autorotation is stored in the height. To replace that is a damn big number you're putting somewhere else. Maybe landing rockets like the soyuz.

In theory, yes. In practice the blades don't have enough angular momentum for this to work longer than a very short time, which is why the first step a pilot must take upon engine failure is to slam the collective down so the air flowing past the blades will keep them spinning in the same direction.

Some helicopters with very heavy blades with lots of angular momentum have been built, but it makes the blades heavy which requires a more powerful engine which makes the whole craft heavier which means you need even more angular momentum...you get the idea. It's an engineering nightmare so it's atypical.

If the engine fails in a helicopter, and you have enough altitude, you can autorotate the rotors for a soft landing. It's the helicopter equivalent of gliding in a fixed wing aircraft. The danger potential is similar to a fixed wing aircraft - landing and takeoff are the critical stages, but normal flight gives plenty of opportunity for a safe-ish forced landing.

A multirotor on the other had doesn't have the ability to glide, like a fixed wing or a helicopter. Any safety will have to be engineered in by making sure that a single rotor failure (or even multiple) can be coped with correctly and reliably.

I suffered a battery lead failure on a large hexcopter this summer at ~27 ft. It was spectacularly destructive to the airframe. Had it been a motor/prop/ESC, it would have had enough thrust to land (awkwardly), but cutting power to all 6 motors at the same time (flight controller has a separate battery)... not recoverable.

I know copters have autostability as their benefit - the big rotor has inertia on its side and keeps it aloft and even a loss of power would fall slower.

Multirotors I believe require active control of every individual rotor instead of straightforward big angular momentum. Which I believe is part of why they are seen on drones.

Riffing on this a little bit based on my experiences with multirotors but not having any kind of "autorotation" capability with them:

Most motors on multirotors are BLDC motors that require active control to make them spin. Voltage gets applied to the motor phases sequentially to make it spin. However, when they're completely open-circuit, they actually spin quite freely! I've seen the ones I had (26" prop) windmill quite well in a mild breeze. The trick, though is that you're probably not going to get much descent-arrest out of open-circuit BLDCs. Another possibility is short-circuited BLDCs; in that condition, the rotos are quite tough to turn. My speculation is that the props wouldn't spin much at all in a short-circuit condition. Buuuuut... if ESCs were able to modulate the open/short-circuit condition (even without power to actively spin the motors), I suspect you might be able to get some kind of arrested descent situation.

I've been thinking of building an ESC from scratch this winter... this might be a useful feature to experiement with!

Helicopters have a height-velocity diagram with some areas marked unsafe (low speed, low altitude); when operating in the forbidden areas a pilot cannot be reasonably expected to perform an emergency landing using autoration following complete engine loss. In all other areas this should be possible. Coincidentally, that's why helicopters avoid taking off vertically (they only do that in video games...); they want to gain some speed before gaining significant altitude, because forward speed helps autorotation. If a helicopter just climbs straight up at zero speed and the engine goes out, it's going to fall pretty much like a rock.

Average multicopter designs would seem to be incapable of autorotation for aerodynamic and technical reasons. The technical reason being that the BLDC motors used are typically permanent magnet synchronous machines, which develop significant holding torque in most (driver) failure modes and there is no clutch to disconnect the motor from the propeller.

There's also typically* no collective pitch control in small multicopters, so to autorotate the props have to have time to stop turning and reverse. Even assuming they could declutch from the motors they would usually crash first.

*There are some exceptions: https://www.droneguru.net/collective-pitch-drones/

Notably - this is what killed the billionaire owner of the Leicester City football club. He liked to land his helicopter on the field after the game and depart from there (against many people's advice) which prevented any forward momentum on takeoff or landing. Last year, he was taking off after a game and had an engine issue which resulted in the helicopter crashing into the parking lot killing everyone on board.

https://www.youtube.com/watch?v=Kvk6rslCDAI

Modern soft/hardware development looks like this: idea -> money. There is no real engineering in between except hammering on things until they get to "money".

Reply on news.ycombinator.com