4 ms·

more specifically - an octocopter scaled up to carry 2-4 peoples, not that big of a problem if you take time to look into it An octocopter will have even worse

more specifically - an octocopter scaled up to carry 2-4 peoples, not that big of a problem if you take time to look into it An octocopter will have even worse range than a normal helicopter, so you cant get very far, and everyone having flying cars based on pushing air will not be able to use them from the street due to the problem of blowing pedestrians over. Flying cars as envisioned in sci-fi require reactionless drives, at least if you want to be able to take off from the street without endangering everyone.

trhway 12y ago
>An octocopter will have even worse range than a normal helicopter, so you cant get very far not really. Octocopters are much easy for a scheme with tilting of a couple of rotors and thus having efficient plane style horizontal flight than tilting the whole machine like helicopter. As result - higher speed, longer range. > and everyone having flying cars based on pushing air will not be able to use them from the street due to the problem of blowing pedestrians over. You statement is true for helicopters. The octocopters are much efficient with pushing of the air. Google e-volo. https://www.youtube.com/watch?v=CzP0Zqxam7E https://www.youtube.com/watch?v=CzP0Zqxam7E (or the old work - Piasecki VZ-8 Airgeep - much less of airwash effect)
- lotsofmangos 12y ago
  Trust me on this. Blade efficiency is not only a swept area game, but also the middle of the blade is extra useless due to horrible vortex effects. One big blade massively outperforms eight little ones. And you cannot get away from the fact that you are hovering around a minimum of a ton. Either you have a really big cross sectional area of downwash from massive blades, or everyone falls over.
  - trhway 12y ago
    > One big blade massively outperforms eight little ones. taking just blades into account - yes, though not that "massively". But the moment we consider the whole system - attach the blades onto a shaft, connect that shaft to reductor, to gas turbine, etc... ie. if that system is helicopter than bigger blade is still better, but if we change the whole system, ie. octocopter built using electric motors - the equation starts to change and we have good efficiency with octocopter and, which is more important, - much better control (eight electric motors with some of them allowed to be tilted gives precise control compare to very sluggish system of one big rotor driven by one big gas turbine), which addresses your point about " hovering around a minimum of a ton" - impossible task to do safely for helicopter in urban environment and pretty easy to do with ducted octocopter (you can't really have ducted helicopter precisely because of the size of the blades) As far as i see, the lack of good control (not efficiency, range, payload, etc..) and direct danger from unshrouded blades is that doesn't allow to fly helicopters in any (even close to)urban environment, and especially for regular people.
- x0054 12y ago
  lotsofmangos is right! I have designed and built RC helicopters, tricopters, and quadcopters, from scratch. More blades is usually a lot less efficiency. There is something to be said for tricopters. From the point of view of pure efficiency, one large blade is always more efficient than 3 or more smaller once. Don't trust me, take a look at any static thrust calculator[1]. However, I found that with large and voluminous loads sometimes a tricopter can be more efficient because it allows you to mount the props away from the center load, so that the thrust of the prop isn't lost by pushing against the frame of the craft. This works in RC vehicles, but probably wouldn't be much of a benefit in full scale vehicles. As for the prop wash of an octocopter, try standing right next to one that's taking off caring just 5 kilos of payload (batteries and DSLR). I have, and trust me, it gets really, really, windy. And that's with a 5 kilo payload, with a copter capable of generating maximum of 12 kilos of trust. No scale that up to 1000 kilos. [1] http://personal.osi.hu/fuzesisz/strc_eng/ http://personal.osi.hu/fuzesisz/strc_eng/
  - trhway 12y ago
    >As for the prop wash of an octocopter, with correct design for the frame and ducts, the same airmass down flow would generate 20-30% more of static trust than the same airmass flow in case of helicopter. And that airflow will not have that blade end vortex which causes helicopter crashes and is just straight efficiency loss until that. I did spend my time with trust calculators and some other systems :)
    - x0054 12y ago
      Yes, but 8 props will be considerably less efficient then one prop in generating that thrust in the first place. For instance: One 300x8 inch prop will generate roughly 1 ton of thrust while spinning at 330RPM and consuming 12kW of energy. So, let's say we go with your prediction, and assign a 30% air flow efficiency to the one prop system. So in reality, according to you, it is generating 700 kilos of thrust. 700/4 is 87.5 kilos. So now we need to figure out how big of a prop we need to generate 1/8 of the thrust of the 300 inch prop, or roughly 88kg. I am going to use 60 inch props, because that will allow us to space 8 props in a circle roughly the same size as the original 300 inch prop. Ok, so a 88x8 prop, spinning at 2350 RPM will generate roughly 88 kg of thrust wile consuming 6.7 kW of energy! That's 6.7 kW x 8 props, or a total of 54 kW, or almost 4.5 times less energy efficient than the one prop system, and that's taking into account your proposed 30% efficiency gain from multi-bladed systems and ducts. I build these things all the time, there is a reason why the industry still uses helicopters with a single prop. It IS more efficient.
zackmorris 12y ago
That's not entirely accurate. Imagine a dozen 20 pound wild turkeys flying together. They are quite a bit more efficient than a single 240 pound craft/passenger because air displacement grows by the surface area of an object (by the square) while weight grows by the cube. That's why birds get off the ground easily while a human powered airplane needs a hundred foot wingspan. Mathematically, the efficiency of a helicopter rotor is roughly proportional to its length. Double the diameter and fuel usage falls by roughly half for the same lift. However the lift for an array of rotors grows by the square of length, even though efficiency stays constant. So if you start with a quadcopter running on batteries that’s 10 times as efficient as a full-scale helicopter running on gas, an array of them with the same lift will still be 10 times as efficient. It will also have a smaller cross sectional area at the cost of greater complexity. So the future of high efficiency lift is almost certainly in using arrays of rotors. The reason that hasn't happened yet is that most aircraft are designed for high speed travel, not hovering. I still want my reactionless drive though!
- lotsofmangos 12y ago
  "So if you start with a quadcopter running on batteries that’s 10 times as efficient as a full-scale helicopter running on gas" This I have never heard of. In a ratio of energy efficiency against carrying capacity I was under the impression that a normal helicopter massively outperformed a small quadcopter. My impression with doing lifts with lots of small drones was that you get around this by constantly cycling the swarm. Also, this still doesn't get around the problem of downwash. Either you shift a massive cross section of air for your force and get very little downwash, but then you end up with something much bigger than a car, or you focus it and get lots. You cannot get around the fact that to suspend a mass you have to move something equivalent downward. edit - I get what you are saying with the turkeys. However, in a vehicle carrying four people, you have replaced the turkey bodies with a giant blob that has much less lifting potential, especially at low speed, than lots of turkey bodies. You have to dump a lot of useful surface area. Also I note that turkeys do not hover much.
  - trhway 12y ago
    > a normal helicopter massively outperformed a small quadcopter. not really. The power loading (power divided by surface of the propeller's circle) is the key factor what matters for efficiency (until of course Reynolds number - air viscosity factor - starts to change - happens when we come to Kolibri scale) In general, helicopter with 3m long blades (6m diameter) on one 500kw turbine or something like 16 of 30kw motors with 1.5m diameter propeller (with the same number of blades as helicopter) each is pretty much equivalent. That is pretty theoretical starting point. After that we can optimize these systems in different ways. 16-copter allows to use somewhat bigger, yet less-bladed, like 2-3 bladed propellers which are more efficient than multi-blades of helicopter (and you can't really have 2 bladed rotor on helicopter). You can use ducts on 1.5m propellers - not a case for helicopter. So all this allows to increase efficiency.

trhway 12y ago

>An octocopter will have even worse range than a normal helicopter, so you cant get very far not really. Octocopters are much easy for a scheme with tilting of a couple of rotors and thus having efficient plane style horizontal flight than tilting the whole machine like helicopter. As result - higher speed, longer range. > and everyone having flying cars based on pushing air will not be able to use them from the street due to the problem of blowing pedestrians over. You statement is true for helicopters. The octocopters are much efficient with pushing of the air. Google e-volo. https://www.youtube.com/watch?v=CzP0Zqxam7E https://www.youtube.com/watch?v=CzP0Zqxam7E (or the old work - Piasecki VZ-8 Airgeep - much less of airwash effect)

lotsofmangos 12y ago
Trust me on this. Blade efficiency is not only a swept area game, but also the middle of the blade is extra useless due to horrible vortex effects. One big blade massively outperforms eight little ones. And you cannot get away from the fact that you are hovering around a minimum of a ton. Either you have a really big cross sectional area of downwash from massive blades, or everyone falls over.
- trhway 12y ago
  > One big blade massively outperforms eight little ones. taking just blades into account - yes, though not that "massively". But the moment we consider the whole system - attach the blades onto a shaft, connect that shaft to reductor, to gas turbine, etc... ie. if that system is helicopter than bigger blade is still better, but if we change the whole system, ie. octocopter built using electric motors - the equation starts to change and we have good efficiency with octocopter and, which is more important, - much better control (eight electric motors with some of them allowed to be tilted gives precise control compare to very sluggish system of one big rotor driven by one big gas turbine), which addresses your point about " hovering around a minimum of a ton" - impossible task to do safely for helicopter in urban environment and pretty easy to do with ducted octocopter (you can't really have ducted helicopter precisely because of the size of the blades) As far as i see, the lack of good control (not efficiency, range, payload, etc..) and direct danger from unshrouded blades is that doesn't allow to fly helicopters in any (even close to)urban environment, and especially for regular people.
x0054 12y ago
lotsofmangos is right! I have designed and built RC helicopters, tricopters, and quadcopters, from scratch. More blades is usually a lot less efficiency. There is something to be said for tricopters. From the point of view of pure efficiency, one large blade is always more efficient than 3 or more smaller once. Don't trust me, take a look at any static thrust calculator[1]. However, I found that with large and voluminous loads sometimes a tricopter can be more efficient because it allows you to mount the props away from the center load, so that the thrust of the prop isn't lost by pushing against the frame of the craft. This works in RC vehicles, but probably wouldn't be much of a benefit in full scale vehicles. As for the prop wash of an octocopter, try standing right next to one that's taking off caring just 5 kilos of payload (batteries and DSLR). I have, and trust me, it gets really, really, windy. And that's with a 5 kilo payload, with a copter capable of generating maximum of 12 kilos of trust. No scale that up to 1000 kilos. [1] http://personal.osi.hu/fuzesisz/strc_eng/ http://personal.osi.hu/fuzesisz/strc_eng/
- trhway 12y ago
  >As for the prop wash of an octocopter, with correct design for the frame and ducts, the same airmass down flow would generate 20-30% more of static trust than the same airmass flow in case of helicopter. And that airflow will not have that blade end vortex which causes helicopter crashes and is just straight efficiency loss until that. I did spend my time with trust calculators and some other systems :)
  - x0054 12y ago
    Yes, but 8 props will be considerably less efficient then one prop in generating that thrust in the first place. For instance: One 300x8 inch prop will generate roughly 1 ton of thrust while spinning at 330RPM and consuming 12kW of energy. So, let's say we go with your prediction, and assign a 30% air flow efficiency to the one prop system. So in reality, according to you, it is generating 700 kilos of thrust. 700/4 is 87.5 kilos. So now we need to figure out how big of a prop we need to generate 1/8 of the thrust of the 300 inch prop, or roughly 88kg. I am going to use 60 inch props, because that will allow us to space 8 props in a circle roughly the same size as the original 300 inch prop. Ok, so a 88x8 prop, spinning at 2350 RPM will generate roughly 88 kg of thrust wile consuming 6.7 kW of energy! That's 6.7 kW x 8 props, or a total of 54 kW, or almost 4.5 times less energy efficient than the one prop system, and that's taking into account your proposed 30% efficiency gain from multi-bladed systems and ducts. I build these things all the time, there is a reason why the industry still uses helicopters with a single prop. It IS more efficient.

zackmorris 12y ago

That's not entirely accurate. Imagine a dozen 20 pound wild turkeys flying together. They are quite a bit more efficient than a single 240 pound craft/passenger because air displacement grows by the surface area of an object (by the square) while weight grows by the cube. That's why birds get off the ground easily while a human powered airplane needs a hundred foot wingspan. Mathematically, the efficiency of a helicopter rotor is roughly proportional to its length. Double the diameter and fuel usage falls by roughly half for the same lift. However the lift for an array of rotors grows by the square of length, even though efficiency stays constant. So if you start with a quadcopter running on batteries that’s 10 times as efficient as a full-scale helicopter running on gas, an array of them with the same lift will still be 10 times as efficient. It will also have a smaller cross sectional area at the cost of greater complexity. So the future of high efficiency lift is almost certainly in using arrays of rotors. The reason that hasn't happened yet is that most aircraft are designed for high speed travel, not hovering. I still want my reactionless drive though!

lotsofmangos 12y ago
"So if you start with a quadcopter running on batteries that’s 10 times as efficient as a full-scale helicopter running on gas" This I have never heard of. In a ratio of energy efficiency against carrying capacity I was under the impression that a normal helicopter massively outperformed a small quadcopter. My impression with doing lifts with lots of small drones was that you get around this by constantly cycling the swarm. Also, this still doesn't get around the problem of downwash. Either you shift a massive cross section of air for your force and get very little downwash, but then you end up with something much bigger than a car, or you focus it and get lots. You cannot get around the fact that to suspend a mass you have to move something equivalent downward. edit - I get what you are saying with the turkeys. However, in a vehicle carrying four people, you have replaced the turkey bodies with a giant blob that has much less lifting potential, especially at low speed, than lots of turkey bodies. You have to dump a lot of useful surface area. Also I note that turkeys do not hover much.
- trhway 12y ago
  > a normal helicopter massively outperformed a small quadcopter. not really. The power loading (power divided by surface of the propeller's circle) is the key factor what matters for efficiency (until of course Reynolds number - air viscosity factor - starts to change - happens when we come to Kolibri scale) In general, helicopter with 3m long blades (6m diameter) on one 500kw turbine or something like 16 of 30kw motors with 1.5m diameter propeller (with the same number of blades as helicopter) each is pretty much equivalent. That is pretty theoretical starting point. After that we can optimize these systems in different ways. 16-copter allows to use somewhat bigger, yet less-bladed, like 2-3 bladed propellers which are more efficient than multi-blades of helicopter (and you can't really have 2 bladed rotor on helicopter). You can use ducts on 1.5m propellers - not a case for helicopter. So all this allows to increase efficiency.