Why haven't quadcopters been scaled up yet?

  • Why haven't quadcopters been scaled up yet? ratsimihah

    Why are quadcopters not flown by human pilots yet?

    Wouldn't they be more stable and easier to control than helicopters?

  • 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.

  • It has (sort of) been done:

    (not a quad)copter lifting a pilot

    The company that did this is working on a more useful version.

    It's important to note that scaling up is extremely difficult in aviation. Model airplanes have performance numbers the full scale folks can only dream about.

  • 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 only effect that can make quadcopter stable is having centre of gravity below centre of lift just like normal helicopter.

    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.

  • 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.

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