# Computer Vision in Autonomous Unmanned Aerial Vehicles--A Systematic Mapping Study
## 3 Results
### RQ3: What Types of UAVs Have Been Used in the Proposed Solutions?
#### 3.3.2 Concerning the Vision-Based Task and Type of UAV  
If we consider the first classification regarding the computer vision task in each paper, the distribution again shows that the most frequent type of UAV belongs to the rotatory-wing class. Rotary-wing UAVs are the majority in all categories, and more clearly for the first two focused on navigation and control. The papers that do not specify the type or employ a less frequent type of UAV belong to categories _1. Visual Navigation_ and _4. Vision-Based Sense-and-Avoid (SAA)_. These papers are usually validated by means of tests using images obtained from databases or satellite maps. Furthermore, a more in-depth analysis has been performed considering the specific types of aircraft within each class. Figure 13 shows the UAV class distribution over the four categories detailing the type of UAV. This information is provided as \"Class: Type of UAV\"; for example, Rotatory-Wing: Helicopter. When the type of UAV is not available, only the name of its class is indicated.  
Figure 11: UAV class distribution over the database.  
Figure 12: Annual trend per UAV class.  
Starting with the most common _Rotatory-Wing_ class of UAV, the types of UAV found in the analysis are _Helicopter_, _Quadrotor_, _Multirotor_ and _Sit-On-Tail VTOL_. Quadrotors are clearly the most widely used, highlighting 18 papers in the control of maneuvers subcategory. In this respect, quadcopters are vehicles with four rotors, relatively simple to control, with high maneuverability and the ability to hover. All this, together with the wide range of low-cost quadrotors on the market, has made a significant contribution to their clear predominance in the research developed to date. The second most frequent type of rotary vehicles are helicopters, distantly followed in third place by multirotors. A Sit-On-Tail VTOL aircraft, which is used in a paper on the control of maneuvers, is in the last position.  
With respect to the second most common class, _Fixed-Wing_, we should point out that it is usual not to provide details of which model of fixed-wing aircraft has been used. In fact, only three papers provide this information. Two of them address the process of autonomous landing by using a _Blended Wing-Body_ aircraft, whereas a _Shadow_ model is used in one paper on the detection and avoidance of an intruder aircraft. Furthermore, the only UAV grouped in _Airship_ is a _Blimp_, which is used in a paper concerning visual navigation based on SLAM.  
An additional fact is that these graphs once again show the predominance of rotary-wing air vehicles over the fixed-wing category, not only as regards the number of articles, but also as regards the fact that this type of air vehicle is present in all the subcategories considered, while fixed-wing  
Figure 13: UAV class distribution over the categories.  
air vehicles are used in only some of them. However, it should be highlighted that fixed-wing UAVs are more used in operations related to high-altitude flight and large distances due to the features of such airplanes. For instance, in the case of autonomous aerial refueling operations, whose purpose is, in effect, to increase flight autonomy (the ability to perform the complete flight without the need of making stops to refuel on the ground). Fixed-wing UAVs are also used frequently in stabilization of the UAV's flight regarding the skyline, and in detection and avoidance of other aircraft.  
Finally, in order to conclude this extensive analysis of UAV platforms, the complete list of the 144 papers considered in this mapping study, classified according to the class of UAV and also detailing the type of aircraft, is shown in Table 5.