What is eVTOL?
eVTOL stands for electric vertical take-off and landing. As the name suggests, the core of eVTOL is electric + vertical take-off and landing. The earliest concept model of eVTOL appeared around 2010. In 2009, JOBY, the world’s first eVTOL company, was established. In 2011, the world's helicopter giant August Westland of Italy proposed the eVTOL concept. In 2014, the American Helicopter International Association (AHA) and the American Aeronautics and Astronautics Association (AIAA) officially introduced the eVTOL concept at the Virginia Convention. In 2016, Uber proposed the Uber Elevate air taxi plan, triggering a wave of eVTOL in the European and American markets. In January 2016, EHang Intelligent released the world's first unmanned manned eVTOL aircraft "EHang 184" at CES in the United States. In 2017, the first eVTOL aircraft catalog compiled by the Vertical Flight Association was released. In 2019, the European Aviation Safety Agency (EASA) issued new aviation management regulations for the airworthiness certification of small eVTOL aircraft. Since then, the eVTOL concept has been officially recognized, and players such as startups, automobile and aviation industry giants have begun to enter the eVTOL market. In 2019, China's EHang Intelligent was listed on Nasdaq, becoming the world's first listed UAM (Urban Air Mobility) company. In 2021, companies such as Joby in the United States, Lilium in Germany, Archer in the United States, and Vertical Aerospace in the United Kingdom will be launched one after another, and the commercialization of eVTOL is accelerating.
Currently, eVTOLs used by major OEMs can be divided into three types according to flight principles: multi-rotor type, composite airfoil type, and tilt-rotor type. The principle of multi-rotor is similar to that of traditional civilian drones and helicopters. It is relatively easy to develop, has a simple design configuration, stronger stability, and is small in size. It can be flexibly deployed in urban environments and has lower infrastructure requirements; composite wing The composite type has two sets of rotors, one vertical rotor controls take-off and landing, and one horizontal propulsion rotor controls cruise. The composite type's take-off and landing are separated from the propulsion power system, so it has high flight safety; the principle of the tilt-rotor type is similar to that of a traditional tilt-rotor. For a helicopter, during the take-off phase, the rotor provides lift like a helicopter propeller. During level flight, the rotor tilts forward to provide forward thrust. The tilt-rotor type has the greatest advantages in terms of speed and range, but the mechanical design is too complex. , the rotor tilting is prone to failure during high-speed flight, following the high accident rate of traditional tilt-rotor helicopters.
All eVTOL technical routes go hand in hand, each anchoring applicable demand scenarios.
The principle of the multi-rotor type is similar to that of traditional civilian drones, with low efficiency. The current flight mileage is short and it cannot be widely used in low-altitude transportation in a short period of time. The multi-rotor type has high redundancy, stability and safety, and can be deployed flexibly. , has the advantages of low infrastructure requirements, low airworthiness risks, good economics, and rapid market access. It is expected to be the first to expand in the tourism market where eVTOL density is low and flight demand is small.
The composite airfoil take-off and landing is independent of the propulsion system. When encountering wind shear, loss of control, stall, etc. during flight, the independent lift system can be immediately activated to hover the eVTOL in the air and make a controlled descent to land. Preparation of the power system and response time is very short, so it is safe, has low noise, considerable range, and the composite airfoil has excellent overall performance. It has a higher safety factor than other types and is expected to be used in fields such as short-term rescue and emergency medical transportation.
According to "Safety Assessment of Power Systems of Electric Vertical Takeoff and Landing Aircraft of Different Configurations" (Liu Jujiang, [Journal of Harbin Engineering University], 2023), the tilt-wing configuration eVTOL power system has the characteristics of insufficient power redundancy. After losing two thrusts, it is not enough to perform thrust trimming to complete vertical landing; when encountering wind shear, loss of control, stall, etc. during flight, the tilting aircraft airfoil requires a tilting process of several seconds to tens of seconds, and the tilting wing speed It is slow and cannot realize the protection function. The failure probability is 1.265×10-6, which is much higher than the characteristic accident rate standard established by the aviation industry for large passenger aircraft and large general aviation aircraft of 10-9 flight hours. If the tilting airfoil eVTOL solves the problem of flight safety Problem, with its advantage of fast speed, tilt-rotor and composite airfoil eVTOL will be the main force for large-scale low-altitude urban transportation in the future. It is expected to open up a larger market after the low-altitude travel policy is further released, with high potential.
Multi-rotor eVTOL: The principle is derived from multi-rotor UAVs. It is easy to achieve lifting and cruising. The multi-rotor design is stable and safe.
According to "Main Progress in Electric Vertical Takeoff and Landing Aircraft in 2022" (Han Yuqi, [Aerodynamics], 2023), multi-rotor eVTOLs provide lift from multiple rotors, and control attitude and cruising by adjusting the speed of each rotor. There are no wings. The principle is derived from traditional multi-rotor civilian drones, and the technical difficulty is minimal. Taking a small quad-rotor UAV as an example, functions such as hovering, vertical lift, pitch or roll, and yaw movements can be achieved by controlling different rotor speeds. Multi-rotor eVTOLs mostly have a four-axis or six-axis layout. The EH216-S adopts a coaxial twin-propeller 16-rotor layout, which has high redundancy and is flexible, stable and safe in flight.
Composite airfoil eVTOL: Originated from compound vertical take-off and landing fixed-wing helicopters, efficiency is improved under electric drive
According to "Analysis of Compound Helicopter Technology Development" (Li Jianbo, [Journal of Nanjing University of Aeronautics and Astronautics], 2016), as early as 1954, the former Soviet Union's Kamov Company Ka-22 invented a compound helicopter, which used dual rotors. With a horizontal layout, the engine under the rotor also drives the propeller. It achieved a flight record of 356km/h in 1961. This type of compound vertical take-off and landing fixed-wing helicopter is the inspiration for the compound airfoil eVTOL. Benefiting from lightweight motors, the composite airfoil eVTOL has reduced dead weight compared to traditional compound helicopters and improved flight efficiency.
Tilt-rotor eVTOL: aerodynamics, dynamics, and flight control issues need to be solved urgently
In 1972, NASA and the Army launched a tilt-rotor aircraft program. Bell Helicopter won the development contract in 1973 and named the prototype XV-15 (the prototype of the V-22 Osprey aircraft). It is the originator of the tilt-rotor helicopter; the flight principle of the tilt-rotor eVTOL is similar to that of the tilt-rotor helicopter, and it also inherits the high accident rate characteristics of the tilt-rotor helicopter. Since the test, the V-22 "Osprey" There have been 5 fuselage damage accidents involving aircraft, resulting in a total of 54 deaths. The main reason is that the tilt-rotor aircraft has rotor-wing aerodynamic interference during flight, aerodynamic characteristics during the rotor tilting process, flight dynamics, Flight control in the transition corridor and a series of difficult problems.
Why eVTOL?
The low-altitude economy is a comprehensive economic form driven by various low-altitude flight activities of various manned and unmanned aircraft, radiating and driving the integrated development of related fields. With the development of intelligent unmanned aircraft in recent years, urban air mobility (UAM) has become popular around the world. UAM uses short take-off and landing or vertical take-off and landing aircraft and related system facilities to realize urban low-altitude airspace. The transportation activities of people and goods are an important part of the future intelligent transportation system.
China's urban low-altitude traffic is a strongly regulated industry. The Civil Aviation Administration of China is responsible for air traffic management and provides unified management and airspace division for domestic and foreign civil aircraft. The lower limit of cruising altitude for civil aviation aircraft is 3,000 meters, which is managed and deployed by the air traffic control center. Flight operations carried out at medium and low altitudes below 300 meters belong to the category of low-altitude general aviation.
There is a natural connection between low-altitude economy and general aviation. According to my country's Civil Aviation Law, general aviation refers to civil aircraft engaged in civil aviation activities other than public air transportation, including operational flights in industry, agriculture, forestry, fishery and construction, as well as medical and health care, emergency and disaster relief, meteorological detection, ocean monitoring, Flight activities in scientific experiments, education and training, culture and sports, etc.
In May 2016, the General Office of the State Council issued the "Guiding Opinions on Promoting the Development of the General Aviation Industry". General aviation was listed as a national strategic emerging industry and was given a history of adjusting the economic structure, transforming the mode of economic development, improving people's livelihood and upgrading consumption. mission. The National Development and Reform Commission also issued the "Notice on Doing a Good Job in the Demonstration and Promotion of General Aviation" and the "Implementation Opinions on the Construction of a Comprehensive Demonstration Zone for the General Aviation Industry" in November 2016 and January 2017, proposing the construction of the first batch of 26 general aviation demonstration zones. Aviation industry comprehensive demonstration zone and 41 general aviation demonstration projects.
Despite policy support, my country's general aviation still faces development difficulties, mainly reflected in the insufficient supply of low-altitude airspace resources, cumbersome approval procedures for low-altitude flight activities, and the lagging construction of general airports. As an important national strategic resource, low-altitude airspace is under the unified leadership of the National Air Traffic Control Commission. There are few low-altitude airspaces developed for general aviation. It is difficult to adapt to the multi-point, convenient and efficient characteristics of general aviation. The number of airspaces suitable for private and recreational flights is limited. Small scope, scattered distribution, and poor availability. Some civil airports refuse or restrict the entry of general aviation aircraft for operations. They lack dedicated low-altitude routes. The approval procedures for general aviation flights are cumbersome and the approval efficiency is low. Planes need to taxi long distances to take off and need a large enough area to pave the runway. The overall construction standards of general airports are too strict and the cost is too high. It costs hundreds of millions of yuan to build a runway of a few hundred meters. At the same time, in order to avoid repeated approvals, several kilometers of runways will be built at a time. Each flight requires a high route subsidy to operate. Continuity is unsustainable.
Fixed-wing aircraft require long runways to assist in take-off, and urban centers with tall buildings and dense crowds cannot provide them with complete take-off conditions. They can only perform large-scale passenger and cargo transportation tasks between cities, and cannot serve point-to-point services within cities or adjacent cities. transportation.
Helicopters have the advantages of heavy load and strong maneuverability, and do not require special runways and airports for take-off and landing conditions, which can save the infrastructure costs of general aviation airports. However, it is noisy during flight, has high operating costs, and is expensive to maintain, making it difficult to further popularize it.
Compared with the first two, eVTOL integrates autonomous driving, electric power, and low-altitude aviation elements. It is quieter, lower cost, and can take off and land vertically, eliminating the cost of building an airport. It can take off and land anytime and anywhere.
How to view the barriers to the complete eVTOL machine: sub-system supporting and aerodynamic layout can be "migrated", focusing on the design and integration capabilities of the complete machine
Various types of UAM carriers vary greatly in terms of aircraft models, aerodynamic layouts, and power modes. Carriers are the key to whether UAM can operate. Among UAM carriers, helicopters were the first to be used, but received far less attention than eVTOL aircraft. The main reason is that eVTOL aircraft have the characteristics of low noise, environmental protection, safety, low maintenance costs, and diverse aircraft models.
In terms of aerodynamic layout, whether it is multi-rotor, composite wing or tilt-rotor, there are helicopters, drones and other models with similar aerodynamic layouts that can be used as reference, and the technology has strong traction.
eVTOL aircraft are mainly based on electrical and automatic control. The technical basis is flight control, motors, lithium batteries, etc. These factors are closely related to automatic control and electrification technology, and these related technologies have only matured in recent decades and can be applied in On board a manned aircraft. It is precisely for this reason that eVTOL aircraft design has begun to appear in the past ten years. Compared with conventional aircraft, eVTOL has the following technical characteristics:
1) Vertical take-off and landing technology. Most eVTOL aircraft use an electric drive system, usually a multi-rotor design, and are equipped with advanced flight control systems, including sensors, electric adjustments of rudder surfaces and propellers, etc. to achieve precise flight control and attitude adjustment. They usually have autonomous flight and automatic ability to drive;
2) Battery technology. Battery is the core of eVTOL technology, and the mainstream is lithium-ion battery. Battery technology will not only affect the endurance of the aircraft, but also the shape of the aircraft. Improving battery specific energy and power, and achieving fast charging and long service life are the main technical challenges facing eVTOL batteries;
3) Distributed electric propulsion technology. Distributed electric propulsion technology can provide higher thrust control accuracy and flexibility and is commonly used in eVTOL. By distributing multiple electric engines at different locations on the aircraft to achieve propulsion and control of the aircraft, it has significant advantages in improving flight performance, enhancing maneuverability, and providing redundancy.
eVTOL uses batteries to provide energy for the power system and onboard systems. Battery performance is a key factor restricting the development of eVTOL. At present, lithium battery technology is relatively mature and stable, with the highest energy density ratio, and most manufacturers use lithium batteries. The four major parameters of battery energy density, battery instantaneous charge and discharge rate, battery charge and discharge life, and battery safety are key indicators in eVTOL batteries, which significantly affect the technical performance of eVTOL such as range, lifting stability, replacement life, and ride safety. At present, as the application demand scenarios of eVTOL continue to expand, in order to break the development limitations of low payload, short range, and short endurance time faced by eVTOL, high energy density, high charge and discharge life, high instantaneous charge and discharge rate, and high safety are needed. The demand for aviation batteries from the “four highs” continues to expand, and batteries have become a key technology limiting the development of eVTOLs.
Existing battery technology is still lacking, and indicators such as energy density and safety need to be further improved. Otherwise, eVTOL will face development limitations such as low payload, short range, short endurance time, and unsafe rides. Current battery technology cannot fully meet the range, life cycle, fast charging technology and energy density of eVTOL operating scenarios. At present, the most used and fastest growing field of lithium batteries is the field of new energy electric vehicles, but the batteries used in eVTOL (taking the lithium battery used in Joby S4 as an example) are leading in terms of energy density, instantaneous charge and discharge rate, cycle life, and safety. Ternary lithium batteries used in passenger cars.
The aviation battery market is developing rapidly, gradually breaking the bottleneck of eVTOL performance limitations. At present, the biggest problem limiting the performance of eVTOL is aviation batteries. Currently, all parties have a comprehensive layout in the field of aviation batteries. The Green Aviation Manufacturing Development Outline (2023-2035) of the Ministry of Industry and Information Technology and other four departments points out that green aviation lithium should be vigorously developed. Batteries, breakthroughs in high-energy-density lithium batteries, achieving mass production of aviation lithium battery products with an energy density of 400Wh/kg, and small-scale verification of 500Wh/kg-level products; NASA and CATL have developed new high-energy-density aviation lithium batteries with high stability Batteries are designed to solve range anxiety and safety anxiety in the field of manned electric aircraft; Zhengli New Energy has released a new aviation battery with high energy density and high charge-discharge rate, solving the industry problem of incompatible high energy density and high rate. The comprehensive and high-level development of aviation batteries will fully drive the performance end of eVTOL, making eVTOL application scenarios more extensive, and the field of airspace electrification is expected to be fully rolled out.
According to a report by SMG, a consulting company in the field of aerospace, defense and automotive technology in the United States, it scores UAM manufacturers based on five dimensions: financing situation, leadership team, technology, certification progress, and production status, and obtains a ranking of mainstream UAM vehicle manufacturers. ranking.
