The computational tools to help design airfoils that produce specific aerodynamic characteristics first became available in the 1920s. ![]() Wind tunnel work to measure airfoil characteristics was soon followed by the first development of validated numerical methods to predict chordwise pressure distributions and airfoil characteristics without making as many measurements in the wind tunnel. Early airfoil shapes were improved systematically but empirically by trial and error over many decades using a combination of testing in the wind tunnel and theoretical developments. The figure below shows a relatively rapid evolution of airfoil shapes tailored to aircraft applications between 19, with the thin and highly cambered airfoil sections used on early airplanes being relegated to history. The Wright brothers also built and used an open-return wind tunnel that would prove instrumental to the success of their 1903 Flyer, recognizing that not only the airfoil cross-section was important for wing efficiency but also the wing span-to-chord ratio, now known as the aspect ratio. Not long after Phillips, Gustave Eiffel conducted more experiments using a wind tunnel of the open-return (single passage) type. Some of the earliest known “concavo-convex” airfoil shapes were patented in the late 1880s. Phillips also tested these airfoils in one of the very first wind tunnels. Notice the thin, highly cambered profile shapes, which are now known to have poor aerodynamic efficiency compared to modern airfoils, at least under the operating conditions of most flight vehicles. Taking inspiration from nature is nothing new in engineering, but history shows that it should not necessarily be a basis for our engineering. Some of the earliest known airfoil sections considered 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. Taken from “ Aeronautical & Miscellaneous Note-Book of Sir George Cayley,” Cambridge University Press, 1933. Cayley’s sketch of the cross-section of a trout looks remarkably like a modern airfoil section. Cayley obtained the profile shown in the drawing below by measuring the cross-sectional shape of a trout, which, interestingly enough, conforms closely to modern low-drag “laminar” airfoil sections. Cayley made essential observations about drag, including “It has been found by experiment that the shape of the hinder part of the spindle is of as much importance as that of the front in diminishing resistance.” Cayley referred to the shape of a wing as spindle-shaped. ![]() Sir George Cayley, often revered as the “Father of Aeronautics,” delineated the problem of sustentation, i.e., aerodynamic lift, from that of drag, i.e., the component of aerodynamic resistance. In this regard, theory and experimentation (e.g., wind tunnel testing) have been used to design airfoils to meet specific operating requirements for different aircraft types, including low-speed airplanes, high-speed airplanes, helicopters, propellers, wind turbines, etc. Historically, the most suitable airfoils for most practical engineering applications were obtained through an evolutionary process. Understand the differences in the shapes between subsonic, transonic, and supersonic airfoil sections.Know how to construct a NACA airfoil profile geometrically using a camberline shape and a thickness envelope.Be able to identify and explain the significance of the critical geometric parameters that define the shape of an airfoil. ![]()
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