Читать книгу Flight Theory and Aerodynamics - Joseph R. Badick - Страница 92

Most airfoil development in the United States was done by the National Advisory Committee for Aeronautics (NACA) starting in 1929. NACA was the forerunner of the National Aeronautics and Space Administration (NASA). The first series of airfoils investigated was the “four‐digit” series. The first digit gives the amount of camber, in percentage of chord. The second digit gives the position of maximum camber, in tenths of chord, and the last two give the maximum thickness, in percentage of chord. For example, a NACA 2415 airfoil has a maximum camber of 2% C, located at 40% C (measured from the leading edge), and has a maximum thickness of 15% C. A NACA 0012 airfoil is a symmetrical airfoil (has zero camber) and has a thickness of 12% C.

Further development led to the “five‐digit” series, the “1‐series,” and, with the advent of higher speeds, to the so‐called laminar flow airfoils. The NACA’s 23000 series created in 1935 were very popular and are still in use today. The laminar flow airfoils are the “6‐series” and “7‐series” airfoils and result from moving the maximum thickness back and reducing the leading edge radius.

Figure 3.17 NACA airfoils (NACA data).

Two things happen with this treatment. First, the point of minimum pressure is moved backward, thus increasing the distance from the leading edge that laminar (smooth) airflow exists, which reduces drag. Second, the critical Mach number is increased, thus allowing the airspeed of the aircraft to be increased without encountering compressibility problems. In the 6‐series, the first digit indicates the series and the second gives the location of minimum pressure in tenths of chord. The third digit represents the design lift coefficient in tenths, and the last two digits (as in all NACA airfoils) show the thickness in percentage of chord. For example, NACA 64‐212 is a 6‐series airfoil with minimum pressure at 40% C, a design lift coefficient of 0.2, and a thickness of 12% C. Sketches of NACA subsonic airfoil series are shown in Figure 3.17. National Aeronautics and Space Administration’s (NASA) FoilSim interactive simulation software developed by the Glenn Research Center comprises an excellent tool to explore the various shapes of airfoils and how air flows around them when airfoil characteristics are manipulated.

A modern design used worldwide on corporate, military, and air transport aircraft is the supercritical airfoil, which is flatter on top and more rounded on the bottom than a conventional wing. The upper trailing edge has a downward curve to restore lift lost by the flattening of the upper surface. The benefit of this design in the high‐speed realm of flight, as well as other supersonic airfoils, is discussed in Chapter 14.