Because quadcopters are naturally unstable, so you need complex electronics(and sensors and electric motors) to control it.
You can make helicopters very stable just with weights in the rotor, like any RC fan knows.
Until very recently the inertial motion control complex elements were very expensive.
Accelerometers in cars and gyros in smartphones later had made those sensors inexpensive. The iPhone was released in 2007, that was 7 years ago.
Every air device carries gyros inside, but for controlling the plane they had to give you a digital signal, it is not enough with the artificial horizon sphere.
The simple helicopter design has been mass produced for decades(and wars like Vietnam subsidized them more), and it is well tested and reliable. Any new design has to compete in price and will have to iron lots of bugs at first.
The second most important reason is that you need to use electric motors as normal heat engines could not respond as fast as needed.
So in order to make a quadcopter you need to generate electricity onboard, which means a big hybrid engine.
I see quadcopters as the future, but in order to compete in price, they will have to evolve from small UAVs.
PS: In order to understand how much wars influenced the development of helicopters, my father used to work manufacturing helicopters in Spain.
When Vietnam war ended, the market become so saturated and the prices went down so much that the company my father worked in could not compete and closed its doors.
You touch on a number of good points here. It's true you need accelorometers measuring and giving feedback a few hundred times a second. Electric motors are the only motors that can respond to feedback that fast. So the batteries will be a big deal on a scaled up quad.
The most important aspect you didn't mention is that they are inherently inefficient. The more blades you have, the more inefficiency is introduced. that's why scaling them up doesn't make much sense.
Why couldn't you use a turbine to generate electricity that is then fed to electric motors controlling the blades? Is there actually a requirement for large, efficient batteries?
You've just added a bunch of weight and lost a whole bunch of energy in the process. (EDIT: vs. just not using electrical motors. vs. batteries it really depends on the specifics but if you could get what you get out of a battery with a turbine of lesser weight then presumably we'd all have little turbines in our iPhones and electric cars)
An airplane is a vehicle that wants to fly, a helicopter wants to crash. A helicopter is inherently unstable and requires constant adjustment. It has multiple single points of failure, any one of which will result inevitably in a crash. This is dealt with by insanely expensive quality control, constant inspections, and constant maintenance.
The quadcopter doesn't improve on this, it makes it worse. It has 4 engines - if one engine fails, it crashes. It has inherently 1/4 of the reliability of a single engine helicopter. Losing one rotor blade will also cause a crash, so its 1/4 of the reliability there as well. QA, maintenance, and overhauls will cost 4x as much.
I always thought that thanks to this http://en.wikipedia.org/wiki/Autorotation helicopters are safer than even some airplanes - unless you manage to turn them upside down midair, that is :-).
http://www.youtube.com/watch?v=YlkKfqUTnyk I know it's not full scale, but apparently autorotation can be achieved even inverted. No idea if the swashplate on a real helicopter could invert all the way, but I seem to recall the apache AH-64d could fly inverted.
That kind of stable flight is ok for an unmanned vehicle, but I don't think a passenger/pilot would be delighted abut that kind of accelerations and spin.
I guess if it's a choice between smashing into the ground and short period of spinning whilst an automated system landed the vehicle, I think I know what I'd go for.
It might be better to include a parachute above the rotors.
The Cirrus SR22 single engine aircraft has this (their system's called CAPS.) It lets you sail to the ground in the event of some catastrophic failure preventing you from landing the aircraft safely.
http://en.wikipedia.org/wiki/Cirrus_SR22
Would the airplane parachute be scalable to large commercial jets? I assume the fuel costs of carrying the extra parachute weight are too great but curious about the engineering.
A quadcopter changes direction by spinning up or down the appropriate set of propellers. The smaller the propellers, the less energy (time) it takes to slow them down. Meaning a huge quadcopter is increadibly unstable and can't change direction fast enough for anything. They're uselessly inefficient other than the smallest ones (flying circuit board, ~4cm long) and the bigger ones just use bigger batteries and have the same flight time (mainly for lifting cameras, but they too are slower or use much more energy to go the same speed as the small ones).
> A quadcopter changes direction by spinning up or down the appropriate set of propellers.
To extend on this further... a quadcopter uses the motors to quickly change the speed of the blades rotating. This requires a lot of power, depending on the inertia of the rotor. The inertia of the rotor is roughly proportional to the fifth power of the blade length. Double the blade length, 32x the inertia. The power requirements of this forbids scaling up.
Contrast this with a traditional helicopter, where the rotor is spinning at stable speed (low engine torque requirement) and the helicopter is controlled by changing the pitch of the blades. The pitch of the blades changes during every rotation and is controlled with a mechanism called a "swashplate".
There have been "hybrid" helicopter designs involving a variable pitch propeller in a quadcopter as well as supplementing a quad with a large propeller for lift.
But the most promising research on quadcopters is focusing on using several independent quadcopters co-operatively.
That's what I meant. I'm bad at talking, I mainly just play with quadcopters and "study" their flight. IMO, I like them as they're really cheap, much cheaper than fixed pitch or collective pitch single rotor nano helicopters, and coaxials are really slow to respond and are incredibly bad at handling even minor wind.
You can have collective pitch setting in a quadcopter, with which you can scale up quite a lot without having to go to full cyclic pitch control (helicopters).
You could change the pitch on the blades a lot faster than you can change their rotation speed. But, at that point a helicopter becomes not just more efficient and stable, but easier to build as well.
- Large rotors are more efficient than small rotors.
- Because of the square-cube law, bigger, heavier aircraft require higher fuel energy densities than can be delivered by batteries. Therefore you need a relatively complex engine rather than simple motors, and you can't afford to replicate it four times.
- A variety of handling and safety benefits of helicopters compared to quad rotors.
Most people are still missing the real reason, which is that quadrocopters vary thrust by changing the speed of the rotors. You can only do that with small rotors due to inertia, so with large rotors you'll need variable pitch to change the thrust. And if you have variable pitch you only need one rotor.
The only exception to this is to have many small fixed pitch propellers, which has been done and is probably quite attractive from a price point of view, and to a certain extent safety (although you can't autogyro). But it is undoubtedly less efficient.
Trains require a HUGE engine, yet they relay their power via electrical linkage. This is actually more efficient than other systems, because it allows the engine to run at its optimum power generation cycle, without worrying about converting RPM down to the appropriate amount via mechanical linkage.
An electrical linkage would be relatively straightforward to split in to 4 components.
It's still a lot of weight to carry around a high-power generator, large-gauge power cables (or heavy transformers and smaller-gauge cables), and four electric motors. This is less of a problem on a train, because the train only has to deal with the additional friction, and doesn't have to worry about generating more lift to make up for the increased weight.
Being able to individually step wheels is also a big win -- if a given wheel starts to lose traction, it's much easier to cut electrical power than mechanical. Steel-wheel-on-steel-rail has fairly low static friction (which is why you find electrically-powered trains with many powered wheelsets, and virtually _all_ trains with track-sanding capabilities to increase traction).
It sounds wrong to me as well. I would think you would want the center of lift as close as possible to the center of gravity, and that you would also want the flight controllers accelerometers and gyroscopes in the same spot.
It must be the costs... Multirotors have enjoyed open source development by enthusiasts so far, and they built heavily on the experience of the RC enthusiasts, and they repurposed tons of hardware that is suitable for small applications. A few thousand dollars is within the budget of many developers, so the small UAV improvements could proceed very much like software-based innovation curve does. But, any size/budget/project bigger than that is bound by a different progress curve. I bet it can be done, and if it is practical, it will be done too. Aerospace engineers were always a very innovative bunch; check out autogyro (https://en.wikipedia.org/wiki/Autogyro).
It's not the cost. The problem is electrical planes. Helicopters are known for their agility, coming at the cost of extreme energy use (and, imho, extreme danger. Any mechanical failure in any part of the helicopter WILL result in contact with the ground in 2 minutes or less. Redundancy (in your standard helicopter craft) is effectively zero). Getting a normal plane to fly electrical is a huge challenge. Getting a helicopter to fly electrical is a bigger challenge.
In 2011, for the first time in history, a manned helicopter took flight. More than 1/4th of the flight mass was batteries and that afforded the craft ~2 minutes of powered flight (hover, not maneuvering). [1]
We just don't have the electrical storage solution to pull it off easily. It's not impossible at these efficiencies either, but it's bloody hard.
It's not just electrical power that falls short. For the longest time, planes were quite picky fuel wise and gasoline also fell short. Even today, it's not gasoline that's used, even though most planes will fly fine on your car's gasoline (not diesel though), even corn ethanol, but less efficient (range reduction of ~20-30%).
The answer to most of the "can we do X with electricity" is simply : find a way to store 4 times (or more) the electrical power in something the size and weight of our best batteries (take lithium-polymer), and it'll work. Without that, it's barely possible.
We do have things that store electrical power more effectively than a battery, but you won't like them. Plutonium batteries would easily allow for an electrical helicopter and would allow it to remain flying constantly for 40 years straight (or more if you like, thousands of years wouldn't be out of the question, really). There are a few other nuclear options that would provide similar performance. But I don't think I need to explain why this isn't done. Plutonium batteries have the advantage that they do direct electricity generation, making them very, very small and efficient. Also, in space, temperature is supposedly -271 degrees celcius, but because there's no gas colliding with the craft, heat will take weeks to leak away from the spacecraft. So anything with a heat based generator (which is nearly everything) is out. Which is why plutonium batteries are pretty much the only answer.
A couple of factual issues: most planes fly on Jet-A aka Kerosine which is actually closer to diesel than to gasoline.
You're talking about 'avgas' which is used for piston engined planes, the turbines typically run on cheaper fuel (they can be run on just about anything but typically you'd run them on Jet-A rather than on single malt scotch).
Second, plutonium 'batteries' (you mean a thermoelectric generator powered by nuclear decay) aka an RTG is not feasible for aircraft due to the weight of the shielding that such a solution would require as well as the risks associated with flying such a device (it can crash!), not to mention the proliferation headaches and waste disposal issues.
So even though they are used in space they have little or no chance to ever see deployment in commercial aviation or to power drones.
There's alternative to the common arrangement of a motor per blade, like the http://curtisyoungblood.com/V2/products/quadcopters/stingray... - one motor, variable pitch. It's clearly more complex, but you gain agility, motor efficiency and it seem to give a better running time.
One major reason for multi rotor machines is to avoid the classic helicopter problem of the rotor blade's leading edge approaching the speed of sound while the trailing blade is simultaneously stalling.
Unfortunately, this would require such a scaling up of existing quadcopter designs that it's unlikely to ever happen for all the reasons already listed.
As many of the other answers say, the current multicopters are very simple, mechanically: an electric motor directly coupled to a fixed-pitch propeller. Everything is controlled by the amount of current the motors receive.
Any other alternate arrangement means a more complex mechanical control: servos, gears, larger propellers, variable-pitch propellers... This makes them less reliable, harder to mend and more dangerous (that's one of the reason model helicopters never really caught on, even with modern electronic stabilizers - even a smaller 450-class blade can break bones if you hit it).
Scaled up insects work just fine when the atmosphere has 50% extra oxygen. Birds figuratively ate them for lunch, but for several million years 70cm wingspans were not uncommon for dragonflies.
Electrical fixed rotors. Once you have a swashplate to give you variable pitch that can be used to give you the same kind of control that you get by changing the RPM and the electrical advantage disappears.
You can make helicopters very stable just with weights in the rotor, like any RC fan knows.
Until very recently the inertial motion control complex elements were very expensive.
Accelerometers in cars and gyros in smartphones later had made those sensors inexpensive. The iPhone was released in 2007, that was 7 years ago.
Every air device carries gyros inside, but for controlling the plane they had to give you a digital signal, it is not enough with the artificial horizon sphere.
The simple helicopter design has been mass produced for decades(and wars like Vietnam subsidized them more), and it is well tested and reliable. Any new design has to compete in price and will have to iron lots of bugs at first.
The second most important reason is that you need to use electric motors as normal heat engines could not respond as fast as needed.
So in order to make a quadcopter you need to generate electricity onboard, which means a big hybrid engine.
I see quadcopters as the future, but in order to compete in price, they will have to evolve from small UAVs.