Cambered aerofoils, such as the NACA4412, shown in Figure A3.23, have curved chord lines and this allows them to produce lift at zero angle of attack. Generally, cambered aerofoils have higher maximum lift/drag ratios than symmetrical aerofoils for positive angles of attack and this is the reason for their use.
The classification of the NACA four-digit range of aerofoils, which were commonly used on wind turbines, is very simple and is illustrated in Figure A3.24: from left to right, the first digit represents the amount of camber as a percentage of the chord length, the second digit represents the percentage chord position, in units of 10 percent, at which the maximum camber occurs and the last two digits are the
maximum thickness to chord ratio, as a percentage of the chord length, which, in this family of aerofoils, is at the 30 percent chord position. The cambered chordline, now called the camber line, comprises two parabolic arcs that join smoothly at the point of maximum camber. For other ranges of aerofoils the reader should refer to Abbott and von Doenhoff (1959).
The angle of attack a is measured form the chord line which is now defined as the straight line joining the ends of the camber line. Note that the lift at zero angle of attack is no longer zero; zero lift occurs at a small negative angle of attack. With most cambered aerofoils the zero lift line is approximately at — A° where A is the percentage camber. The behaviour of the NACA4412 aerofoil is shown in Figure A3.25 for angles of attack below and just above the stall. Note that the lift at zero
Figure A3.25 The Characteristics of the NACA4412 Aerofoil for Re = 1.5 ■ 106
angle of attack is no longer zero; zero lift occurs at a small negative angle of attack of approximately 4°.
The centre of pressure, which is at the | chord position on symmetrical aerofoils lies aft of the | chord position on cambered aerofoils and moves towards the trailing edge with increasing angle of attack. However, if a fixed chordwise position is chosen then the resultant force through that point is accompanied by a pitching moment (nose-up positive, by convention). If a pitching moment coefficient is defined as
C = Pitching/unit span (3
2 PU2 c then there will be a position, called the aerodynamic centre, for which dCm/dQ = 0. Theoretically, the aerodynamic centre lies at the 4 chord position and is close to this point for most practical aerofoils. The value of Cm depends upon the degree of camber but for the NACA4412 the value is —0.1, note that pitching moments are always negative in practice (nose down) despite the sign convention. Above the stall there is no aerodynamic centre, as defined, and so the pre-stall position continues to be used to determine the pitching moment coefficient, which then becomes dependent upon a.
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