An airfoil is a specifically designed shape used in wings, blades, or similar structures to generate lift or thrust when interacting with an airflow. Airfoils are fundamental to the science of flight, playing a pivotal role in how aircraft achieve and sustain lift.

What is an Airfoil

An airfoil (or aerofoil in British English) refers to the cross-sectional shape of a wing, blade, or propeller designed to produce a desired aerodynamic effect. When air flows around an airfoil, it creates differences in pressure that generate forces such as lift or thrust.

An airfoil is a surface designed to obtain lift from the air through which it moves.
Thus, it can be stated that any part of the aircraft that converts air resistance into lift is an airfoil.

Key applications of airfoils include:

  • Aircraft wings
  • Helicopter rotor blades
  • Propellers (airplanes, ships, and wind turbines)
  • Racing car spoilers

Anatomy of an Airfoil

Understanding the components of an airfoil is essential to appreciate its function:

  1. Leading Edge:
    • The front edge of the airfoil where airflow first meets the surface.
    • Typically rounded to ensure smooth airflow.
  2. Trailing Edge:
    • The rear edge of the airfoil where airflow rejoins after passing over the top and bottom surfaces.
  3. Chord Line:
    • An imaginary straight line connecting the leading edge to the trailing edge.
    • Used as a reference for measuring other airfoil properties.
  4. Camber:
    • The curvature of the airfoil’s surface, often expressed as the upper and lower camber.
    • Positive Camber: Produces more lift; commonly seen in aircraft wings.
    • Symmetrical Airfoil: Has no camber, producing no lift at zero angle of attack; used in aerobatic planes.
  5. Mean Camber Line:
    • The mean camber line is an imaginary line that runs midway between the upper and lower surfaces of an airfoil. It is equidistant from both surfaces at every point along the chord length.
    • It represents the curvature of the airfoil.
    • If an airfoil has no camber (i.e., a symmetrical airfoil), the mean camber line coincides with the chord line.
  6. Thickness:
    • The distance between the upper and lower surfaces of the airfoil.
    • Affects structural strength and aerodynamic performance.

How an Airfoil Works

Airfoil performance is primarily based on Bernoulli’s Principle and Newton’s Third Law of Motion.

  1. Bernoulli’s Principle: Pressure Differences
    • The air flowing over the curved upper surface travels faster than the air beneath the flatter lower surface.
    • Faster airflow creates lower pressure above the airfoil, generating lift.
  2. Newton’s Third Law: Action and Reaction
    • As the airfoil deflects airflow downward, the air pushes upward on the airfoil in response, contributing to lift.

Factors That Affect Airfoil Performance

Several factors influence how well an airfoil performs:

  1. Shape of the Airfoil
    • Cambered Airfoils: Generate high lift at low angles of attack.
    • Symmetrical Airfoils: Generate less lift but offer better balance and control.
    • Supercritical Airfoils: Designed for high-speed aircraft, delaying shockwave formation.
  2. Angle of Attack (AoA)
    • The angle between the chord line and the direction of the relative airflow.
    • Increasing the AoA generally increases lift—up to a point.
    • Beyond the critical AoA, the airfoil stalls, causing a sudden loss of lift.
  3. Airfoil Size
    • Larger airfoils produce more lift, suitable for heavy aircraft.
  4. Airflow Velocity
    • Higher airspeed increases lift, as the dynamic pressure on the airfoil grows.
  5. Surface Smoothness
    • Smooth surfaces reduce drag and enhance lift.
  6. Air Density
    • Lift increases with denser air, such as at lower altitudes or cooler temperatures.

Note: Within limits, the lift can be increased by increasing the angle of attack (AOA), wing area, velocity, density of the air, or by changing the shape of the airfoil.

Shape of the Airfoil

Individual airfoil section properties differ from those of the wing or aircraft as a whole because of the effect of the wing planform. A wing may have various airfoil sections from root to tip, with taper, twist, and sweepback. The resulting aerodynamic properties of the wing are determined by the action of each section along the span.

The shape of the airfoil determines the amount of turbulence or skin friction it produces, consequently affecting the efficiency of the wing. Turbulence and skin friction are controlled mainly by the fineness ratio, which is defined as the ratio of the chord of the airfoil to its maximum thickness. If the wing has a high fineness ratio, it is a very thin wing. A thick wing has a low fineness ratio. A wing with a high fineness ratio produces a large amount of skin friction, while a wing with a low fineness ratio produces a large amount of turbulence. The best wing is a compromise between these two extremes to minimize both turbulence and skin friction.

The efficiency of a wing is measured in terms of the lift-to-drag ratio (L/D). This ratio varies with the angle of attack (AOA) but reaches a definite maximum value at a particular AOA. At this angle, the wing has reached its maximum efficiency. The shape of the airfoil determines the AOA at which the wing is most efficient and also influences the degree of efficiency. Research has shown that the most efficient airfoils for general use have their maximum thickness occurring about one-third of the way back from the leading edge of the wing. High-lift wings and high-lift devices have been developed by shaping airfoils to produce the desired effect.

The amount of lift produced by an airfoil increases with an increase in wing camber. Camber refers to the curvature of an airfoil above and below the chord line. Upper camber refers to the upper surface, lower camber to the lower surface, and mean camber to the mean line of the section. Camber is positive when the departure from the chord line is outward and negative when it is inward. Thus, high-lift wings have a large positive camber on the upper surface and a slightly negative camber on the lower surface. Wing flaps allow an ordinary wing to approximate this same condition by increasing the upper camber and creating a negative lower camber.

It is also known that the larger the wingspan, compared to the chord, the greater the lift obtained. This comparison is called the aspect ratio. The higher the aspect ratio, the greater the lift. Despite the benefits of increasing the aspect ratio, definite limitations arise due to structural and drag considerations.

On the other hand, an airfoil that is perfectly streamlined and offers little wind resistance may not have enough lifting power to get the aircraft off the ground. Thus, modern aircraft have airfoils that strike a balance between extremes, with the shape depending on the purpose for which the aircraft is designed.

Angle of Incidence

The acute angle between the wing chord and the longitudinal axis of the aircraft is called the angle of incidence, or the angle of wing setting. In most cases, the angle of incidence is a fixed, built-in angle. When the leading edge of the wing is higher than the trailing edge, the angle of incidence is said to be positive. Conversely, when the leading edge is lower than the trailing edge, the angle of incidence is negative.

Angle of Attack

The AoA is defined as the angle between the chord line of the wing and the direction of the relative wind. The AOA changes as the aircraft’s attitude changes.

When the angle of attack (AOA) increases to the angle of maximum lift, the burble point is reached. This is known as the critical angle. At this point, the air ceases to flow smoothly over the top surface of the airfoil and begins to burble or eddy. This means the airflow breaks away from the upper camber line of the wing. What was previously the area of decreased pressure is now filled with turbulent, burbling air.

When this occurs, lift decreases, and drag increases significantly. As a result, the force of gravity takes over, causing the aircraft’s nose to drop. This is a stall. Thus, the burble point is the stalling angle.

Types of Airfoils

Different airfoils are optimized for specific applications:

  1. Cambered Airfoils:
    • Common in general aviation and commercial aircraft.
    • High lift-to-drag ratio.
  2. Symmetrical Airfoils:
    • Used in aerobatic planes and helicopter rotor blades.
    • Balanced performance in both directions.
  3. Supercritical Airfoils:
    • Flattened upper surface for transonic speeds.
    • Reduces drag caused by shockwaves.
  4. High-Lift Airfoils:
    • Incorporate slats and flaps to maximize lift during takeoff and landing.
  5. Laminar Flow Airfoils:
    • Designed to minimize drag by maintaining smooth airflow over a longer surface area.

Key Aerodynamic Forces on an Airfoil

  1. Lift: Generated perpendicular to the direction of airflow.
  2. Drag: Resistance to motion, acting parallel to airflow.
  3. Moment: A rotational force acting about the aerodynamic center, affecting stability.

Stall: When Airfoils Fail

A stall occurs when the angle of attack exceeds the critical value, disrupting smooth airflow over the upper surface.

  • Effects of Stall:
    • Drastic reduction in lift.
    • Increased drag.
    • Loss of control.
  • Prevention and Recovery:
    • Use of stall warning systems.
    • Adjusting the AoA or deploying flaps/slats.
    • Increasing throttle to regain airspeed and restore lift.

Airfoil Design in Modern Aviation

  • Computational Aerodynamics
    • Engineers use computational fluid dynamics (CFD) to optimize airfoil shapes for specific aircraft.
  • Wind Tunnel Testing
    • Scaled models are tested in wind tunnels to validate performance.
  • Materials
    • Lightweight, durable materials like composites and aluminum are used to construct airfoils.
  • High-Lift Devices
    • Flaps: Extendable surfaces on the trailing edge to increase lift during takeoff and landing.
    • Slats: Devices on the leading edge that delay stall by improving airflow at high AoA.

Applications of Airfoils

  • Aircraft Wings
    • Primary source of lift.
    • Optimized for speed, range, and fuel efficiency.
  • Propellers and Rotor Blades
    • Generate thrust and lift, respectively, using similar aerodynamic principles.
  • Wind Turbines
    • Use airfoils to convert wind energy into rotational motion.
  • Automobile Aerodynamics
    • Spoilers use inverted airfoil designs to increase downward force and improve traction.

Innovations in Airfoil Technology

  • Morphing Airfoils: Adaptive surfaces that change shape in flight for optimal performance.
  • Blended Wing Body Designs: Airfoil integrated into the entire structure of the aircraft for greater efficiency.
  • Energy-Efficient Designs: Airfoils tailored for electric and hybrid propulsion systems.

Conclusion

The airfoil is the cornerstone of flight, with its shape and characteristics determining the performance and efficiency of any aircraft. By carefully balancing lift, drag, and other forces, airfoil designs have evolved to meet the demands of modern aviation. As technology advances, airfoils will continue to be at the forefront of innovation, enabling faster, safer, and more efficient flight.

AviationHunt Team

Our team at AviationHunt is a group of aviation experts and enthusiasts. We aim to provide aviation insights, technical notes, best aircraft maintenance practices, and aviation safety tips.