source -- image shows Volocopter's design
I find that in the designs of aerial taxis, particularly in Volocopter's design, the design consists of a system of multirotors.
What is the main reason for not substituting this bunch of rotors with fewer bigger blades (helicopter)? As far as I know, a helicopter is energy efficient for horizontal displacement, and the multirotor is at hovering. Does a quadrotor taxi provide any other dynamics advantage?
There are a few main reasons most electric air taxi concepts have multiple rotors.
1) Mechanical simplicity. Helicopters are controlled by varying the angle of attack of the blades as the rotate, and the mechanical mechanisms to do this (swashplates) are complex and expensive to build and maintain. The thinking is that with electric motors, you can stabilize the vehicle using differential thrust and eliminate the complexity of those systems. This has implications for vehicle cost.
2) Noise. One of the main reasons there aren't any helicopter airlines around now (and there used to be) is that helicopters are very noisy. Noise in rotorcraft is related to a lot of things, but one of the main drivers is blade tip speed. With more, smaller rotors, you can have lower blade tip speeds for the same disk area, thereby reducing noise. Whether this has the potential to reduce noise enough to make a significant difference has yet to be conclusively demonstrated, mostly because there's no clear definition of "enough noise reduction".
3) Safety. Another argument commonly made for eVTOL air taxis is more motors = more safety, because if one motor fails you have lots of others to keep you in the air. The validity of this argument depends on how much redundancy you have in the rest of your powertrain, but it principle the opportunity for more redundancy offers a pathway to improved reliability. There are new failure modes introduced by this multitude of rotors and in general these vehicles have no autorotation capability, making a total power failure much higher consequence; all of which needs to be taken into account.
4) It looks cool There's probably more to this argument than it gets credit for. Most of the investment and momentum behind the air taxi concept is coming from Silicon Valley and not the traditional aerospace players (who, it must be said, are started to get onboard - especially Airbus). This creates pressure to develop a product that looks and feels revolutionary, to raise the billions required for aircraft development. Public acceptance of these systems will also be an issue, and public opinion in many large cities is pretty anti-helicopter.
From a performance standpoint the big difference between most multirotor concepts and current helicopters is that lots of multirotor concepts are battery-powered (for noise, local emission, and infrastructure flexibility reasons). Batteries are much worse than avgas from a energy/weight standpoint. A hybrid-electric multirotor would have broadly similar performance to a hybrid-electric helicopter, assuming they had the same disc loading.
It’s a good question. Single rotor helicopters exist already, and would make a much more boring investment proposition for private equity than a new sort of vehicle which sports claims of revolution, modernity and cleanliness.
Above clip (also posted in this thread of five years ago(!)) has a person controlling an electric multicopter, seeming to have partial success in learning how to control the craft in ground effect hover in 6 minutes, which it looks like is the flight duration that the batteries can provide.
Comparing multiple against a single rotor:
Efficiency. The generic equations for thrust T and power P are:$$T = \dot{m}\cdot v \tag{1}$$ $$P = ½ \cdot \dot{m} \cdot v^2 \tag{2}$$ with $\dot{m}$ the mass flow of the airstream through the rotor(s) and $v$ its velocity. So for a given amount of thrust, a larger total rotor disk area requires less power. A single rotor covering the outside perimeter of the OP photo would be more efficient than multiple small rotors which leave a large amount of disk space open.
Controllability. The multicopter in the clip has fixed pitch rotors. To control the craft in pitch and roll some of the rotors can be sped up, which tilts the craft and therefore the thrust vector. A simple mechanism if the differential thrust can be supplied rapidly enough, as with current supply to electric motors. Slowing down some of the rotors can always be done rapidly but lowers total thrust, the craft descends while controlling.
The multicopter way of control is simpler than that of the cyclic pitch adjustment of the traditional helicopter. An alternative could be a large rotor without cyclic pitch control, with four small rotors to speed up/down for control.
Translational lift. As a helicopter speeds up, it requires less power to drive the rotor(s) at a given thrust because of the airflow streaming in. As in this answer: the required induced power reduces.
In order to fully profit from translational lift the rotors should be able to adjust their pitch: like in a propeller aeroplane, the angle of attack of a fixed pitch blade reduces as speed increases, ultimately resulting in zero thrust at blade AoA zero. But that requirement negates the simplicity of multiple fixed blade propellers, an arrangement which severely restricts maximum speed.
Noise. Thrust is proportional to tip speed. from expanded impulse theory: $$T = C_T \cdot \rho \cdot A \cdot {(\Omega R)}^2 \tag{3}$$ with ${(\Omega R)}$ being the tip speed. In order to maximise thrust for a given rotor the maximum tip speed should be used: thrust scales with the square value.
The main contributor to helicopter noise is the engine, a turboshaft being very noisy while an electric motor is very silent.
Scalability. Upscaling heavier-than-air aircraft is subject to the square-cube-law. What is relatively easy to do at low weight becomes very problematic at higher weight, and all aircraft focus on weight reduction. A vertical take-off craft even more: thrust must be higher than weight, unlike in a fixed wing aeroplane. Batteries are a major spoiler here since they are heavy.
A possible solution. Noisy turboshaft and piston engines have very good power-to-weight ratios. Power a multi-copter with a silent piston engine generator, and make the rotors have adjustable collective pitch.
Why are aerial taxis multirotored, instead of a single rotor?
There are not many good technical reasons. Simplicity of control is not an urgent problem to be solved, cyclic pitch technology is mature and has been around for almost a century. The main reason seems to be marketing, because the image of a multicopter automatically associates silent, clean electrical actuation, while that of a helicopter associates with loud engine noise.
The real reason is that the current concept for aerial taxis came from people who don't understand or don't care about aerodynamics. This is one of ten business ideas that Uber is/was pushing for their IPO, and they have had success with the money they put into its marketing. It originated from extrapolation of camera drone technology to human passenger size, but the tech won't scale.
You are absolutely correct that one large rotor is more efficient that many small rotors, but for both hovering as well as lateral motion. Hovering is very inefficient, requiring twice as much energy as a translating takeoff. Small is not necessarily quieter as supersonic airflow at the tips generates noise, and rotors turn at whatever speeds they are designed to turn.
Quadcopters dominate for camera drones as control is simple. Multiple rotors fit small electric motors, which are quieter than small gas motors. Unfortunately, the electricity has to come from very inefficient batteries, or you need a gas electricity generator and are back to noise. Simplicity gets blocked when dealing with failure, so duplicating everything means a quadcopter turns into an octocopter.
Multicopters are an inefficient means to accomplish VTOL, an inefficient flight profile, using batteries, an inefficient energy store. You can expect anything having this configuration to fail.
Perhaps not the main reason, but an important one in addition to the other answers, is IP. The established players in helicopter design & manufacturing are just that, established; they are all firms that have been refining their technology for decades, often with substantial defence funding, so that all of the performance & efficiency gains in their engineering are subject to stridently contested patents. These are worth many, many millions of dollars.
As is often the case with research by these organisations, there will be patents for things they developed that they didn't end up using, that they keep simply to stop anyone else from using. And they will stop anyone from using them for free.
I don't know the status of the patents that cover what started out as novelty toys, which is where these electric-powered, microprocessor-controlled & stabilised multicopters have developed from - presumably they are not held by Boeing, Lockheed, Bell, Sikorsky, Agusta, et al.
The history of aviation is littered with advances that have happened simply as a result of avoiding patent infringement, however. For example, the Wright Brothers were consumed with stopping anyone else, particularly Curtis, from using any of their ideas. They spent as much effort in litigation as they did in building & flying. Rolls Royce & GE are in a constant dance of patent infringement with their engines to this day, as are Boeing & Airbus. They really hate having to licence technology from or to their competitors.
The concept drawing shows a safety advantage of the multiple rotor design: You can see the beams that support the multiple rotors. The beams are beneath the rotors. This makes it less likely that passengers will accidentally put themselves in the way of the rotor(s).
From the point of view of the concept artist, it is much easier to make the beams that support multiple rotors look both solid and circular than it is to make a single rotor look solid and circular.
