6 ms·

>That has four sets of contra-rotating props, so there's some redundancy That is no redundancy. With only four booms, it only takes a single boom failure (elec

by dTal 7y ago

>That has four sets of contra-rotating props, so there's some redundancy That is no redundancy. With only four booms, it only takes a single boom failure (electrical or mechanical) to crash. And with co-axial counter-rotating props, a prop failure will almost certainly cascade to its partner - so again, you really only have four props. There is almost no survivable failure mode with that design.

de_Selby 7y ago
If just one wheel fails on a car travelling at 100km/h on a road with oncoming traffic you're very unlikely to survive either.
- Ntrails 7y ago
  A 200km/h head on collision with equivalent modern cars is absolutely survivable. Gravity is pretty potent at getting you moving faster than that, and with extremely uncertain impact point
  - underwater 7y ago
    Terminal velocity for a person is around 195kph.
    - Retric 7y ago
      That’s minimum arms out speeds for a human, you can hit 400+kph in a streamlined dive. Failing aircraft can go well past that into ripping of the wings of an aircraft.
  - jfindley 7y ago
    I know we're veering somewhat off topic here, but I wonder how true this is in practice? I can't find a lot of testing done at this sort of speed with regular cars. Racing cars and some high-end sports cars are exceptionally strong and allow you to survive a 200 km/h crash - often with only very minor injuries - but I'm really uncertain that this is true for a regular family car. Do you have any further data on this? Given how many regular cars I see driven at this sort of speed when I visit Germany, it'd be interesting to know!
  - marcinzm 7y ago
    It's actually a 100km/h collision done twice. Each car goes from 100 to 0, not from 200 to 0. There's twice the energy but it's distributed across the two cars so for each one it's equivalent to hitting a stationary wall. That said, based on this video I don't think an actual 200km/h is anywhere near survivable for the average modern car: https://www.youtube.com/watch?v=LmRkPyuet_o https://www.youtube.com/watch?v=LmRkPyuet_o Possibly higher end sports cars with cages and five point harnesses may fare better.
    - todd8 7y ago
      A 100km/h head-on with a similar car going 100km/h should be similar to a 100km/h head-on collision with a solid concrete wall. However a car traveling at 200km/h has four times the kinetic energy as a car traveling at 100km/h so a collision at that speed is much less likely to be survived than a collision at 100km/h.
      - matt-attack 7y ago
        Wow you are the first person to point out what I’ve been saying for decades. A car hitting another car at the same speed is indistinguishable from a car hitting an immovable wall. Intuition wants to think it’s twice as bad. As an aside can I pose another physics car challenge to you that 100% of people I’ve asked get wrong? You’re the only car at stop light waiting for the light to turn green. You look up in your rear view mirror and see a car bearing down on you presumably with a distracted driver. You know you’re about to get rear-ended. In that split second do you: A) Let up on the brakes and put the car in Neutral or B) slam in the brake as hard as you can.
        deleted 7y ago
        [deleted]
        dzdt 7y ago
        The brake will increase survivability for you; neutral will increase survivability for the other driver. What hurts you is the sudden acceleration from the impact, and the brake will decrease that. What hurts the other driver is the sudden deceleration of impact; your brake will make that a harder stop for him while being in neutral would make it a bit less of a hard stop.
- Cthulhu_ 7y ago
  If it falls off that axle is only dropping 20cm though, instead of however high this thing flies. Plus, wheels are a lot more solid than props.
- marcinzm 7y ago
  Wheels are pretty solidly mounted to cars with large pieces of metal. Tires do blow out however then you still have some control. Props on the other hands can have electrical component failures, wiring failures, engine failure, bird ingestion induced failures, control instrument failures, etc. Remember that each vehicle has four times these components (ie: four times the failure rate) and the failure of any is a crash. Just a lot more potential things that can fail and cause a catastrophic accident.
- dsfyu404ed 7y ago
  Actually you're pretty likely to survive as long as you're wearing a seat-belt and don't crash into anything that only hits the upper half of the cabin (like the side of a big truck)
fooker 7y ago
>There is almost no survivable failure mode with that design. Parachute?
- nradov 7y ago
  Parachutes can help, but even with rocket assist there can be a "dead zone": too high for passengers to survive a crash but too low for the parachute to fully deploy. The aircraft also has to be fairly stable for the parachute to deploy successfully; if the aircraft has a sharp pitch or roll speed due to thrust imbalance then the parachute might not work.
  - tim333 7y ago
    Maybe a modified version of this skyscraper escape system https://i.redd.it/5ltd9qizlh621.gif https://i.redd.it/5ltd9qizlh621.gif ?
  - mrfusion 7y ago
    There should be ways to deploy parachutes faster. Maybe with springs or pressurized gas. Otherwise having large airbags deploy under the vehicle could be another option.
Erlich_Bachman 7y ago
I'm pretty sure that it is generally considered a more reliable design in both small drones and such vechicles. Yes, there are 4 booms, but there are 8 ESCs, 8 cable sets, 8 motors, 8 motor holders(they are the ones experiencing most vibrations), 8 props. All those 8x parts are much more likely to fail than the boom. How would the boom even fail mechanically if it's just a solid holding mechanical part that is well-built and tested? Would it just break off? Because of what? If so, in any regular normal airplane or a helicopter there are many other similar parts which are usually not redundant. This must be due to the fact that such solid parts fail very rarely, and perhaps more importantly, can be somewhat easily tested on the ground to check their structural integrity. Motors, ESCs, etc can fail due to many other reasons and are sometimes harder to test properly, there is much more risk in them. So yes there is redundancy. This configuration of props is called coax, and is specifically less effective than just putting 4 motors of higher power and bigger props. If there were no gains in reliability, no one would build drones like this, because they would just be less efficient and fly for less time. There is basically no other advantage in this setup than their better reliability due to redundancy. >And with co-axial counter-rotating props, a prop failure will almost certainly cascade to its partner Do you have a source for that? If a prop breaks, it will likely fly off onto sides due to centrifugal forces, not on the vertical axis into the other prop. Additionally if a bottom prop breaks, there is already a downward flow of air from the top prop, which will act as additional downward force on those breakaway parts which will force them to fly off downwards, not upwards into the remaining prop.
- dTal 7y ago
  My source is bitter experience designing and testing counter-rotating electric ducted fans for distributed propulsion drone applications. You have to consider specific failure modes. You might have an object strike, like a bird. You might lose sealing and get water in somewhere, frying anything electronic inside. You might have a loose bolt, a fatigued bracket, or a frayed wire. When you start considering scenarios rather than parts lists, it becomes clear that consolidating things together spatially costs you most of the advantage of redundancy. This is especially true in the case of props. In point of fact, if a prop breaks the net force is upwards, because that's the direction it was pushing in when it failed. However it's basically irrelevant - unless you perfectly and flawlessly detach the blades symmetrically with explosive bolts, with impeccable timing such that no detached blade hits the airframe, so much energy is released with a moving-part failure that anything in the vicinity is pretty well destroyed. Experience you might have with normal-sized drones is not applicable if you scale it up even a little - big props make a big mess, and you're lucky if the airframe survives at all. You certainly won't be able to count on that shared pod. We can only guess at why they chose to design it this way - personally I think they're a bunch of cowboys who will kill someone the way they're going. I suspect it's because they want to keep the footprint small, and because it's kind of uncool to have to pick your way through a dense ring of props to get to the cockpit (if you look at some of their videos you'll see that they've experimented with non-coaxial x8 configurations in the past). But stacking the props two-by-two like that only costs you reliability.
mhandley 7y ago
I would think that the most likely failure mode would be bird ingestion. A large bird would likely break the top prop, and the fragments of that prop+bird would hit the cowel, and then be sucked down through the bottom prop. Not to mention the high forces exerted on the top-prop motor mounting from losing only one blade. I'm not sure whether you want the top prop motor mount to fail at that point to save the airframe, but if so, then you'll get the top motor ingested into the bottom prop too. Unless you can actually prevent large bird ingestion, or design props that can survive it intact, I agree you don't really have any redundancy from co-axial contra-rotating props.
matt-attack 7y ago
A traditional helicopter had no redundancy either. The single motor could fail or a single blade. What’s the difference?
- mceachen 7y ago
  See https://en.wikipedia.org/wiki/Autorotation https://en.wikipedia.org/wiki/Autorotation
cal5k 7y ago
Ballistic parachutes. They've been a safety feature in composite aircraft like the Cirrus SR22 for a long time. Useless on commercial aircraft, great for small, lightweight personal transportation vehicles.

by dTal 7y ago

de_Selby 7y ago

If just one wheel fails on a car travelling at 100km/h on a road with oncoming traffic you're very unlikely to survive either.

Ntrails 7y ago
A 200km/h head on collision with equivalent modern cars is absolutely survivable. Gravity is pretty potent at getting you moving faster than that, and with extremely uncertain impact point
- underwater 7y ago
  Terminal velocity for a person is around 195kph.
  - Retric 7y ago
    That’s minimum arms out speeds for a human, you can hit 400+kph in a streamlined dive. Failing aircraft can go well past that into ripping of the wings of an aircraft.
- jfindley 7y ago
  I know we're veering somewhat off topic here, but I wonder how true this is in practice? I can't find a lot of testing done at this sort of speed with regular cars. Racing cars and some high-end sports cars are exceptionally strong and allow you to survive a 200 km/h crash - often with only very minor injuries - but I'm really uncertain that this is true for a regular family car. Do you have any further data on this? Given how many regular cars I see driven at this sort of speed when I visit Germany, it'd be interesting to know!
- marcinzm 7y ago
  It's actually a 100km/h collision done twice. Each car goes from 100 to 0, not from 200 to 0. There's twice the energy but it's distributed across the two cars so for each one it's equivalent to hitting a stationary wall. That said, based on this video I don't think an actual 200km/h is anywhere near survivable for the average modern car: https://www.youtube.com/watch?v=LmRkPyuet_o https://www.youtube.com/watch?v=LmRkPyuet_o Possibly higher end sports cars with cages and five point harnesses may fare better.
  - todd8 7y ago
    A 100km/h head-on with a similar car going 100km/h should be similar to a 100km/h head-on collision with a solid concrete wall. However a car traveling at 200km/h has four times the kinetic energy as a car traveling at 100km/h so a collision at that speed is much less likely to be survived than a collision at 100km/h.
    - matt-attack 7y ago
      Wow you are the first person to point out what I’ve been saying for decades. A car hitting another car at the same speed is indistinguishable from a car hitting an immovable wall. Intuition wants to think it’s twice as bad. As an aside can I pose another physics car challenge to you that 100% of people I’ve asked get wrong? You’re the only car at stop light waiting for the light to turn green. You look up in your rear view mirror and see a car bearing down on you presumably with a distracted driver. You know you’re about to get rear-ended. In that split second do you: A) Let up on the brakes and put the car in Neutral or B) slam in the brake as hard as you can.
      - deleted 7y ago
        [deleted]
      - dzdt 7y ago
        The brake will increase survivability for you; neutral will increase survivability for the other driver. What hurts you is the sudden acceleration from the impact, and the brake will decrease that. What hurts the other driver is the sudden deceleration of impact; your brake will make that a harder stop for him while being in neutral would make it a bit less of a hard stop.
Cthulhu_ 7y ago
If it falls off that axle is only dropping 20cm though, instead of however high this thing flies. Plus, wheels are a lot more solid than props.
marcinzm 7y ago
Wheels are pretty solidly mounted to cars with large pieces of metal. Tires do blow out however then you still have some control. Props on the other hands can have electrical component failures, wiring failures, engine failure, bird ingestion induced failures, control instrument failures, etc. Remember that each vehicle has four times these components (ie: four times the failure rate) and the failure of any is a crash. Just a lot more potential things that can fail and cause a catastrophic accident.
dsfyu404ed 7y ago
Actually you're pretty likely to survive as long as you're wearing a seat-belt and don't crash into anything that only hits the upper half of the cabin (like the side of a big truck)

fooker 7y ago

>There is almost no survivable failure mode with that design. Parachute?

nradov 7y ago
Parachutes can help, but even with rocket assist there can be a "dead zone": too high for passengers to survive a crash but too low for the parachute to fully deploy. The aircraft also has to be fairly stable for the parachute to deploy successfully; if the aircraft has a sharp pitch or roll speed due to thrust imbalance then the parachute might not work.
- tim333 7y ago
  Maybe a modified version of this skyscraper escape system https://i.redd.it/5ltd9qizlh621.gif https://i.redd.it/5ltd9qizlh621.gif ?
- mrfusion 7y ago
  There should be ways to deploy parachutes faster. Maybe with springs or pressurized gas. Otherwise having large airbags deploy under the vehicle could be another option.

Erlich_Bachman 7y ago

I'm pretty sure that it is generally considered a more reliable design in both small drones and such vechicles. Yes, there are 4 booms, but there are 8 ESCs, 8 cable sets, 8 motors, 8 motor holders(they are the ones experiencing most vibrations), 8 props. All those 8x parts are much more likely to fail than the boom. How would the boom even fail mechanically if it's just a solid holding mechanical part that is well-built and tested? Would it just break off? Because of what? If so, in any regular normal airplane or a helicopter there are many other similar parts which are usually not redundant. This must be due to the fact that such solid parts fail very rarely, and perhaps more importantly, can be somewhat easily tested on the ground to check their structural integrity. Motors, ESCs, etc can fail due to many other reasons and are sometimes harder to test properly, there is much more risk in them. So yes there is redundancy. This configuration of props is called coax, and is specifically less effective than just putting 4 motors of higher power and bigger props. If there were no gains in reliability, no one would build drones like this, because they would just be less efficient and fly for less time. There is basically no other advantage in this setup than their better reliability due to redundancy. >And with co-axial counter-rotating props, a prop failure will almost certainly cascade to its partner Do you have a source for that? If a prop breaks, it will likely fly off onto sides due to centrifugal forces, not on the vertical axis into the other prop. Additionally if a bottom prop breaks, there is already a downward flow of air from the top prop, which will act as additional downward force on those breakaway parts which will force them to fly off downwards, not upwards into the remaining prop.

dTal 7y ago
My source is bitter experience designing and testing counter-rotating electric ducted fans for distributed propulsion drone applications. You have to consider specific failure modes. You might have an object strike, like a bird. You might lose sealing and get water in somewhere, frying anything electronic inside. You might have a loose bolt, a fatigued bracket, or a frayed wire. When you start considering scenarios rather than parts lists, it becomes clear that consolidating things together spatially costs you most of the advantage of redundancy. This is especially true in the case of props. In point of fact, if a prop breaks the net force is upwards, because that's the direction it was pushing in when it failed. However it's basically irrelevant - unless you perfectly and flawlessly detach the blades symmetrically with explosive bolts, with impeccable timing such that no detached blade hits the airframe, so much energy is released with a moving-part failure that anything in the vicinity is pretty well destroyed. Experience you might have with normal-sized drones is not applicable if you scale it up even a little - big props make a big mess, and you're lucky if the airframe survives at all. You certainly won't be able to count on that shared pod. We can only guess at why they chose to design it this way - personally I think they're a bunch of cowboys who will kill someone the way they're going. I suspect it's because they want to keep the footprint small, and because it's kind of uncool to have to pick your way through a dense ring of props to get to the cockpit (if you look at some of their videos you'll see that they've experimented with non-coaxial x8 configurations in the past). But stacking the props two-by-two like that only costs you reliability.

mhandley 7y ago

I would think that the most likely failure mode would be bird ingestion. A large bird would likely break the top prop, and the fragments of that prop+bird would hit the cowel, and then be sucked down through the bottom prop. Not to mention the high forces exerted on the top-prop motor mounting from losing only one blade. I'm not sure whether you want the top prop motor mount to fail at that point to save the airframe, but if so, then you'll get the top motor ingested into the bottom prop too. Unless you can actually prevent large bird ingestion, or design props that can survive it intact, I agree you don't really have any redundancy from co-axial contra-rotating props.

matt-attack 7y ago

A traditional helicopter had no redundancy either. The single motor could fail or a single blade. What’s the difference?

mceachen 7y ago
See https://en.wikipedia.org/wiki/Autorotation https://en.wikipedia.org/wiki/Autorotation

cal5k 7y ago

Ballistic parachutes. They've been a safety feature in composite aircraft like the Cirrus SR22 for a long time. Useless on commercial aircraft, great for small, lightweight personal transportation vehicles.