# Fast Airfoil Selection Methodology for Small Unmanned Aerial Vehicles
## 1 Introduction
Optimizing processes can also be applied for airfoils of any type, such as wind turbine airfoils [34]. As long as all of the internal details of Figure 1 are now discussed, it is comprehensible that the aircraft's design will result in an airfoil, more or less efficient for the given needs, whatever possible route is followed by the designers. This study focuses on a certain path, which is marked in green color within Figure 1. Specifically, it concerns selecting an airfoil with the strict application of the methodology described in Dr. [PERSON]'s textbook _Design of Unmanned Aerial Systems_[21]. For the readers willing to acquire more information regarding this method, the same author's textbook _Aircraft Design: A Systems Engineering Approach_[8], is the appropriate source. This study offers the solution to the following problem:  
Any high Reynolds number-operating aircraft designer will not face any difficulty applying the aforementioned airfoil selection method, due to the fact that the literature already offers all of the required data to do so. In reverse, a low Reynolds number aircraft designer, who is willing to select an airfoil for a small UAV, will discover that the data available in the literature are not sufficient. This is one of the reasons that the overlapping simulations of the same airfoils are present in the indicative literature review previously conducted. The designer will eventually be forced to execute a new cycle of airfoil simulations, which will cost in terms of development time. The novelty of this study consists in the following facts:  
1. It fills the gap that exists in the low Reynolds section when applying the airfoil selection methodology according to [8; 21], as it provides the only bibliographically available \(\left(C_{l_{max}}-C_{l_{i}}\right)\) diagram until now that presents the following unique characteristics in comparison with the similar diagrams that exists, of course, only for the high Reynolds section. 1. The diagram additionally displays the \"bucket width\" each studied airfoil has in its \(\left(C_{l_{max}}-C_{l_{i}}\right)\) plot. The usefulness of this characteristic is discussed later in this study. 2. The diagram's x and y axes are expressing different Reynolds numbers comparing to each other for increased accuracy, while the x axis corresponds to the cruise phase and the y axis corresponds to the take-off phase, where the vehicle's velocities are different by nature;
2. It provides supplementary airfoil tables containing the full set of the necessary variables that procedure [8; 21] needs, for later use on airfoil comparisons. The numerical performance gain that a small UAV designer will obtain by using the current study's findings is not related to the performance of the produced UAV, however it is exclusively related to the reduction in the development times because:  
1. If the designer is satisfied with current study's participating airfoils, no additional time-consuming simulations are needed. 2. If the designer is willing to expand the airfoil database or include other Reynolds numbers, reduced times will also be needed due to the fact that the current study offers its own expansion, step by step, methodology. To express the time gain in a percentage manner, the authors, after implementing the overall procedures into Euclid Small UAV, estimate that 85% of engineering time will be economized in the above circumstance outlined in 1. The time gain for the above circumstance 2 varies, as it is related to the expansion depth that the designer is willing to apply.