As shown in the figure below, airfoils can be symmetric, which is an airfoil with the same shape and curvature on the upper and lower surfaces, or cambered, which has a different upper and lower surface shape. The basic geometry of an airfoil is described in terms of a profile shape or envelope that defines the curvature of its upper and lower surfaces. Unfortunately, airfoil characteristics at low Reynolds numbers are usually quite different from those found at higher Reynolds numbers, often showing remarkably low aerodynamic efficiencies. There has recently been much interest in designing efficient airfoils for use at the low flow speeds and low Reynolds numbers found on UAV systems, which require detailed knowledge of boundary layer developments. A high critical Mach number, i.e., the free-stream Mach number when supersonic flow first develops over the airfoil.The ability to reach high values of the lift-to-drag ratio, perhaps also at specific angles of attack.The attainment of a particular value of nose-up or nose-down pitching moment.The minimization of drag over a broad range of operating conditions.Obtaining high values of the maximum attainable lift coefficient before flow separation and stall occurs.Typical design requirements for airfoil sections include: There are 1000s of airfoils in current use, most being selected or otherwise adapted to optimize their performance for their specific aircraft application(s). Some of the earliest known “concavo-convex” airfoil shapes were patented in the late 1880s. Phillips tested these airfoils in one of the very first wind tunnels. Notice the very thin, highly cambered profile shapes compared to most modern airfoils. The earliest known airfoil sections for aircraft concepts were patented in the 1880s by Horatio Phillips, as shown in the figure below, which were inspired by the wings of birds.
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