One can clearly see the recent surge and wide applications of unmanned aerial vehicles in many areas, such as agriculture, forestry, surveillance, rescue operations, security, aerial photographing, and so on. The most widespread use of the commercial drones is a multi-copter type of unmanned aerial vehicle. For example, most aerial photography drones are this type. It is because this type of drones doesn’t require a length runway and it can hover in the air while filming. This kind of convenience is necessary, especially in the congested areas like city centers. However, it has a disadvantage that the flight time is too short (typically ranging between 15 to 30 minutes). The fixed wing type drones can stay afloat for a much longer time, typically over 1 or 2 hours. However, it is not always easy to find a safe landing space, and it is not easy to manually control this type of drones   . Due to these reasons, the most commercial drones are multi-copter types.
The third type of unmanned aerial vehicle is often referred to as a VTOL drone, which can vertically takeoff and land, while flies like a fixed-wing aircraft once airborne. It doesn’t require a runway, and land like a helicopter. The VTOL drone can hover in the air, yet capable of flying a much longer time. Due to these advantages, the VTOL drone is taking a great interest among researchers and commercial applications. In order for VTOL drones to hover and make a transit flight like airplane, its propulsion system should be very different than those of multi-copter type drone or fixed-wing type drones. Our initial analysis shows that there are so many varieties already developed, while a thorough study of analyzing VTOL types is not present until this time. In this regard, our goal is to collect the different types of VTOL propulsion technologies and analyze the advantages and disadvantages of each type. Such study will provide valuable insights for the drone community.
2. Analysis of VTOL Propulsion Systems
The analysis classifies each type of VTOL into 3 categories. This include: 1) TYPE1, which has a wing and a TILT-ROTOR, 2) TYPE 2, with no wings and similar to helicopters, and 3) TYPE 3 with wings and no tilting mechanism functions. The details are defined in Table 1  -  .
Tables 2-5 show the specifications of VTOL drones in major countries. In fact, there are over 35 VTOL drones we have identified. Due to similarities, in this paper, we only show the representative types of VTOL drones. Those are illustrated in the tables below.
3. Analysis Contents
First, the percentage of VTOL types has been investigated, as shown in Figure 1. It shows that the Type 2 and Types 3 take the majority of the VTOL drones. VTOL with a tilt mechanism only takes about 18%. The reason can be conjectured that Type 1 drone can be unstable during the transit flight (i.e., from hover to forward flight), and the tilting mechanism can add substantial weights as well as increase the complexity in the system. Due to these reasons, the Type 1 VTOL drones are comparatively limited. Type 2 and Type 3 VTOL drones are quite
Table 1. Classification of VTOL types.
Table 2. There presentative VTOL drones in Korea and USA.
Table 3. There presentative VTOL drones in Taiwan and Canada.
Table 4. There presentative VTOL drones in Japan and USA.
similar in terms of numbers among the drone that we have investigated. Each type bears specific merits and also disadvantages, so each type takes about the same percentages. Second, we have analyzed each type according to the number of criteria investigated. The results are organized in Table 5 and Table 6. The table mainly shows the engine types (electric motors, gasoline powered, diesel, or hybrid engines), and flight durations (less than 2 hours of flying time, less than 4 hours, and over 4 hours).
By analyzing the content of the payload specification, the larger the size of the airframe, the more likely it was to see the airframe size increase. It appears to affect the flight time with drag on airframe, depending on whether or not the airframe is aerodynamically well balanced. Third, we have classified the VTOL drone in accordance with their size and payload. It appears that the size and payload will continue to increase in the future.
The analysis shows that the higher the payload, the bigger the size of the aircraft.
Table 5. Analysis of VTOL propellant technology.
Figure 1. The percentage of VTOL types.
Table 6. Analysis of payload and wing span of VTOL drones.
It is more likely that a bigger air frame affects the flight time. For drones that use electric motors, the battery capacity is directly related to the flight time. However, a bigger battery size inversely affects the payload capacity. According to the analysis of three types, it appears that the fixed wing type VTOL drones with no tilting mechanism are getting more and more popular. It is due to the facts that this type of VTOL drone can provide a stable transit flight while reducing the complexity associated with tilting mechanism. So the drones can be built at a lower cost and a lot lighter. It is judged that the surge in global demand for a wide range of VTOL drone is expected, and a growing competition is expected around the world.
This work was supported by the Ajou University research fund.