How Is Airfoil Design Defined?

An airfoil is an aircraft control surface that is designed to react to air moving through and around it. When looking at a typical airfoil profile, there are various specifications to consider. For instance, there is a major difference in the curvatures, or cambers, of the upper and lower surfaces. Furthermore, the leading edge is rounded in appearance, while the trailing edge is characterized by its narrow, tapered look. 

When discussing airfoils, the term chord line is common and is used to describe the straight line drawn through the profile connecting the extremities of the leading and trailing edges. The distance from this chord line to the upper and lower surfaces of the wing determines the magnitude of the camber. Additionally, another reference line is drawn from the leading edge to the trailing edge, and this is called the camber line. This line is equally distant at all points on the upper and lower surfaces. 

Airfoils are usually constructed to take advantage of the air’s response to certain physical laws. Air operates on two actions: a positive pressure lifting action from the air mass below the wing, and the negative pressure lifting action from lowered pressure above the wing. As air streams strike the flat, lower surface of the wing or rotor blade, the air is forced downward, generating positive lift. As an airstream strikes the upper surface of the wing, the air is deflected upward. Together, both wings are able to produce the lift necessary to maintain flight due to the airfoils. 

Different airfoils have varying flight characteristics, though they are all subjected to similar testing to ensure they work optimally. Generally, the weight, speed, and purpose of each aircraft determines the shape of the airfoil. Efficient airfoils are those which produce the greatest lift. These are usually characterized by a concave or “scooped out” lower surface. However, this design is not ideal for high-speed flight. Advancements in aerospace engineering have made it so high-speed jets can take advantage of concave airfoils' high lift characteristics. 

Leading edge (Kreuger) flaps and trailing edge (Fowler) flaps, when extended from the basic wing structure, change the shape of the airfoil into a classic concave form in order to generate greater lift during slow flight conditions. On their own, airfoils that are streamlined offer little wind resistance which makes it hard for takeoff. As such, modern aircraft have airfoil designs that possess a combination of the aforementioned characteristics. More than that, the shape varies based on the needs of the aircraft. 


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