# 22 Airfoil Geometries

# Introduction

Engineers must know how to select or design suitable cross-sectional wing shapes (often called airfoil *profiles *or* airfoils*) for use on a diverse range of flight vehicles such as airplanes, various types of space launch and re-entry vehicles, as well as helicopter rotors, propeller blades, wind turbines, UAVs, etc. To this end, not all airfoils are created equally, and different airfoils will be better suited for one application relative to another.

For example, airfoils for use on the wings of low-speed airplanes are generally thicker (in terms of their thickness-to-chord ratio) and have more curvature or *camber.* Airfoils for use on high-speed aircraft are much thinner with pointed leading edges and use less camber. Historically, the design of airfoil shapes for specific applications has proceeded in an evolutionary manner, with wind tunnel experiments, theoretical analysis, and flight testing all being used synergistically to develop the best airfoil shapes for specific types of flight vehicles.

Objectives of this Lesson

- Better understand the historical evolution of airfoil sections for aircraft applications.
- Be able to identify the key geometric parameters that define the shape of an airfoil.
- Know how to geometrically construct a NACA airfoil profile using a camberline shape and a thickness envelope.
- Understand the differences between subsonic, transonic, and supersonic airfoil sections.

The earliest known airfoil sections 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. Notice the very thin highly cambered profile shapes compared to what most modern airfoils look like.

There are literally 1000s of airfoils in current use, most airfoils being selected or otherwise adapted to optimize their performance for their specific aircraft application(s). Typical design requirements for airfoil sections include:

- Obtaining high values of the maximum attainable lift coefficient before flow separation and stall occur.
- The minimization of drag over a broad range of operating conditions.
- The attainment of a particular value of nose-up or nose-down pitching moment.
- The ability to reach high values of the lift-to-drag ratio.
- A high critical Mach number.

There has recently been much interest in designing efficient airfoils for use at the low flow speeds and low Reynolds numbers found on UAV systems, which require detailed knowledge of boundary layer developments. Unfortunately, airfoil characteristics at low Reynolds numbers are usually quite different from those found at higher Reynolds numbers, often showing remarkably low aerodynamic efficiencies.

# Shape of an Airfoil

The basic geometry of an airfoil is described in terms of a profile *shape* or *envelope* that defines the curvature of its upper and lower surfaces. As shown in the figure below, airfoils can be *symmetric*, an airfoil with the same shape and curvature on the upper and lower surfaces, or *cambered*, which has a different upper and lower surface shape. The mean camber of the airfoil profile is a measure of its curvature, and the shape and amount of the mean camber will also affect the shapes and curvature of the airfoil’s upper and lower surfaces. In addition, some airfoils have camber in which the trailing edge region has an upward or negative camber, called reflex camber, which are often used on flying wings and helicopters.

Airfoils are geometrically constructed to form a shape or *envelope* with upper and lower surface shapes, as shown in the figure below. The key length dimension of an airfoil profile is defined in terms of its chordline; the *chord* is the distance measured from the leading edge of the airfoil profile to its trailing edge. However, in the geometric construction of airfoil profiles, it is necessary to be very precise about how exactly the airfoil shape is defined, including the value and position of the maximum thickness (thickness to chord ratio), the value and position of the maximum camber, as well as the nose radius.

In most geometric constructions of airfoil profiles, the thickness envelope of the airfoil is defined in such a way that the upper and lower surfaces of the envelope evolve if the thickness is plotted perpendicular to the slope of the defined camberline. This is a formalized geometric process to trace out the envelope in terms of the coordinates of the upper and lower profile shapes, which can be tabulated for various purposes. In addition, the leading-edge shape of the airfoil is often defined geometrically in terms of a nose radius, which also affects some of the airfoil’s aerodynamic characteristics.

Other geometric parameters of interest for airfoils are the maximum thickness and maximum camber, usually defined as a ratio relative to the chord, i.e., the maximum thickness to chord ratio and the maximum camber ratio. The chordwise position of these latter parameters may also be defined and used to describe the shape of the airfoil profile, especially as they subsequently relate to the effects on the aerodynamic characteristics of the airfoil. For example, it is known that increasing the camber at the leading edge of an airfoil can increase its maximum lift coefficient.

# Early History of Airfoils

Historically, the most suitable airfoils for most practical engineering applications have been obtained through an evolutionary process. In this regard, both theory and experimentation (e.g., wind tunnel testing) have been used together to design airfoils to meet specific operating requirements for different types of aircraft, including airplanes, helicopters, propellers, wind turbines, etc. The computational tools to help design airfoils that produce specific aerodynamic characteristics first became available in the 1920s. The development of the thin-airfoil theory by Max Munk (in the U.S.) and Hermann Glauert (in the U.K.) during the 1920s led to an understanding of how the camber affected the lift and pitching moments of airfoils.

The problem of defining the airfoil pressure distribution for an airfoil with thickness and arbitrary shape was tackled by Theodorsen & Garrick in the early 1930s. The design of practical airfoil profiles was further aided by methods such as the conformal transformation that was first developed by Prandtl & Tietjens. This latter approach made it possible to compute pressure distributions and the resulting lift and pitching moment characteristics of some specially shaped *“Joukowski” airfoils*. The aerodynamic properties of Joukowski airfoils were measured in wind tunnel tests starting in the late 1920s at Gottingen in Germany and by the NACA in the U.S.A. from 1930 onward. As shown in the figure below, there was a relatively rapid evolution of airfoil shapes tailored to aircraft applications between 1908 and 1944, with the thin and highly cambered airfoil sections used on early airplanes being relegated to history.

This experimental 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. Today, it is possible to predict the aerodynamic characteristics of airfoils with a high confidence level, with several popular computer codes such as XFoil being available. Even today, however, measurements of airfoil characteristics in wind tunnels have proven to be much more reliable than results from calculations, mainly when the airfoils operate at higher angles of attack, higher subsonic and transonic Mach numbers, or lower Reynolds numbers.

# NACA Method of Drawing Airfoil Shapes

As early as 1920, research institutions in Europe and the U.S.A. had embarked on the systematic measurement of the aerodynamic characteristics of airfoils that were already in practical use. This work led to the organization of the results into *families* of airfoils known to produce specific aerodynamic characteristics. By having a catalog of airfoils with measured aerodynamic characteristics, aircraft designers could quickly choose the most appropriate airfoil profile from the catalog for a given application.

The National Advisory Committee for Aeronautics (NACA) conducted the most comprehensive and systematic study of the effect of airfoil shape on aerodynamic characteristics. Existing cambered airfoils, such as the Clark-Y and Gottingen sections, were known from the earliest wind tunnel experiments to have good aerodynamic characteristics. Therefore, the NACA used these airfoils as a basis; in fact, these two airfoils were found to have geometrically similar profiles when the camber was removed, and the airfoils were reduced to the same thickness-to-chord ratio. A polynomial curve fit defined the resulting thickness shape, which then became fundamental to many of the subsequent NACA airfoil families, i.e., what has become known as the classic NACA 00-series symmetric airfoils.

In the NACA method of defining the shape of an airfoil, a coordinate system is placed at the nose of the airfoil and is defined in terms of the and distances, as shown in the figure below. The airfoil profile is then constructed of a series of upper and lower points by using a thickness shape distributed around a camber line and by plotting the thickness perpendicular to the slope of the camberline, as detailed in the lower part of the figure below.

The basic steps of the process are straightforward geometry and easily programmed on the computer. If the slope of the camberline makes an angle with the chord line, as shown in the figure above, then the upper coordinate and lower coordinates of the airfoil can be obtained by using the equations

(1)

where and , i.e., coordinates non-dimensionalized with respect to the chord, . The slope of the camberline is given by

(2)

where expresses the shape of the camberline. For airfoils with small camber, i.e., small values of surface slope , a reasonable approximation is to apply the thickness along the axis. The various upper and lower surface points can be exported to a data file and so the final shape of the airfoil is then obtained by connecting the points. Generally, more points will be needed in the nose region of the airfoil because of the higher curvature of the section.

The nose radius must also be formally located on the profile, obtained with an inscribed circle. The center for the leading edge radius (defined by a circle) is found by drawing a straight line through the end of the chord at the origin of the axes, but with a slope equal to the slope of the camberline at , and then marking off a distance along this line that is equal to the leading edge radius. This point then becomes the origin location for the leading-edge nose circle. For a symmetric airfoil, the center of this circle lies on the axis. The nose circle is then drawn and geometrically blended into the upper and lower surface coordinates.

Another way of drawing an airfoil section graphically is to draw a series of circles of radius (or ) if done non-dimensionally as a fraction of chord) centered on the camberline, as shown in the figure below. The upper and lower surfaces of the airfoil are then formed from curves drawn tangential to all of the circles. While this method offers a simple method of graphical construction, which can be a helpful approach in visualizing the overall approach of drawing an airfoil section, it is best to construct the shapes of the surfaces and tabulate the data points, as mentioned previously.

The forgoing NACA approach also allowed the systematic construction of several families of airfoil sections differing only by a single geometric parameter, such as the location of maximum camber. The various families of airfoils developed by NACA were then tested in the wind tunnel to measure the effects of varying the important geometrical parameters on the lift, drag, and pitching moment characteristics as a function of angle of attack, as well as in some cases, the chord Reynolds number and Mach number. The primary geometric characteristics that affect the airfoil characteristics include the maximum camber and its distance aft of the leading edge and the leading-edge nose curvature (nose radius) airfoil. A summary of the results is documented in considerable detail in the NACA report (later a book) “Theory of Wing Sections: Including a Summary of Airfoil Data,” by Ira H. Abbott and A. E. von Doenhoff.

# Symmetric NACA Airfoils

Symmetric airfoil sections are often selected for horizontal and vertical tail surfaces. The upper and lower surfaces of the NACA 00-series or four-digit symmetrical sections are described by the polynomial

(3)

where , , and , i.e., the geometry of the airfoil and the coordinates are expressed as a fraction of its chord or for . The shape of the airfoil is obtained by plotting as a function of and for any number of points; at least 50 points and more typically 100 points will be required to define the airfoil shape to good fidelity. The corresponding leading-edge radius of the airfoil is , which is smoothly blended into the upper and lower surfaces as previously described. Examples of the NACA 00-series symmetric airfoils is shown in the figure below.

# Cambered NACA Airfoils

Cambered airfoils are constructed by distributing the thickness envelope (as defined above) around a camberline shape, as also previously discussed. The camberline can be specified as . Specifically, the thickness envelope is plotted perpendicular to the camberline to trace out the profiles of the upper and lower surfaces. There are many camberline profiles in the NACA portfolio, including the two-digit and three-digit camberlines, some examples of which are shown in the figure below.

The simplest cambered airfoils are used to form the NACA 4-digit series, which is comprised of the standard NACA four-digit thickness envelopes and the following camberline based on two coefficients, i.e.,

(4)

where is the maximum camber (100 is the first of the four digits), is the location of maximum camber with 10 being the second digit in the NACA 4-digit airfoil description. The slopes of the camberline are

(5)

The NACA three-digit mean (camber) lines are also very popular and are given in this case in terms of three coefficients, i.e.,

(6)

where the coefficients of the camberline are given in the table below.

Mean Line | |||

210 | 0.05 | 0.0580 | 361.4 |

220 | 0.10 | 0.1260 | 51.64 |

230 | 0.15 | 0.2025 | 15.957 |

240 | 0.20 | 0.2900 | 6.643 |

250 | 0.25 | 0.3910 | 2.230 |

# Other NACA Airfoils

Modifications to the NACA four-digit and five-digit series of airfoil sections include reflex camber to produce zero pitching moment and changes in the nose radius and position of thickness to improve the maximum lift capability. The latter sections are denoted by a two-digit suffix, such as the NACA 0012-64 and NACA 23012-64. After the dash, the first integer indicates the relative magnitude of the nose radius, with a standard nose radius denoted by 6 and a sharp radius by 0. The second digit indicates the position of the maximum thickness in tenths of the chord.

The camberline for the NACA 3-digit 231-series reflexed airfoils are of some interest because they are designed to give zero-pitching moment about the 1/4-chord axis. In this regard, they are considered suitable for rotor blades (e.g., for a helicopter) because of the need to keep torsional twisting moments to a minimum. The camberline of these airfoils is defined by

(7)

and

(8)

where , , , and .

Another set of NACA airfoils that have seen some use on various aircraft is the six-digit series. These airfoils were designed to achieve lower drag, higher drag divergence Mach numbers, and higher maximum lift coefficients. Their profiles are such that they are conducive to maintaining an extensive run of laminar flow over the leading-edge region, thereby lowering skin friction drag, at least over a range of angle of attack limited to low lift coefficients.

This latter goal is achieved by using camberlines that produce a more uniform pressure loading from the leading edge to a distance . After that, the loading decreases linearly to zero at the trailing edge. The favorable pressure gradients tend to give the airfoils lower drag than other airfoils, at least over a limited range of attack angles. Unfortunately, surface contaminants or other transition-causing disturbances quickly spoil the characteristics of laminar flow types of airfoils, sometimes resulting in significant adverse characteristics.

Many designator combinations are used in the six-digit airfoil number system, which tends to become rather complicated. For example, consider the NACA 64-215 section. In this case, the number 6 denotes the airfoil series, and the 4 denotes the position of minimum pressure in tenths of the chord for the basic symmetric section. The 3 denotes the range of lift coefficient in tenths above and below the design lift coefficient for which low drag may be obtained. The 2 after the dash indicates a design lift coefficient of 0.2, and 15 denotes a 15% thickness-to-chord ratio.

# Grid generation for CFD

The numerical generation of airfoil coordinates can also be used to generate input points or grids, or meshes to calculate their aerodynamic characteristics using programs like XFoil or other methods such as computational fluid dynamics(CFD). CFD grids are composed of discrete cells over which the conservation laws of fluid mechanics can be applied. An example of a grid about an airfoil section is shown in the figure below.

The resulting flow solution can then be used to calculate various properties around the airfoil, including local pressure, local Mach number, etc. CFD methods can also be used to design the shape of an airfoil to obtain a specified level of performance. However, this tends to be lengthy because of its iterative nature and slow convergence. Nevertheless, the ability to design airfoil shapes on the computer, to a point, is much quicker than the repetitive testing of many prospective shapes in the wind tunnel.

Numerical grid generation for CFD solutions can take on a variety of types, including *structured* and *unstructured*. Structured grids are geometrically regular, whereas unstructured grids have more randomly generated points, which is a valuable approach that can reduce the computational time needed to find a flow solution. Several software tools are available to engineers, e.g., Cadence, that can help create grids about particular airfoil shapes. The fidelity of the resulting aerodynamic solution strongly depends on the grid, especially the number of grid points, which can reach many millions. Of course, the numerical cost (and time) to obtain a solution increases commensurately with the number of grid points.

# Supersonic Airfoils

The well-rounded, cambered airfoil sections well-suited to subsonic flight speed are generally not appropriate for high-speed and supersonic flight. Supersonic airfoils are distinctive in their geometric shapes in that they are thin (i.e., have a low thickness-to-chord ratio) with sharp leading edges. Supersonic airfoils generally have thinner sections formed of either angled planes called *double wedge airfoils*, or opposed circular arcs called *biconvex airfoils*, as shown below.

The sharp leading edges on supersonic airfoils prevent the formation of a detached bow shock in front of the airfoil, which is a high source of drag called *wave drag*. For example, in the schlieren image shown below, the supersonic flow passes smoothly over the sharp nose of the airfoil profile with minimum drag. For a supersonic airfoil, the thickness and camber shapes are designed to minimize energy losses associated with the flow’s compression and expansion. However, often only this can be achieved over a small range of operating angles of attack.

However, a significant problem with thin supersonic airfoils is at low flow speeds, when they tend to produce leading-edge flow separation and stall even at moderate angles of attack. Therefore, most aircraft that employ supersonic airfoils must use high-lift devices for takeoff and landing, such as large leading-edge slats along the entire wing. Nevertheless, supersonic aircraft tend to have high takeoff and landing speeds and may need to employ drogue parachutes to reduce their landing distances.

# Transonic Airfoils

Because commercial airliners have been designed to reach higher and higher cruise speeds approaching the speed of sound, i.e., for flight at transonic Mach numbers, this requirement has led to the design of a unique wing shape called a *supercritical wing*. A supercritical wing also uses a *supercritical airfoil *to reduce the strength of shock waves, thereby reducing wave drag. The principle used in transonic wing and airfoil design is to control the expansion of the flow to supersonic speed and its subsequent recompression.

As shown in the figure below, a supercritical airfoil shape is distinctive in that it has a point of maximum thickness fairly aft on the chord, with a relatively flat upper surface with a slight amount of camber. However, such airfoils also tend to have significant camber at their trailing edges. Supercritical airfoils were extensively studied and refined during the 1960s by Richard Whitcomb.

# Examples to Try

Clearly there are many airfoils to choose from, but for the student it is valuable to understand the NACA method of airfoil geometric construction; it is very systematic, quite easy in terms of the mathematics, and the algorithm lends itself naturally to the computer.

For example, the shape of a NACA 0018 airfoil takes no more than to plot the upper and lower surfaces using

(9)

where .

The shape of the airfoil is obtained by plotting as a function of ; the results can be tabulated for any number of specified discrete points along the chordline but 50 to 100 is usually enough, as shown in the two figures below. The corresponding leading-edge radius of the airfoil is , which is blended into the upper and lower surfaces. Recall the all of the NACA airfoils have finite thickness at the trailing edge, which is for practical reasons.

The NACA 23018 airfoil section is a cambered airfoil comprised of the NACA 0018 thickness envelope (described above) wrapped around the NACA 230 camberline, the equations again being

(10)

where the slope of the camberline is .

The resulting NACA 23018 airfoil is shown in the two figures below. It is apparent on the enlarged plot of the nose region that the leading-edge part of the nose radius protrudes very slightly forward of the origin at ; this is an artifact of the construction technique and is of no practical significance when building a wing with such an airfoil.

A MatLab code to draw these airfoil shapes is given below. The student is encouraged to try using some other NACA camberlines to better understand the process of drawing NACA airfoil shapes.

# Summary & Closure

The use of the most suitable airfoil section or sections is fundamental to the success of the design of a wing as a whole. To this end, many different types of airfoil sections have been geometrically tailored to give the best aerodynamic performance at the conditions of flight. For example, thicker and more cambered airfoils with rounded nose shapes are more suitable for slower flight speeds and low Mach numbers. In contrast, very thin airfoils with sharp leading edges are much more suitable for high speeds and supersonic Mach numbers. In addition, for transonic flight conditions, where many airliners fly, special supercritical airfoil sections have been developed to reduce wave drag and prevent boundary layer separation behind the shock wave, so allowing airliners to cruise closer to the speed of sound.

5-Question Self-Assessment Quickquiz

For Further Thought or Discussion

- Do some research into laminar flow airfoil sections. What are particular geometric features incorporated into these airfoils to produce laminar flow?
- What kinds of airfoil shapes are likely to be used for supersonic flight? Are there any NACA supersonic airfoil sections?
- Research the types of airfoils used on propeller blades. Why is it that different airfoils with different thicknesses along the blade’s span are used on propellers?
- What kind of airfoils are likely to be used on wind turbines, and why?

Other Useful Online Resources

There many more resources on airfoils to explore:

- All about airfoils on Wikipedia.
- The University of Illinois at Urbana-Champagne has complied airfoil coordinates and related links. Here is the guide to many uses from the UIUC Airfoil Data site.
- Go here for some interesting airfoil drawing tools.
- A NASA site as to how the shape of an airfoil affects lift.
- YouTube video on how airfoil shapes can affect lift generation.
- Airfoil shape basics video on YouTube.
- Information about grids and CFD calculations about airfoils.
- Learn about Richard Witcomb’s contributions to transonic airfoil design here.