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NASA defines an airfoil to be a “streamlined surface designed in such a way that produces useful motion.” The useful motion being referred to in aerospace applications is lift or propulsion depending on where the airfoil is utilized. Other sources define airfoils to be any shape or surface designed to turn flow. In aerospace applications airfoils are not only utilized on the wing. All control surfaces are in essence are airfoils. When the vertical tail induces a rudder deflection the local flow is turned and results in a yawing motion.
Airfoil Nomenclature Edit
An airfoil is best visualized as the cross-section of a wing as shown in Figure 1. Further investigating this cross-section, Figure 2, illustrates several design features.The most important design feature is the mean camber line, shown in figure as a dashed line spanning the length of the chord. The mean camber line is equidistant from the upper and lower cross section, essentially a dividing line where the thickness is equal above and below. The chord line is a straight line drawn from the leading and trailing edges of the airfoil. The distance then between the leading edge and trailing edge is simply the chord and it is denoted by the letter c. When the mean camber line and the chord line lie directly on top of each other the airfoil is symmetric. The camber is then said to be zero since it is the maximum distance between the chord line and the mean camber line. The camber of the airfoil is the dominant characteristic which influences the lift, drag, and moment produced by the airfoil. Changing the angle of attack also influence the behavior of an airfoil.
Airfoil Naming Convention Edit
These cross-sections come in all variants of thickness, camber, and chord length. In 1884 Horatio F. Phillips patented his first airfoil shapes. The Wright brothers experimented and tested their own shape designs as well. Airfoil shape design was a much customized art early on, however in 1930’s the National Advisory Committee for Aeronautics (NACA) performed an exhaustive amount of design and testing of various airfoil shapes. NACA developed a large database and produced several naming conventions for different classifications using a numbering system.
“four –digit” Edit
The NACA “four-digit” airfoils were the first developed in the early 1930’s. Using the NACA 4415 shown above in figure 2 as an example, the first digit (4), represents the maximum camber in hundredths of chord, the second digit (4) is the location where the maximum camber is located along the chord from the leading edge in tenths of a chord. The last two digits (15) give the maximum thickness in hundredths of a chord. Thus for the NACA 4425 the maximum camber is 0.04*c located at 0.4*c from the leading edge, and has a maximum thickness of 0.15*c. The alternate way of describing this airfoil is through percent of chord, for example: 4 percent camber, 40 percent chord with 15 percent thickness.
This family of airfoils was developed during World War II; they are laminar flow airfoils which have no sections of turbulent airflow. In this family the first digit implies the series; the second digit specifies the location of minimum pressure in tenths of chord from the leading edge. The third number represents the design lift coefficient tenths, and the last two digits give the maximum thickness in hundredths of a chord. This family of airfoils became widely popular due to the ability to increase the critical Mach number compared to other NACA airfoils at the time. This airfoil was also the stepping stone to the supercritical airfoil.
Supercritical Airfoil Edit
This section describes a completely different family of airfoils that were developed during the 1960’s and 1970’s by NASA strictly for the purpose to reduce the onset of drag due to the formation of standing shocks from local supersonic flows over the airfoil. The goal was to increase the drag-divergence Mach number and allow for more efficient flight in the transonic regime. Figure 3 illustrates the shape of a supercritical airfoil. The most obvious feature of this airfoil is the tail and the flat top. These two features cause for the supersonic flow to occur closer to the surface as well as maintain low local supersonic Mach numbers, and finally the terminating shock wave occurring over the and below the airfoil are much weaker than other airfoil families. The supercritical airfoil shown in figure three is the SC(2)-0412. The airfoil designation is broken down into two segments, the phase which is 1, 2 or 3 and the characteristics. The first two digits for the latter segment are the design lift coefficient in tenths; the last two are the thickness in percent chord.
- Phase 1 Supercritical Airfoils: Early Supercritical airfoils that increased the drag divergence Mach number beyond the “6-series” NACA family.
- Phase 2 Supercritical Airfoils: The extension of Phase 1 Supercritical Airfoils with “target pressure distributions”
- Phase 3 Supercritical Airfoils: Airfoils developed for studies to reduce phase 2 leading edge radii. The study was eventually abandoned.
Airfoil Characteristics EditAirfoil performance is generally depicted by a series of coefficients for lift, drag, pressure, and moment. These coefficients vary for each airfoil and angle of attack, Figure 3 shows a general lift curve plot for an airfoil falling under thin airfoil theory. This plot shows the effect of angle of attack on lift coefficient, which is generally linear. The slope of this line is called the lift slope. During this segment flow is attached to the airfoil, as the angle of attack increases the flow begins to separate and the lift generation decreases, this is called stall. Prior to stall the airfoil exhibits the maximum lift. A symmetric airfoil has a lift coefficient of zero for zero angle of attack any change in camber directly affect the lift curve.
Many codes exist to design custom airfoils to meet specific aerodynamic needs.
See Also Edit
- Abbott,Ira H. and Von, Albert E. Theory of wing sections, including a summary of airfoil data.
- Harris, Charles D. NASA Supercritical Airfoils:A Matrix of Family-Related Airfoils. Langley Research Center Hampton, Virginia
- Anderson, John David. Introduction to Flight. New York: McGraw-Hill, 1985.
- Anderson, John David. Fundamentals of Aerodynamics. New York: McGraw-Hill, 1984.
- NASA Aerodynamics for Beginners
- History of NACA