Why are quadcopters not flown by human pilots yet?
Wouldn't they be more stable and easier to control than helicopters?
Wouldn't they be more stable and easier to control than helicopters?
No, they would not.
Quadcopters don't have any special inherent stability. When you increase power of one of the rotors to pitch, the increasing pitch will not do anything to the power difference and therefore the pitching moment.
The advantage of quadcopters is that the rotors can be fixed pitch while single (or double) rotor helicopter needs complex control mechanism. While this is huge advantage for the small scale devices where each rotor can be powered by its own simple electric motor, the complexity of either additional engines or long transmission shafts would outweigh any advantage from the simpler rotors in full-scale vehicle.
And why can't full-scale helicopters use electric motors like the small ones? The reason is that when you scale an airfoil up, the lift it produces increases with its area, which grows with the second power of size, but its weight increases with volume, which grows with the third power of size. Therefore models have much more lift for weight and can afford simple but relatively heavy batteries while full-size aircraft need propulsion systems with higher power density.
And then there is also the factor of safety. In case of power failure, helicopters can still glide to the ground and still land vertically using autorotation. But since the rotor rotating speed can't be changed without power, controlling the helicopter during such manoeuvre requires variable pitch rotor. So there goes the main advantage of quadcopters.
Quadcopters are not an efficient design -- one large rotor is far more efficient than four smaller rotors. The reason quadcopters became popular is that they are mechanically simpler, safer (due to the smaller rotors) and far easier to control by software.
That said, recent advances in machine learning have made so that helicopters can be controlled by software. I'd expect to see a resurgence in helicopter-style drones because of this.
Finally, I'm not an aeronautics engineer but I suspect there's considerable merit in combining attributes from a quadcopter and a helicopter. Imagine one large fixed-pitch rotor driven by an electric motor. A few small additional electric-motor driven rotors could provide the needed pitch, yaw, and roll control. Electricity would be generated by an efficient gas engine spinning at near constant speed. Enough backup power to land could be provided by small batteries. This design would be extremely reliable due to the greater reliability and simplicity of motors and fixed pitch rotors.
Advancements will come, but slowly since the costs and dangers of human-carrying vehicles are so high.
Scaling up the quadcopter design would make them very large. Only having one (or even two) rotor allows the helicopter to be smaller and even fold up the rotors and remain fairly compact. Since by definition helicopters are supposed to get into more difficult landing areas, increasing the footprint is generally undesirable.
Also, the power system would be complicated. RC quadcopters use an electric motor on each rotor for independent control. A larger version would probably use turbines like most helicopters, and they would either need 4 turbines or a way to gear fewer turbines to 4 rotors. This just adds complexity to the system.
The Chinook is an example of a helicopter that uses two rotors, but the added complexity makes it undesirable unless the added lifting power is needed.
There are three challenges for making a market-competitive full scale quadrocopter.
1) Energy efficiency
The quadrocopter is not an energy-efficient design. Helicopters increase their range by using relatively under-powered engines that take a long time to spool up throttle and try to keep it at relatively constant RPM while changing the pitch of the blades to control the helicopter. While this can be used to control pitch and bank, the quadrocopter controls yaw by changing the RPMs of the motors, which I would think would be difficult, and inefficient to do on a full scale helicopter without some sort of CVT to adjust the RPM of the blades without changing the RPM of the engine.
2) Structural integrity and weight
Tail rotor helicopters essentially have a fuselage and a tail boom. The tail boom doesn't even need to be that beefy, because the load on the boom is rather small compared to the load of the main rotor pulling up on the frame of the fuselage. Coaxial helicopters can probably get away with just a fuselage, and no tail boom, and probably have less forces acting on the entire structure in normal operating conditions than conventional helicopters. Quadrocopters have four main rotors. Granted, each of the rotors is approximately 1/4 of the power of the main rotor of a conventional helicopter, but the booms would still need to be able to withstand more than 1/2 of the power of a conventional main rotor - think of the extreme case: you're full yaw - that means two rotors diagonally across from one another are either off, or spinning in the opposite direction, creating either loss of lift, or even down force that the other two rotors have to compensate for by producing two times the thrust/lift. It's a rather common phenomenon among miniature R/C planes and helicopters to be completely overpowered, and much beefier than their full-sized counterparts.
3) Cost of manufacturing
It's worth mentioning that a larger rotor is easier, and cheaper to manufacture than three additional engines/motors. I think (I could be wrong) the only reason military helicopters have two engines instead of one big one, is for redundancy. If one gets shot, you still have the second one that you can use to limp back to base, or a friendly location. So when you're looking at a whopping four overpowered engines (see point #1) compared to 1-2 underpowered engines, I would imagine you would be looking at something that costs more.
Those are the reasons why a quadrocopter has not been scaled up, and become commercially available by a large manufacturer. I'm sure each manufacturer is trying to be the first in that market, but I don't think QUADROcopters will ever become human-piloted. If you were talking about making a MULTIcopter (more than 4 rotors) however, there are a few DIY prototypes out there, and I think they have potential.
4) Addressing the proposals for hybrid systems:
As things stand now, conventional helicopters run 1-2 turbines for power. They spin in their optimal RPM ranges, and the pilot uses the collective to change the pitch of the rotor blades to control flight. A hybrid system is appealing for cars because it allows the ICE to run in its optimal RPM range rather than constantly changing RPMs. In rotorcraft this isn't an issue - since they already spin at optimal RPM ranges, a hybrid system would introduce a generator, and batteries to generate and store power for the electric motors to consume. This would add weight, without any particular added benefits.
The big question is - why bother? What makes quadrocopters better than conventional helicopters? For drones the biggest benefit of quadrocopters is their ease and cost of production. It's cheaper to produce four small electric motors than all the mechanisms for a collective. It's also more robust and easier to service. However, we're talking about different scales and missions. Rotorcraft are already inherently inefficient. Their niche - close air support, and transport to locations where it's difficult to land an airplane. The tendency is to create a hybrid, or make a plane with VTOL capabilities. (Osprey, Yak-141, Harrier, F-35 aka US Yak-141). Quadrocopters may find their niche as cheap utility drones, or surveillance drones, but I think a much more likely design, is the likes of the one shown in Avatar - two counter-rotating shrouded rotors on either side of the fuselage, which can be vectored to control flight.
Technology is ever-changing. Scientists in Russia have miniaturized nuclear reactors and fit them inside missiles. This negates the need for batteries and hybrid systems. If anything, this is the powerplant of choice for large quadrocopter systems, but only time can tell how technology will evolve. Right now, I don't see the benefit of quadrocopter configurations for large rotorcraft.
Amendment, based on new tech in the market (23.03.2018):
A Chinese company is actively marketing their EHang 184 autopilot drone air taxi, that uses the quadrocopter design. Specs:
Basically, one can compare it to conventionally designed Mosquito helicopters. They fly slower, but duration of flight,range, and useful load are significantly higher. Also note that while fueling a helicopter these sizes takes five minutes, the air taxi will spend 2 times longer charging, than it will flying. But this is more of a EM vs IC type issue. Time will tell if quadrocopters can prove they're worth up-scaling.
Adding from recent professional experience (09/19/2018) I was recently asked to develop a quadrocopter concept. While attempting to use electric power, I found that payload weight, and mission flight time drastically increased the weight of the batteries required to fulfill mission parameters, and you ended up with a 3-4 ton four-seat quadrocopter (yikes!). Converting to turboprop solved the weight issue, but dramatically increased cost and complexity. As a result, the decision was made to drastically reduce mission parameters, making for a very short range vehicle, which under normal circumstances would be quite useless. You would need to charge the batteries for several hours to get 15-30min flight time. Either your quadrocopter would be sitting on the tarmac charging, or you would need to swap batteries after every hop, requiring expensive infrastructure at every landing pad. Ultimately the idea was proposed with infrastructure and battery swap, and is awaiting its fate at the hands of upper management, but everyone on our team has already scrapped the project as unfeasible.
There ARE a number of upcoming battery technologies that would drastically change the potential of such products, but they're not market-ready yet, mostly in theoretical, conceptual, and testing phases at the moment. Can't say much more on the subject without an NDA, sorry.
Below is the info about one of the early protoypes by P Moller, author also of the: 'Volantor', now he's head of Freedom-Motors, a company focused on Wankel Rotary Combustion Engines. Moeller built his flying machines with fans driven by Wankel rotary engines, however, it sounds better having electrical engines for lifting and propulsion fans, and some kind of hybrid system for generating power, Wankels are very good in weight to power ratio and safety. I'd say the Moeller and Helikar are exactly a Quadcopter, and as this machine, and the Utility model/patent backing it are from earlier times as the toys we all know, probably it was the chicken of contemporary toy Quadcopters, the today toys being the eggs.
The rope and the crane behind the Moller: 'flying saucer', Avro Canada failed in having one airborne, are not for keeping the machine in the air, but a safety measure, if you look, you'll notice that the rope is not under tension while the machine on the air, meaning that the machine flies by itself, and is not hung on the crane.
The prototypes Bell Textron/X-22 and Curtiss-Wright X-19 can be considered also very close to a quadcopter. An Spanish team: FuVex, designed a prototype named: 'HeliKar', also close to a big size Quadcopter. YouTube includes a video about a German invention named: 'Volocopter VC200', Terrafugia proposed another 'Flying Car', as well as Zee.Aero did, they draw a concept VTOL flying car that could be parked between two ordinary cars in a mart parking lot (see: 'SlashGear'). Aero-X by Aerofex is another one. Fipsi is involved in developing a four-fan flying car, as cited in 'Advanced System Engineering', SUSB Expo 2014.
'Future transportation' cites what they consider: 'The top 10 flying cars'. Krossblade proposed the SkyProwler and the SkyCruiser, the big problem in a flying car seems fullfilling in the same design the weight and safety requirements for both a car and an airplane. Ford released drawings for an: 'Aero-car'. CNN reports about Fly Citycopter, by E Galvani. Mechanix Illustrated, March 1957 includes a: 'Flying saucer', design by Peter Nofi, power comes from a flat-six reciprocating engine, and Popular Mechanix, French editon, Sept 1961, a similar design: 'Rotavion', by Ben Kaufman. Mechanix Illustrated, Jan 1962, pp 70-73, is about a VTOL device having 3 shrouded fans moved by a turbine. Regarding historical precursors, the machine in the 1961 movie by William Witney: 'Master of the World', based on two novels by Jules Verne: 'Master of the World', and: 'Robur the Conqueror', could be easily considered as a Multi-propeller: 'Quadcopter'. If: 'Fly by wire', succeeded in keeping airborne intrinsecally unstable and unsteerable designs, there can be no limits in what could be accomplished with a digital control technology that currently is sold in toys starting at around $20.
A Hungary based company: 'Flike', and Hungary's name doesn't come from 'Hunger', but from: 'Huns', just presented a 'piloted' three propeller copter, video is signed by Bay Zoltan.
Do the flying platforms, single person devices, ducted fan below, tested by the US Army, belong to the: 'Quadcopter' class?. Also several 'tilting wings' and annular wing flying machine arrangements were tested, the list of 'Concept' and 'Unconventional' aircraft made the pave for many books, but the web is killing this type of paper publications.
'SkyProwler', is another approach, a mixed Quadcopter and fixed wing design. Bensen B-12 (see 'Aerofiles') is a 1961 example of Multi-Rotor (rather multi-propeller) Quadcopter style machine, patent US49820151 is about the multi fan P Moller design, and patent CA1264714 is about a: 'Robotic or remotely controlled flying platform', also by P Moller. All patents are of open and free access and download in: 'Espacenet'
Is there a reason why Quadcopter toys couldn't be increased in size, even using the same software for controlling engines and stability?
Thanks. + salut
Moller M200 test flight:
The reason quadcopters are chosen as platforms for small, computer controlled flying machines is because they are by design more agile and simpler to move in and around all three axes. This is because they are by design very very unstable.
It is in fact impossible for a human to control a quadcopter (that can move in and around all three axes) without the aid of a computer or some other kind of artificial stabilizer. The reason computers can control quadcopters is because they are fast enough to produce control input that counters any small destabilizing forces that act on the frame.
One way to imagine the inherent stability of a flying platform is to consider what would happen if you would let go of the controls. Normal airplanes and helicopters will tend to just keep flying to the same direction. If you let go of the controls of a quadcopter (and have no stabilizing mechanism installed) the quadcopter will very quickly just chaotically tumble towards the ground. This means that in a helicopter or an airplane the design "helps" you and forces the frame to fly in a stable manner (forwards.) In a quadcopter there is no such help but there are also no forces that hinder you from moving towards any direction you want.
This principle is also deliberately on frames such as the F-117. The F-117 is impossible to fly without the help of computers (it was designed to be unstable), but this has made it a much more agile airplane than its frame would normally have allowed.
The other (bigger) reason quadcopters have not been scaled up is because they consume (a lot) more fuel than other types of aircraft. Why would anyone build a quadcopter when a helicopter or an airplane does the job using less fuel. They are also slow and noisy.
Keep in mind that the "job" for scaled up airframes is usually to move stuff from point A to point B while the "job" for small, computer controlled quadcopters is to be agile.
For clarification on some of the answers, technology in the area of distributed propulsion is moving towards systems that would allow hybrid quadcopters to be built that eliminate the problems of a gas quadcopter and remove the energy density limitations of electric systems. I suspect we will see experimental human scale quadcopters in the next 20-30 years.
e-Volo claims it will use range extender in VC200: http://www.e-volo.com/information/how-long-can-you-fly
I was really curious about that technology, and did small research. My conclusion is, if they consider using Bladon Jets micro turbine to generate electricity it could be a total win!
So, it seems, the most advanced turbine they have now is 50 kW, 40-kg jet turbine for concept car: http://www.bladonjets.com/news/bladon-jets-at-the-geneva-motor-show/
According to wikipedia, Volocopter motros consume ~36 kW (18x2 Kw, see E-volo_VC2) which means, they can be powered directly by electricity produced by turbine without a need for batteries! Of course there must be small battery pack which provides energy buffer for safe operation.
There is also a datasheet on bladonjets website about the fuel consumption of stationary 12 kW turbine, it's 5 liters/hour. Let's say, their 50 kW turbine will consume 5 times more. Which means, 50 kg of fuel will be enough to fly vc200 for at least 2 hours (not 20 minutes, as they say for pure electric e-Volo).
Also, total weight of system is not that much exceeded by turbine: 40kg turbine + 50 kg fuel + 50 kg chassy. Well, e-volo can alredy carry 2 people=150 kg, right? So existing prototype can lift the generator as is. 50% stronger motors (18*3 kW=54 kW) is not a problem to lift another 150 kg. Although total weight might exceed 450kg - category of ultralight rotor aircraft, but who says that specially designed turbine can't be integrated in e-volo structure?
My point is, the technology is there. Just apply it alltogether, and it will make a revolution.
The only answer that matters is safety, because it makes all the others unimportant. Assuming fixed pitch rotors, if you Lose power to a rotor, you're going to crash, and you will crash in an uncontrolled way.
You could gain more control by adding more rotors, but the more rotors you add, the higher the likelihood of a failure of one rotor. Also, for each rotor you add, you'll lose rotational inertia in each rotor, the result being that in a loss of power event, even collective pitch rotors will be unable to provide sufficient lift to keep you from dying of rapid deceleration sickness.
They have been scaled up - perhaps not in rotor size but in number of rotors. If this can be done, I can see no practical limit to producing a manned platform with four rotors.
Controllability seems to be a bit of an issue, but maybe that is because the pilot has mounted a milky glass dome around his head.
There's a number of comments above about the various reasons that quadcopters don't exist in full size, but there's one I don't see mentioned. The quadcopters themselves use 4 rotors that have a constant pitch (the angle of the rotor blades is constant, and doesn't change). It then varies the speed of the various motors to control the lift provided by each rotor, so that it can coordinate flight. At a small scale, it is possible to use both an electric motor, and additionally, to adjust the speed of the rotors very quickly. On a normal, human sized helicopter, the rotors would need to be significantly larger. Once the rotors get scaled up, they also weigh more, which gives them more inertia than a smaller motor. For example, in a Robinson R-22, which has relatively lightweight rotors and can only carry 2 people, it takes me something on the order of 3-4 minutes for the rotors to stop spinning once I remove power (on the order of 30 seconds to a minute if I'm using the rotor brake). As the size and weight of the helicopter scales up, this problem gets more significant. You're unable to quickly adjust the speed of the spinning rotor because of the inertia it carries, so you'd end up having to add something in addition that lets you control the pitch of the blades. Once you've added the mechanical complexity of being able to control the pitch of the blades, most of the advantages of a quad copter setup go away, and you have significant disadvantages (like having to deal 4 rotors instead of a single one, without the advantages of mechanical simplicity).
There are heavier-than-air firefighting (mine service, etc.) lift frames with four lift props. Ones without whose adaptations and use cases one would find rides:
Pitchy; trim requires pitching or rolling the crew, or at least jinking the evacuee 'hard mount.' Blendy; Four sources of chop, no particular service cowling (emergency fixup, water landing kit, etc.) in air or not, or guarantee of no inflow at ground. Noisy; reaction lift is from outer extent of blades; perhaps orbit of blades can be altered to optimize pattern? Harder to service-harden by 3x or more (for commercial suitability, nevermind commercial war duty. Dual-engine certified? Sorry.)
The e-volo multirotor movie has a nice white seemingly unloaded 2-fore heli frame with a DC patch panel, no rotor or tail, and instead a rack of smaller props. It goes up in a test hangar/messe and doesn't self-invert or evert...at some point reminds of the incidental score from that drone repairman flick where Jony Ive designed the humankind-wrecking drones' looks, and sounds like a lot of fans; performance is not up front in this production (though batteries were surely lofted.) They should at least cut the reel to appear to have a white cat onboard?
I think that both single rotor than quadcopters will cohesist, at least for some time. The reason for creating manned quadcopters are countless. A quadcopter could be driven like a car, while real helicopters need hundreds of flight hours for a pilot to master. this will lead to mainstream production and lower manufacturing prices to make them available to the mass. about usefullness: a quad copter can have the blades ENCLOSED inside protecting structures, making their flight viable for cramped areas, where trees, buildings and poles would otherwise destroy normal helicopters. this makes a quadcopter much a better choice for rescue operations. probably normal helicopters will stay a better choice for long range operations and speed.
Noise and Safety
Noise - If you live in a big city, take a look out nearest window at the traffic snarl below. Now imagine all those cars (or at least half) replaced by flying human-sized quad-copters. To scale up, those quads will need piston engines (or expensive turbines) and large rotors, and if successful there will be literally millions of them in the sky above any medium to large city. It would be a constant, deafening drone which would significantly reduce the livability of the city.
The Black Knight Transformer mentioned by @menjaraz is very, very noisy, as are conventional helicopters.
Safety - Unless you're a qualified pilot, you can't manually operate an aircraft. This means they'll need to be able to take off, fly and land fully automated to their destination, navigate along the way, communicate with and avoid other traffic and have some kind of interaction with ATC, all done for you by computer(s).
Both these problems are surmountable, but will require significant R&D, which means investing big dollars in a shakey economy. And who's going to buy all these multi-million dollar machines?
TL;DR: The limitations on human scale quad copters are primarily economic, regulatory and environmental, but those are significant limitations.
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