VOL. 14, NO. 11, JUNE 2019 ISSN 1819-6608 ARPN Journal of Engineering and Applied Sciences ©2006-2019 Asian Research Publishing Network (ARPN). All rights reserved. www.arpnjournals.com 2087 CLASSIFYING NACA AIRFOILS BASED ON THIN AIRFOIL THEORY Ahmad Faiz Aiman Faisal, Aslam Abdullah, Sofian Mohd., Mohammad Zulafif Rahim and Bambang Basuno Department of Aeronautical Engineering, Faculty of Mechanical and Manufacturing Engineering, Universiti Tun Hussein Onn Malaysia, Parit Raja, Batu Pahat, Johor, Malaysia E-Mail: aslam@uthm.edu.my ABSTRACT Thin airfoil theory idealizes the flow around a thin airfoil, and addresses an airfoil of zero thickness and infinite wingspan. It is particularly notable in providing a sound theoretical basis for the important properties of airfoils in two- dimensional flow. This paper gives attention to the gap between the theory and the numerical experiment data in order to make the classification of NACA airfoils and then proposing a simple guideline on the use of the theory. The two- dimensional viscous and incompressible flow around the airfoils is assumed. Several NACA airfoils are considered in the theoretical calculation against the numerical solution. It is found that the theory is applicable for real airfoils within specific range of angle of attack and defined accuracy. Keywords: thin airfoil theory, NACA airfoils, lifts approximation, airfoils classification method. 1. INTRODUCTION Thin airfoil theory is a theory that relates the angle of attack to lift in incompressible and inviscid flows. This theory was developed by Prandtl during World War I [1]. It is still used today because it offers a very simple but reliable and practical method of calculating airfoils properties. It idealizes the flow around a thin airfoil and addresses an airfoil of zero thickness and infinite wingspan. It is particularly notable in providing a theoretical basis for the important properties of airfoils in two-dimensional flow [2]. The basis of the theory may be described as follows. When the angle of incidence is low, the boundary layer growth on an airfoil is thin and remains connected to the airfoil [3]. This allows the assumption of the in viscid and irrotational flow. This study aims at the recognition of the gap between the thin airfoil theory and the numerical experiment. The numerical experiment data have been compared against the well-established experiment data. Classification of airfoils based on the theory validity has also been made. Such classification can be utilized for quick access to approximate lift coefficient profiles in relevant research as ground and vortex shedding effects on airfoils aerodynamic performance [4]-[7], or even more general studies simply involving flows over airfoils [8]- [10]. We considered the aerodynamic property in a 2- dimensional, incompressible and viscous flow. The numerical experiments were conducted with the use of ANSYS Fluent. The geometries were symmetrical airfoils and cambered airfoils. The symmetrical airfoils of interest are NACA 0003, NACA 0006, NACA 0012, NACA 0024 and NACA 0030, while the cambered airfoils are NACA 1112, NACA 1212, NACA 1812, NACA 2412, NACA 4812, and NACA 23012. The comparison between for both types of airfoil should be within the scope of fundamental airfoil theory, by taking the lift coefficient C l and angle of attack α into account. 2. GEOMETRY, GRID AND DOMAIN The flow of interest is that over symmetrical airfoils and cambered airfoils. The geometry of the airfoils is shown in Figure-1 and Figure-2. Respective grid, domain and boundary conditions are shown in Figure-3. (a) (b) (c) (d) -,1 ,0 ,1 ,0 ,2 ,4 ,6 ,8 1,0 -,1 ,0 ,1 ,0 ,2 ,4 ,6 ,8 1,0 -,1 ,0 ,1 ,0 ,2 ,4 ,6 ,8 1,0 -,15 -,05 ,05 ,15 ,0 ,2 ,4 ,6 ,8 1,0