Shapes! We have been dealing with a plethora of shapes from our kindergarten to advanced studies. However, there is one shape in the world that shapes the aviation industry to its finest positions. Yes, you guessed it correctly; it is none other than the Airfoil shape. Let us take a deeper dive into the airfoil shape and see how it redefines the aviation industry.
What is an Airfoil Design?
FAA defines an airfoil as a “Structure designed to obtain reaction upon its surface from the air through which it moves or that moves past such a structure.” Definitions are always hard to keep in mind, therefore when simply put; Airfoil is any shape that generates a pressure difference when directed into an airflow.
All the fancy diagrams we come across as airfoil shapes are the cross-sections of wings.
• Leading Edge and Trailing Edge: These are the two extremities of the airfoil shape.
• Camber: Curvature of the airfoil in upper and lower surfaces.
• Chord Line: The straight line joining the two extremities of the leading edge and trailing edge.
• Mean Camber Line: Line drawn connecting the two extremities, running equidistant from the upper and lower surfaces.
• Angle Of Attack (AOA): Acute angle between the chord line of the airfoil and the relative airflow.
• Center of Pressure (CP): Point where the resultant pressure acts on the airfoil.
Types of Airfoil Shapes
From the early days of aviation to the modern days of sophisticated flying machines, hundreds of airfoil shapes have been designed and even more, are waiting to join the list. We can divide an airfoil shape into two categories: Symmetrical and Non-Symmetrical. Symmetrical airfoil shapes have equal camber in upper and lower surfaces while non-symmetrical airfoil shapes have different cambers. In symmetrical airfoils, the chord line and mean camber line become identical.
Symmetrical airfoils generate zero lift at zero angles of attack making them well suited for inverted flying. Non-symmetrical or cambered airfoil shapes are widely adopted into commercial aviation airliners.
Airfoil Shapes in Rotary Wings
Despite the rotating wing, the theory of lift generation remains the same. Symmetrical airfoil shapes have been extensively used in rotor wing applications. In symmetrical wings, Center of Pressure (CP) movement remains relatively constant for varying angles of attack by reducing the blade twist.
How an Airfoil Generates Lift?
You already know the answer- Bernoulli’s principle. However, two more principles should accompany Bernoulli’s theorem when talking about the lift: the Coanda Effect and Newton’s Third law of motion.
Coanda Effect based on the flow attachment states that the upper surface of an airfoil pulls the airflow downwards as it sticks with the wing surface. According to Newton’s Third law, when the wing pulls airflow downwards, airflow pulls the wing surface upwards creating a lift.
The Efficiency of an Airfoil
Lift to drag ratio (L\D) or the coefficient of lift (CL) decides how efficient an airfoil is: higher the better. The coefficient of lift varies with the angle of attack and peaks at the stall angle. From general aviation aircraft to commercial jumbos, various methods are in use to achieve the best lift to drag ratio. Secondary flight controls such as slats and flaps are used to alter the airfoil shape depending on the flight phase to achieve desired lift to drag ratios.
Does Airfoil Shape Change Along the Wing?
Yes, it has to! A wing cannot maintain the identical airfoil shape from the wing root to the tip. Many factors including wing taper, sweptback, and wing loading governs the airfoil shape along the wing.