1. Introduction

The concept of vertical flight dates back to the early days of aviation. Pioneers such as Leonardo Da Vinci first imagined that machines could take vertical take-off and landing (VTOL). In the early 20th century, several inventors and engineers began to experiment with VTOL concepts, including helicopters and rotorcraft. According to Wang Guanlin and Wu Zhe, a drone using this technology has many benefits, including taking off within a limited range and reaching its destination quickly [1].

Helicopters are the most famous vertical take-off aircraft. The development of the helicopter began in the 1930s and 1940s, this development was contributed by some inventors such as Igor Sikorsky. Through development, the helicopter has become a complex machine which can do many tasks. More importantly, the vertical takeoff and landing of fixed-wing Unmanned Aerial Vehicles (UAVs) overcomes the common problem that the range and speed of UAVs are always limited [2]. Also, the UAVs gets the rid of the shortcomings of the conventional fixed-wing UAV launch and recovery method, insufficient stability, and high personnel requirements.

Recently, the new concept of vertical landing and takeoff has already emerged. Such as tilt-rotor aircraft, duct fan systems and multi-rotor helicopters [3]. These technologies have unique advantages and are being explored for a variety of applications. These technologies also have many benefits a bright future.

2. Applications of VTOL

In the field of aviation today, vertical takeoff and landing (VTOL) technology is attracting extensive attention with its unique advantages and broad application prospects. To be more specific, VTOL technology enables aircraft to take off and land directly from the ground vertically without the need for a long runway. With the continuous progress of technology in that field, this technology has shown great potential in both military and civilian fields.

Firstly, from a military perspective, vertical takeoff and landing aircraft possess high mobility and flexibility. In that way, on the battlefield, people can be quickly deployed in a confined space and then respond promptly to the rapidly changing situation. Especially in a frontline airport with complex terrain or on a limited-space combat platform such as an aircraft carrier, vertical takeoff and landing fighters can play an important role. In other words, without relying on traditional long runways, the requirements for base facilities are greatly reduced and combat effectiveness and survivability are improved.

Secondly, in the civilian field, VTOL technology also brings new possibilities to urban air transportation [4]. With the acceleration of urbanization, traffic congestion is becoming an increasingly serious problem. However, vertical takeoff and landing aircraft can take off and land on the tops of high-rise buildings in urban centers or other limited spaces, in that way, providing people with an efficient and fast travel mode. In addition, VTOL technology can also be applied in fields such as emergency rescue and material transportation, which can provide strong support for dealing with emergencies.

Thirdly, there are various implementation methods of VTOL technology, mainly including jet vertical takeoff and landing and rotorcraft vertical takeoff and landing. Different implementation methods have their advantages and disadvantages. Researchers have been constantly exploring and improving to enhance the reliability, safety and performance of the technology [5]. As an important innovation in the field of aviation, VTOL technology is leading a new direction for future aviation development.

3. The types of VTOL aircrafts

3.1. Rotor VTOL aircrafts

The work principle of the helicopter is that the helicopter generates lift and thrust through the rotation of the main rotor and the main rotor blades are designed to produce a downward flow force to the air. This will create the lift force. The tail rotor is used to offset the torque generated by the main rotor [6]. The benefit of the helicopter is that this kind of aircraft has high maneuverability, hover, vertical takeoff and landing ability. However, the limitation is that the helicopter has limited speed and range and high noise levels. What’s more, its complex mechanical systems are considered to be a disadvantage.

Another rotor VTOL is the multi-copter, The working principle of the multi-copter is that a multi-rotor consists of some rotors which can generate lift force. The rotors can be independently controlled to ensure stable flight and maneuverability [7]. It has a simple design and lower cost and is also easy to operate. However, it only has a little time to fly in the air and can be affected by big winds.

3.2. Tilt-rotor VTOL aircrafts

The working principle of tilt-rotor aircraft is that the rotor can take off and land at a tilt from a vertical position to a position while flying directly. This allows it to combine the advantages of helicopters and fixed-wing aircraft [8]. The advantage of tilt-rotor aircraft is that it has high speed, long distance and take-off and landing ability. However, the limitations are that the mechanical system is very complex, the cost is too high, and the hover time is too limited.

3.3. Jet-powered VTOL aircrafts

The working principle is that Jet-powered VTOL aircraft use jet engines with vector nozzles to generate thrust in different directions to achieve VTOL. It has fast speed, good performance and long range. This kind of aircraft costs too much fuel and has a complex engine system which is hard to maintain. What’s more, it is limited in payload capacity.

3.4. Mixed VTOL aircraft

A hybrid VTOL system combines an electric motor with a conventional propulsion system together, for example, a gas turbine or internal combustion engine. Electric motors provide additional power for takeoffs and landings. On the contrary, conventional propulsion systems are used for forward flight [9]. This kind of aircraft has reduced emissions, reduced noise and improved efficiency. However, it is limited by battery life and a complex power management system.

3.5. Hybrid rotor or fixed-wing VTOL aircraft

These systems combine the characteristics of rotorcraft and fixed-wing aircraft together. They can take off and land vertically using rotors and then transition to fixed-wing flight to increase speed and range [9]. It has multi-purpose, high performance, vertical takeoff and landing ability. However, the limitation is that the mechanical system is complex and expensive.

4. The working scheme of VTOL

4.1. Aerodynamics and lift generation

The lift generated by the VTOL system is based on the aerodynamic principle. To be more specific, Different types of VTOL systems use different methods to generate lift, for example, rotor blades, wings, and jet engines have different ways of flying. In that way aerodynamics are crucial to the design and optimization of VTOL systems.

4.2. Propulsion system

Vertical landing systems require a powerful propulsion system to generate thrust for takeoff, hover and forward flight. In that way, propulsion systems may include rotors, engines and electric motors. Thus, the choice of propulsion system relies on the particular needs of the application as well as the type of VTOL system.

4.3. Control system

An effective control system is essential to the safe operation of a VTOL system. The control system must be able to accurately control the position, direction and speed of the aircraft. In that way, advanced control algorithms and sensors are used to ensure a stable flight and precise maneuvers.

5. The benefits and the limitations of VTOL technology

5.1. The benefits

Firstly, VTOL technology increases the flexibility of aircraft. This kind of technology allows aircraft to fly in area in which the space is limited or the runway is too short. In that way, this benefit provides big flexibility to military operations, emergency response and urban air mobility.

Secondly, it doesn’t need much infrastructure to support the aircraft. To be more specific, comparing to the normal airplanes the aircraft with VTOL technology don't need a long runway to take off and don’t need a big airport to support it.

Thirdly, this technology provides a quicker reaction time [10]. Aircraft with this technology can react quicker when it comes to emergency, and save more time, and as a result save more lives.

Fourthly, this technology reduces noise pollution. Aircraft with VTOL can fly with a lower noise, which will increase the living quality of the citizens.

5.2. The limitations

Firstly, VTOL aircrafts have limited carrying capacity. Many aircraft with this technology can only carry a small number of things. This capacity limits its application, and so does the small multi-rotor helicopters and some hybrid VTOL aircrafts.

Secondly, aircraft with this technology can only fly for a short time [11]. Aircrafts are limited by the battery life and the electricity. It is complex to recharge.

Thirdly, this system typically has really complex mechanical and electrical systems, which can increase the risk of failure and require significant maintenance. In that way, it is hard to maintain.

Fourthly, it is difficult to overlook the aircraft. The development and operation of VTOL technology are subject to regulatory requirements, which can be complex and time-consuming.

6. Future challenges and development

As technology advances, innovative materials such as lightweight composites and 3D printing will enable lighter and more efficient VTOL systems. Also, these new materials will improve the aircraft’s robustness. What’s more, the development of electric propulsion systems will make a difference in growing number of electric propulsion systems, which could make the aircraft quieter, and greener. Electric motors offer several advantages over traditional internal combustion engines, for example, fewer emissions, less noise, and greater efficiency [12]. Last but not the least, the development of autonomous flight technology can improve the safety and efficiency of VTOL operations. In that way, autonomous VTOL aircraft can perform tasks such as take-off, landing and navigation without human intervention.

Concerning the difficulties, it is essential to discuss the regulations governing vertical landing and takeoff (VTOL) aircraft. To be more specific, it is really necessary for governments to make policies to restrict the use of these kind of air craft, because there is a serious risk that these aircraft may collide with airplanes. Also, it is necessary for everyone to follow the rules. What’s more, the government should develop the infrastructure for these kinds of aircraft to support the use of it [13]. Lastly, it is also a challenge to make every citizen accept these kinds of aircrafts.

7. Conclusion

VTOL technology has emerged as a revolutionary force within the aerospace domain. In that way, it bears significant implications for many areas such as military and civilian arenas. In the military area, VTOL aircrafts present unparalleled strategic advantages. They can be expeditiously deployed in diverse terrains and scenarios, which could diminish reliance on traditional runways and augment operational flexibility. This facilitates swifter responses to emerging threats and allows for deployment in areas where infrastructure is limited or compromised. VTOL fighters and transport aircraft can operate from aircraft carriers, which could expand the reach and power projection capabilities of naval forces. Additionally, their capacity to take off and land in confined spaces renders them ideal for special operations and covert missions. In the civilian sector, VTOL technology holds the promise of transforming urban transportation. As cities become increasingly congested, VTOL aircraft could offer a rapid and efficient way of travel between locations. Concepts such as air taxis and personal air vehicles are being explored, which could revolutionize commuting and logistics. VTOL systems also have potential applications in emergency medical services, disaster relief, and cargo transportation.

The development of VTOL technology has been propelled by advancements in propulsion systems, materials science, and control algorithms. Hybrid-electric and all-electric propulsion systems are being developed to reduce emissions and noise, which will make VTOL aircraft more environmentally friendly. Advanced materials are being utilized to reduce weight and enhance the strength of that aircraft, while sophisticated control systems ensure stable flight and safe operations. However, there are still challenges that need to be addressed. To be more specific, safety and regulatory issues must be meticulously considered to ensure the safe integration of VTOL aircraft into existing airspace. Infrastructure requirements such as landing pads and charging stations, need to be developed. Cost is also a significant factor for the reason that VTOL technology is currently expensive to develop and operate.

All in all, VTOL technology represents a significant advancement in aerospace technology with broad potential applications. While challenges persist, continued research and development are likely to lead to further enhancements and wider adoption. As this technology matures, it has the potential to reshape the way we travel, conduct military operations, and respond to emergencies.