An airfoil (or aerofoil in British English) is any structure designed to manipulate the flow of a fluid to produce a reaction, which in an aircraft’s case, is aerodynamic lift. The wings of fixed-wing aircraft feature airfoil-shaped cross-sections. Airfoils enable heavier-than-air flight, but are also in found various other vehicle parts like helicopters, turbines, and spoilers.
It’s important to note that there is no single optimal airfoil design. Airfoil design characteristics must be carefully considered when constructing an aircraft, as optimal characteristics can vary depending on the aircraft’s intended purpose.
Important Airfoil Design Terminology
- Chord line – A theoretical straight line between the leading and trailing edges, the airfoil’s front-most and rear edges, respectively.
- Mean camber line – The centerline between the upper and lower surfaces. Camber describes how curved an airfoil is.
- Upper surface camber – The curve of the top of the airfoil that is typically more pronounced than the lower surface.
- Lower surface camber – the curve of the bottom of the airfoil.
- Angle of attack – The angle between the chord line and flow direction.
- Relative wind – Airflow relative to an airfoil created by movement of the airfoil. For example, a wing moving at 100 mph will generate relative wind moving in the opposite direction over and under the wing at 100 mph.
Examples of various airfoil designs, courtesy of Wikimedia Commons.
Airfoil Design Overview
An airfoil’s shape is designed to take advantage of the natural response of air flow when disrupted. When air passes through an airfoil, two things occur: a positive pressure lifting action from the air below the wing and a negative pressure lifting action from lowered pressure above the wing.
An airfoil’s purpose is to reduce drag and generate lift. However, design will always vary for each use case. For example, while a streamlined or flat wing may experience very little drag, it may not generate enough lifting power for an aircraft to take off. Going even further, airfoils designed for low-altitude flights will look different than high-altitude ones. For flight, airfoil design must strike a balance in design expectations.
How Does Aerodynamic Lift Occur?
Aerodynamic lift is commonly explained with two explanations, utilizing Bernoulli’s Principle of Pressure for one and Newton’s Third Law for the other:
Applying Bernoulli’s Principle of Pressure, when an airfoil is moving, the airfoil’s shape causes an increase in velocity of air across the upper surface of an airfoil. This results in a drop in pressure in the area above the airfoil and an increase in pressure below the airfoil. The pressure difference between the upper and lower surface creates lift force.
Explanations using Newton’s Third Law focus less on the velocity of the airfoil and more on the fluidity of the air, as an airfoil moving shifts the air around it. When the airfoil is in action, the flow of air moving upwards over the airfoil is an upwash, and air moving downwards is downwash. The airfoil generates lift from the pressure distribution between the upwash and downwash by turning the incoming air downwards.
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