How does an Airfoil work? Aerodynamics explained

An airfoil is a curved surface designed to generate lift when it moves through the air. Airfoils are crucial components of aircraft wings and are also used in other applications where lift is required, such as wind turbines. The shape of an airfoil and its angle of attack are critical factors in its ability to generate lift, and different airfoil shapes are used for different applications.

In this blog, we will explore the basic structure of the airfoil and various aspects of airfoil design and how they correspond to the fundamentals of aerodynamics.

The above image showcases the profile of the cross-section of an aerodynamic element (the wing of an airplane). Due to the airfoil shape here, the aerodynamic element is offered with a lift force as well as a drag force. Only the airfoil profile can bring about the least drag force out of all the possible profiles. 

Components of an Airfoil

Now let us look at some of the modelling parameters in airfoil theory. It is essential to understand a few terms first.  

 

• Mean camber line

The mean camber line is the locus of midpoints of the lines perpendicular to the line joining the leading edge and the trailing edge (chord line). It divides the airfoil symmetrically.

• Chord line

The leading and trailing edges of the mean camber line are the most forward and rearward points. The straight line connecting the leading and trailing edges is the chord line.

• Camber

The camber is the maximum distance between the mean camber line and the chord line, measured perpendicular to the chord line.

• Thickness

 The thickness is the distance between the upper and lower surfaces, also measured perpendicular to the chord line.

• Leading edge radius

The airfoil's shape at the leading edge is usually circular and the radius of the circular curve is the leading edge radius. The leading edge radius is approximately 0.02c, where ‘c’ is chord length.

Airfoil nomenclature

The NACA (National Advisory Committee for Aeronautics) is an organization that developed various airfoils and gave them systematic names, like the NACA 2412. The NACA followed a logical numbering system to distinguish the airfoils according to their properties. For example, the first family of NACA airfoils, developed in the 1930s, was the “four-digit” series, which included the NACA 2412 airfoil.

Here, the first digit (2) defines the maximum camber of the airfoil, and the second digit (4) defines the location of the maximum camber along the chord from the leading edge. The last two digits (12) define the airfoil's maximum thickness.

For the NACA 2412 airfoil, 

• The maximum camber = 2*(c/100) 
• It is located at the maximum camber of 4*(c/10)
• The maximum thickness T = 12*(c/100), where ‘c’ is the length of the chord line

It is common practice to state these numbers in percent of the chord, that is, the camber of the airfoil is 2 percent of the chord length and is located at 40 percent of the chord length. The thickness is 12 percent of the chord length. 

An airfoil with no camber, that is, if the camber line and chord line coincide, then the airfoil is called a symmetric airfoil. Clearly, the shape of a symmetric airfoil is the same above and below the chord line. For example, the NACA 0012 airfoil is a symmetric airfoil with a maximum thickness of 12 percent of the chord length. 

The second family of NACA airfoils was the “five-digit” series, including the NACA 23012 airfoil. This is similar to the four-digit series but is prefixed with an extra digit before the four digits, giving information about the design lift coefficient. 

The design lift coefficient is determined by considering only the camber of the airfoil. The design lift coefficient compares different airfoils with their capacity to generate the lift force on it. The remaining four digits denote the same values.

In the NACA 23012 airfoil, the design lift coefficient is 0.3, i.e., 

Design lift coefficient = first digit* (3/20) = 2*(3/20) = 0.3 (for NACA23012)