# Fast Airfoil Selection Methodology for Small Unmanned Aerial Vehicles
## 1 Introduction
The purpose of this literature review is to collect all of the common attributes from the examined studies, in order to express some useful conclusions for the current study's significance. It should be noted that not under any circumstances are the authors of this study disputing the methodologies used or the results provided in these studies. In study [24], an airfoil selection procedure for a \"Flying Wing Micro Aerial Vehicle\" was conducted, where 10 low Reynolds number airfoils were examined. A series of airfoil's aerodynamic measured values, were presented in the tables. In study [25], the airfoil selection for a \"Morphing and growing micro Unmanned Air Vehicle\" was conducted, without providing enough information for the airfoil measurements or the process itself that could be reused by others. Study [26] discusses a \"mini Unmanned Aerial Vehicle\" design where three airfoils were subject to selection. Little reusable information is given. Reference [27] examines six NACA airfoils for a \"UAV with m\({}_{\text{tow}}\) = 200 kg\". No reusable information is given regarding the airfoils. In study [28], six airfoils participated in the selection for a \"long-range Vertical Take-Off and Landing (VTOL)\" aircraft. An interesting point in this study was the proposal of the \"bucket width\" characteristic to participate as a variable in the airfoil selection. Our study also gives a great importance to this specific attribute and also displays it in a graphical manner in the \(\left(C_{l_{max}}-C_{l_{i}}\right)\) diagram. The importance of the \"bucket width\" attribute, is discussed later in our study. Reference [29] provided a satisfactory amount of reusable information for 15 under selection airfoils, for a \"Solar-Powered UAV\". It also provided a diagram which can be useful for any designer willing to replicate the study's airfoil selection procedure, in other similar projects. This diagram displays \(C_{l_{\text{\emph{transic}}}}\) in the x axis and \(C_{l^{\prime}}/C_{d}\) in the y axis. This diagram, unfortunately, was not useful for someone applying the base methodology of [8; 21]. Finally, there were some published articles such as [30; 31], where no useful information was given at all regarding the airfoil selection procedure. These studies are written in a manner of an overall vehicle design demonstration, where such details were skipped for practical publishing purposes. All of the above studies surely accomplished their main target, which was to select the best airfoil for the given circumstances. Some common attributes of the examined studies are the following:  
1. For the majority of the studies, the simulation software used was either Xfoil or XFLR5;
2. There are studies where useful information cannot be obtained for reuse, due to the small amount of the participating airfoils;
3. In most cases, studies are not offering well organized and sufficient information regarding airfoil aerodynamic parameters which can be reused in similar new applications. As a result, there are a lot of overlapping simulations for the same airfoils;
4. None of the above reviewed, indicative studies, but also none in a more extensive literature review that took place before taking the decision of conducting the current study, offers a \(\left(C_{l_{max}}-C_{l_{i}}\right)\) diagram along with the necessary supplementary tables for conducting a low Reynolds number airfoil selection, that strictly follows the rules in the literature [8; 21]. Continuing with the literature review of the various processes within Figure 1, it can be noticed that after an airfoil is selected or designed, using any of the possible methods, then this airfoil may be a subject of optimization in order to better suit the original requirements. In [32], the authors propose the use of a Genetic Algorithm for the purpose of optimizing UAV airfoils. An application of similar Genetic Algorithms is carried out in [33], where the authors are optimizing the airfoil of an agricultural aircraft.