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Airfoil definition5/6/2023 ![]() ![]() The analytic form of the construction lends itself easily to computer generation of the airfoil coordinates – see, for example, Ladson et al. The nose radius is then inscribed and faired onto the upper and lower surfaces of the airfoil. ![]() The center for the leading edge radius is found by drawing a straight line through the end of the chord at the origin of the axes and moving a distance along the x axis that is equal to the leading edge radius. The corresponding leading edge radius of the airfoil is rt = 1.101912. For example, for the NACA 0012 airfoil t = 0.12. ![]() Where t/c = t = maximum thickness as a fraction of chord. The upper and lower surfaces of the NACA four-digit symmetrical sections (or thickness envelopes) are described by the polynomial Because these airfoils were also relatively thick, the stiffness of the blade could be maintained while keeping blade weight to a minimum. These airfoils were found to have low pitching moments about the 1 /4-chord and good low-speed as well as high-speed (transonic) performance, giving a relatively high maximum lift and a high drag divergence Mach number. Two of the most popular airfoils used on many early helicopters were the symmetric NACA 0012 and NACA 0015 sections. A summary of the results are documented in considerable detail by Abbott et al. Variables found to have important effects on the airfoil characteristics included the maximum camber and its distance aft of the leading edge and the leading edge nose curvature (nose radius) of the airfoil. The various families of airfoils developed by NACA were then tested in the wind tunnel to document the effects of varying the important geometrical parameters on the airfoil lift, drag, and pitching moment characteristics as a function of AoA, Reynolds number, and Mach number. The use of linearized methods such as thin-airfoil theory also enabled the chordwise aerodynamic loading associated with camber and thickness to be studied, allowing means of designing airfoils to meet specific purposes. This allowed the systematic construction of several families of airfoil sections. Given airfoil could be constructed of a thickness shape that was distributed around a camber line. These airfoils were used by NACA as a basis and were found to have geometrically similar shapes when the camber was removed and the airfoils were reduced to the same thickness-tcbchord ratio. Existing cambered airfoils such as the Clark-Y and Gottingen 398 sections were known from early experiments to have good aerodynamic characteristics. In the United States, NACA conducted a comprehensive and systematic study of the effect of airfoil shape on aerodynamic characteristics. However, it was not until the early 1960s that a more serious effort came about to improve airfoil sections for use on helicopters.Īs early as 1920, a number of different research institutions had begun to examine the characteristics of various airfoils and organize the results into families of airfoils, basically in an effort to determine the profile shapes that were best suited for specific applications. Although symmetric airfoils offered a reasonable overall compromise in terms of maximum lift coefficient, low pitching moments and high drag divergence Mach numbers, they were by no means optimal for attaining maximum performance from the rotor. This event, and the generally low torsional stiffness of early wood and fabric helicopter blades, led to the almost universal use of symmetric airfoil sections on helicopters produced prior to 1960. While having a higher stalling AoA, this airfoil also had a higher pitching moment, which resulted in blade twisting and control problems and finally led to a crash of a Cierva Model C-30 Autogiro – see Beavan & Lock (1939). Cierva originally used a symmetric airfoil section on his Autogiros, but he later changed the airfoil to a highly cambered 17% thick section for better performance. The importance of the airfoil shape on the rotor behavior was well known to Juan de la Cierva – see Cierva & Rose (1931).
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