An airfoil, composed of an upper surface and a lower surface, meets the air at its front section, the leading edge, and releases it at the back, the trailing edge. Its principal function is to generate lift as the aircraft moves forward. Airfoils differ in the curvature of their upper and lower surfaces. Cambered airfoils exhibit greater curvature on the upper surface than on the lower, whereas symmetric airfoils have identical upper and lower profiles. To maintain a smooth flow of air over a portion of the surface, designers may select supercritical or laminar flow airfoils. Common examples of airfoils include the NACA 0012 and other 4 digit airfoils generated with the NACA 4-digit airfoil generator.
What is the definition of airfoil in aircraft?
An airfoil is the cross section of an airplane wing. Airfoils are highly-efficient shapes that generate lift, and wings are examples of airfoils. Airfoils are used in the design of aircraft, propellers, and rotor blades.
An airfoil is a structure with curved surfaces designed to provide lift, propulsion, stability, or directional control in a flying object. It is any surface like a wing, propeller, rudder, or trim tab that provides aerodynamic force when it interacts with a moving stream of air. Examples of an airfoil include the airplane wing, the horizontal stabilizer and tailplane.
In aerodynamics, an airfoil is a body designed to produce lift perpendicular to its direction of motion while generating the best lift-to-drag ratio during flight. The leading edge makes initial contact with the air as the aircraft moves forward, the upper surface is the suction surface, and the lower surface is the pressure surface. This streamlined shape enables the lifting force to act at right angles to the airstream.
Helicopters employ airfoils in rotor blades that face unique aerodynamic issues because of widely varying flow conditions. Supersonic aircraft use thin-profile airfoils with sharp front edges, whereas thick, cambered airfoils are typical in commercial aircraft carrying heavy loads at lower speeds. Airfoils are used in drones, unoccupied aerial vehicles, eVTOL concepts, wind-turbine blades, and space launch vehicles.
How does an airfoil work?
According to Bernoulli's principle, the camber of an airfoil increases the velocity of the air passing over the airfoil, because velocity difference is linked to pressure difference. The upper flow is faster and a lower-pressure region appears above the wing while the higher pressure air on the bottom of the airfoil pushes the wing upwards. Together, pressure differences cause flow to change direction and the pressure field creates forces on the object.
The angle between the chord line of the wing and the relative wind is the angle of attack. Increasing this angle creates lift but also creates drag. Lift and drag are measured by the L/D ratio, which varies with angle of attack and airspeed. Increasing angle of attack eventually causes stall when flow separation begins. Aircraft wings are slightly tilted to produce lift during flight, and the goal is to keep lift greater than drag for good airfoil aerodynamic efficiency.
At zero angle of attack the lift coefficient is zero for a symmetric airfoil, whereas cambered sections already generate lift. Thin airfoil theory predicts a lift slope of 2 per radian of angle of attack, and a symmetric airfoil produces the same lift curve slope. The pressure gradient results in lift, and boundary layer thickness influences skin-friction drag. Flow rate difference produces the pressure gradient that keeps the wing aloft.
What are the dynamics of an airfoil?
Fluid dynamics governs every attribute of airfoil performance. An airfoil moves through fluid and experiences fluid-dynamic force. Pressure and shear integrate to produce forces on airfoil. Those forces are resolved into lift and drag, or into a wind-axis system, or into a chord-axis system. Lift acts perpendicular to freestream velocity whereas drag acts parallel to freestream velocity. Resulting forces acting on the airfoil are resolved into either the wind-axis system or the chord-axis system.
Fluid particles that travel over the upper surface of the airfoil encounter a large radius of curvature. The large radius strongly accelerates fluid and creates fast, low-pressure flow. Higher velocity on the upper surface creates lower pressure. Circulation means differential flow exists between upper surface and lower surface which produces lift. Lift per unit length equals density times velocity times circulation. Lift arises from action-reaction dynamics.
Velocity contours show high-speed flow on the upper surface and wake region behind the airfoil. Boundary layer thickness grows from leading edge to trailing edge. The boundary layer thickness is typical 1/2" near the trailing edge. Airfoil drag consists of skin-friction drag and pressure drag. Friction drag of the laminar boundary layer is .08N whereas the friction drag of the turbulent boundary layer is .12N.
Dynamic stall occurs when flow separation occurs at a high angle of attack. The boundary layer separates at the leading edge when the angle of attack is high which creates a recirculation bubble near the leading edge.
What is the mathematical calculation behind an airfoil?
Bernoulli's equation relates pressure and velocity, so it can be used to calculate pressure distribution once the velocity field around the airfoil is known. From this distribution the lift force follows by integration, and the same equation serves as a tool to obtain lift when the velocity field is provided.
The Kutta-Joukowski theorem relates lift per unit length to circulation: L = -ρVΓ , where ρ is density of the fluid, V is the speed of the body relative to the fluid at infinity, and Γ is the circulation.
To close the mathematical system the Kutta condition is applied at the trailing edge, the point on the airfoil where upper and lower surfaces meet. This empirical rule demands that the vortex sheet strength vanish there, forcing the flow to leave smoothly and fixing the unique circulation for each angle of attack. The Joukowski transform demonstrates how angle of attack, camber, and thickness distribution influence the mapped circle and hence the final pressure distribution and lift generation.
What are the types of airfoils?
The types of airfoils are presented below.
- Symmetrical Airfoils: In symmetrical airfoil, the upper section is identical to that of the lower section. A symmetrical airfoil cannot produce any lift at a zero angle of attack.
- Non-Symmetrical Airfoils (Cambered): A non-symmetrical airfoil has different upper and lower surfaces. The mean camber line (the line halfway between the upper and lower surfaces) is curved and does not coincide with the chord line. In a cambered airfoil lift is created even at a small angle of attack.
- High-Lift Airfoils: In high-lift airfoils, the wings are equipped with a flap system to produce the maximum possible lift, reducing landing speed, producing high lift-to-drag ratio, and reducing weight and mechanical complexity.
- Supercritical Airfoils: A supercritical airfoil is a special airfoil designed to reduce the drag when operating near supersonic speeds. By designing the airfoil so that the top is mostly flat and the bottom has a curve at the back, the point on the wing where the air reaches supersonic speeds is farther back on the wing, which creates a smaller shock wave and leads to less wave drag on the wing.
Low-Reynolds-number airfoils contain short or long laminar bubbles. Named families like NACA 4, 5 and 6 series, Joukowsky, Van de Vooren, Kline-Foglean, Eppler, Selig, Wortmann FX, RAE, RAF, Drela, Onera, Gottingen, Rutan, Clark, Eiffel, Delft University, DLR, Sikorsky, NASA, Boeing, Grumman, and McDonnell Douglas each offer distinct geometries for particular applications.
What are the parts of an airfoil?
The parts of an airfoil are outlined below.
- Leading Edge: The front, rounded point where the airfoil meets the airflow.
- Trailing Edge: The rear, often sharp point where airflow separates.
- Chord Line: A straight line connecting the leading edge to the trailing edge.
- Camber Line: A curve midway between the upper and lower surfaces, defining the airfoil's curvature.
- Thickness: A curve midway between the upper and lower surfaces, defining the airfoil's curvature.
- Leading Edge Radius: The curvature of the leading edge, affecting stall characteristics.
- Trailing Edge Angle: The measurement of the curve or sharpness at the very rear of an airfoil.
An airfoil is bounded by a rounded leading edge at the front and a sharp trailing edge at the aft. The two edges are joined by the curved upper surface (extrados) and the lower surface (intrados). A straight chord line connects the leading and trailing points, slicing the section into upper and lower parts and supplying the datum for angle of attack, thickness distribution, and camber. The airfoil skeleton is the chord line together with the mean camber line, a curve offset above or below the chord that runs midway between the two surfaces. Camber is measured from this mean camber line. Maximum camber and its location define the curvature of the airfoil.
Thickness is the perpendicular distance between upper and lower surfaces at each chord-wise station. The leading edge radius, normally about 1% of the chord for subsonic sections, governs how the oncoming flow accelerates around the nose, while the trailing edge angle sets the sharpness of the back and the point where the upper and lower airstreams rejoin. The direction of an airfoil is aligned with the chord line and the pitch angle is the angle this line makes with any chosen reference plane. Ribs inserted between the skins form the internal skeleton of built-up wings. These ribs are cells that open to the airflow at the leading edge and give the section its structural shape while preserving the defined curvature.
What is an airfoil type cross section?
An airfoil type cross section is a two-dimensional silhouette obtained when a side-view cut is taken through a lifting surface perpendicular to both the leading and trailing edges. This contour is referred to as the airfoil profile or the wing profile, and it characterizes the geometric form used for wings, stabilizers, propeller blades, helicopter rotor blades, sails, and even underwater foils like rudders, keels, and centerboards.
Although the contour is not strictly circular, the radius of curvature is frequently increased just ahead of the point of maximum thickness. This design postpones adverse pressure gradients, thereby minimizing the chance of boundary-layer separation and preserving wing efficiency under a wide range of angles of attack.
Cambered versions, where the camber line-the locus of points halfway between upper and lower surfaces-is arched, are the prevalent choice for aircraft wing cross-sections because they raise the lift coefficient. An airfoil with zero camber is essentially a symmetrical cross-section that produces no lift at zero angle of attack yet offers predictable behavior in inverted flight. Such sections are common on aerobatic and supersonic platforms as well as on many propeller airfoils.
A supercritical wing cross-section is an airfoil developed for transonic conditions. Its flattened upper surface and rear-loaded camber suppress strong shock waves and reduce wave drag. Rotorcraft airfoil sections are specialized to enhance the aerodynamic performance of rotor blades that must operate efficiently across a continually varying spectrum of subsonic speeds and angles. Glider airfoil cross-sections favor high lift-to-drag ratios by combining moderate camber with large thickness-to-chord ratios that permit long wings with low structural weight.
Whether the vehicle is a transport jet, a glider, an autogyro, or a helicopter, the selection of an appropriate airfoil cross-section establishes the pressure distribution, lift, drag, pitching moment, and overall efficiency that govern flight performance.
How to design an airfoil?
To design an airfoil follow the steps explained below.
- Start with an assumed airfoil shape like a NACA airfoil
- Set desired properties and characteristics for the airfoil
- Specify section geometry using direct airfoil design methods
- Define the chord line by drawing a straight line from leading to trailing edge
- Classify methods for airfoil design into direct and inverse design
- Reverse-engineer the airfoil using an optimization method
- Include smooth curve specifications from leading to trailing edge
- Employ XFOIL for inverse design methods
- Use CFD methods to obtain a specified level of performance
- Specify thickness distribution as a fraction of chord
- Use XFLR5 graphical user interface for XFOIL
- Calculate leading radius as rt = 1.1019
Direct design starts with an assumed airfoil shape like a NACA section. Engineers then employ mathematical models to predict aerodynamic performance by calculating pressures, lift, drag and moment outputs (cd, cl, cm). If the performance is unsatisfactory, one modifies the shape iteratively until the desired characteristics are reached. The simplest form of this process involves determining the characteristic of the candidate shape that is most troublesome and fixing that problem through successive adjustments. Inverse design methods reverse the sequence: desired properties and characteristics are set first, and an optimization method reverse-engineers the geometry that will produce them. CFD simulations can likewise be used to obtain a specified level of performance, while empirical data guide geometric refinements.
Whether direct or inverse, the airfoil design process proceeds from knowledge of boundary-layer properties and the relation between geometry and pressure distribution. Design specifications include a smooth curve from the radius at the nose leading edge to the thickest part to the back trailing edge, with leading-edge diameter often set to 3% of chord and leading radius rt = 1.1019. Thickness distribution is given as t = maximum thickness as fraction of chord, and the chord line is defined by drawing a straight line from the leading edge to the trailing edge. Parametric draftsmanship is frequently aided by AutoCAD's parametric features, which can layout key points to the requirements of the airfoil and then create a spline that respects these constraints while preserving a symmetrical or cambered profile as demanded by the mission.
What is the best airfoil for general aviation?
The best airfoil for general aviation is the classic NACA 23015 airfoil as it gives a good all-around performance and a cruise drag coefficient of .01. NACA 4412 and NACA 23012 are used on light planes, while Clark-Y airfoil is widely used in general purpose aircraft designs. All of these target a maximum lift coefficient of 1.4 to 1.5. NASA NLF(1)-0115 is a 15% thick natural laminar-flow airfoil intended for high speed and long range GA applications. Its drag bucket keeps drag coefficient near .01 over a wide lift range. NACA 64A-415 appears on later Cessna 210 models, a thinner section that trades short-field capability for efficient cruise.
Fighter aircraft airfoils are thinner and more highly loaded. Fifth-generation fighters with subsonic leading edges tend to use NACA 65A or NACA 64A sections. Military airfoils differ from general-purpose designs. The latter favor moderate thickness, gentle stall, and benign pitching moment. General aviation airfoils often have 12-15% thickness and low moment, whereas new airfoil shapes like NLF, LS, or GAW show pitching moments almost ten times those of older sections.
Aerobatic airplanes need symmetrical profiles. NACA 0015 airfoil is a symmetrical airfoil giving equal lift and moment in upright or inverted flight. Gliders prefer high lift and low drag and so SD7037 airfoil is popular for gliders and has been refined with XFOIL tweaking. Flying-wing configurations are sometimes built with MH60 airfoil, which is recommended for flying wings.
How to choose an ideal airfoil?
The best way to choose an airfoil is to start by thinking about how you are going to manufacture the wing, then match the intended maximum weight and intended speed range to the wing loading you want. A fairly low value is needed when you need a low minimum takeoff speed. Make the choice by calculating the Reynolds number for the design airspeed. For flying wings you start with MH 60, because this low-drag section is recommended for flying wings. Select it at zero angle of attack with flaps in the neutral position. Finally, align the trailing-edge shape with the specific requirements of aerodynamic design like maximum lift, minimum drag, and stall characteristics.
What is the best material for an airfoil?
The best material for an airfoil depends on the requirements. For the classic NACA 4412 section, the modal analysis made with carbon fibre, Al-Zn-Mg alloy and alpha-beta titanium alloy shows that alpha-beta titanium alloy is the better material choice. It keeps the lower natural frequencies indicated on the accompanying chart while offering the strength-weight balance required for contemporary airframes.
Composites are equally attractive when the objective is an exceptionally smooth, glassy-smooth surface free of contamination. Carbon/epoxy for the wings can provide such a finish with minimal weight. The same carbon fabric, with or without aramid and fibreglass cloth, is used on thinner, more highly loaded profiles where torsional stiffness and fatigue govern the design.
In low-Reynolds-number conditions, thin plates can outperform conventional airfoils by delaying flow separation, yet for full-scale aircraft, the selection narrows to either machined metal or advanced composite materials. Carbon fibre is therefore used in wing construction, alpha-beta titanium alloy in primary structures, and Al-Zn-Mg alloy wherever higher ductility and lower cost are acceptable.
What is the effect of a gap on airfoil performance?
A gap between moving surfaces breaks the two-dimensional ideal, so the model ceases to behave as a two-dimensional airfoil and three-dimensional effects like loss of lift creep in. The joint gap has an adverse effect on efficiency: it reduces the lift coefficient, introduces extra drag, and produces a drop in lift. At low speeds and high angles of attack, the gap increases the drag of the trailing-edge flap and decreases aerodynamic efficiency. The same slot delays stall or suppresses separation if the stagger is chosen correctly, showing that both stagger and gap significantly affect pressure distribution at an angle of attack of twelve degrees for the bottom airfoil.
A very small gap of 0.05% of wingspan delays stall, while an appropriate increase of the gap between the main element and the externally blown flap allows the airflow to resist the adverse pressure gradient, which increases the lift coefficient of the top airfoil. A further gap increase causes lift efficiency to rise only within a limited range, after which the decrease of aerodynamic efficiency becomes apparent. Well-designed gap-region geometry mitigates the efficiency loss, making the decrease of aerodynamic efficiency unapparent, but in generic configurations the gap remains the distance between the airfoil and the flap where dilution of the flow field and vortex destabilization at large angles of attack occur.
What is an airfoil trip?
An airfoil trip is a small protrusion installed spanwise along the upper surface of the wing at the desired chord-wise location where the laminar-turbulent transition is to take place. A small layer of tape is applied along the length of the airfoil near the leading edge, but the same effect is obtained with any roughness element like rivets, bolts, counter-sinks, or a purpose-made turbulator disk. The trip is expected to work inside the boundary layer to force the laminar to turbulent transition, instead of allowing natural transition. The process known as tripping the boundary layer artificially turns the laminar boundary layer into a turbulent one. A tripped airfoil is expected to carry a higher turbulent boundary layer and to separate later when the surface meets an adverse pressure gradient, so stall onset is experienced more gradually than on a clean airfoil.
Because the turbulent layer has higher momentum, flow separation is delayed and stability and control at high angle of attack are boosted, albeit at the price of increased skin-friction drag. Tripping is often employed in wind tunnels to simulate the real-world scenarios of airfoils in turbulent flows, yet the same principle is used intentionally on full-scale wings to suppress laminar separation bubbles that otherwise form near the leading edge. The optimum location is airfoil-dependent: for the SA7024 section the best result is obtained when the trip is placed at 0.4 chord, whereas forward locations at Reynolds numbers above 300,000 raise drag because the earlier loss of laminar flow increases skin friction. The lift characteristics of a tripped airfoil are slightly different from those of the clean section: the maximum lift coefficient reached is lower, but the post-stall behaviour is gentler. Designers must balance these gains and penalties. Airfoils intended to use trips currently need extensive and systematic wind-tunnel experimental investigation because analysis tools cannot yet predict trip effects reliably.
An airfoil trip is a short, deliberate flight whose only purpose is to let the wing speak for itself. I recall my initial aviation teaching: the instructor explained the basic law of lift, then indicated the airplane's cross-section and termed it an airfoil. On that trip the frame shivered, the aircraft bounced, and I could physically sense the delicate contours and the strong inner construction. At once its actual complication came evident. Wind must journey quicker over the curved upper surface, and I could physically sense how the exact configuration of that airfoil was diligently made to maintain us aloft. The exact configuration of that airfoil was a testimony to precise technology, no longer an easy simple idea I had previously assumed for granted.
Jim GoodrichPilot, Airplane Broker and Founder of Tsunami Air
Expert behind this article
Jim Goodrich
Jim Goodrich is a pilot, aviation expert and founder of Tsunami Air.
