3.airfoil-design. Airfoil Design
An airfoil is any surface—such as a wing, propeller blade, rotor blade, or control surface—that produces a useful aerodynamic reaction (lift) as it moves through the air. The shape, planform, and orientation of an airfoil determine how efficiently it generates lift, how much drag it produces, and how it behaves at the edges of its operating envelope.
Airfoil Terminology
Every airfoil section can be described using a standard set of references:
- Leading edge: The forward-most edge of the airfoil that first meets the oncoming air.
- Trailing edge: The aft edge where airflow from the upper and lower surfaces rejoins.
- Chord line: A straight line drawn from the leading edge to the trailing edge.
- Chord: The length of the chord line; a primary reference dimension.
- Mean camber line: A line drawn halfway between the upper and lower surfaces. If this line is curved, the airfoil is cambered; if it coincides with the chord line, the airfoil is symmetrical.
- Camber: The curvature of the upper and lower surfaces. Upper camber is typically more pronounced than lower camber.
- Thickness: The maximum distance between upper and lower surfaces, normally expressed as a percentage of chord.
- Relative wind: The direction of airflow with respect to the airfoil; it is opposite to the flightpath of the aircraft.
- Angle of attack (AOA): The acute angle between the chord line and the relative wind.
How an Airfoil Produces Lift
As an airfoil moves through the air, it deflects the airflow downward and accelerates the flow over the upper surface. By Bernoulli's principle, the higher-velocity flow over the curved upper surface produces lower static pressure than the slower flow beneath the wing. By Newton's third law, the downward turning of the air mass (downwash) produces an equal and opposite upward reaction on the wing. Both descriptions are valid and complementary—lift is the integrated pressure differential across the airfoil, manifested as a force perpendicular to the relative wind.
The lift equation summarizes the variables a designer and pilot can influence:
L = CL × ½ρV² × S
where CL is the coefficient of lift (a function of airfoil shape and AOA), ρ is air density, V is true airspeed, and S is wing area.
Types of Airfoils
Designers select airfoil geometry to match the mission of the aircraft:
- Cambered (asymmetrical) airfoils generate lift at zero angle of attack and produce high CL at low speeds. They are typical of training and transport airplanes.
- Symmetrical airfoils have identical upper and lower surfaces and produce no lift at zero AOA. They are common on aerobatic aircraft and helicopter rotor blades because they perform identically inverted.
- Laminar-flow airfoils have their maximum thickness farther aft (around 50% chord) to delay the transition from laminar to turbulent flow, reducing parasite drag at cruise. They are sensitive to surface contamination such as bugs, ice, or frost.
- High-lift airfoils use generous camber and leading-edge radius to maximize CLmax for short-field operations.
Planform and Aspect Ratio
An airfoil's two-dimensional section is only part of the story; the planform (top-down shape) also matters. Aspect ratio is span² divided by area (AR = b²/S). High-aspect-ratio wings (gliders, U-2) reduce induced drag and improve climb and endurance. Low-aspect-ratio wings (fighters, delta wings) tolerate higher speeds and higher load factors but generate more induced drag at low speed. Common planforms include rectangular, tapered, elliptical, swept, and delta—each with predictable stall and drag characteristics.
Stall Behavior and Wing Twist
Airfoil shape directly influences stall behavior. A wing with a sharp leading edge stalls abruptly; a wing with a generous leading-edge radius stalls more gently. Designers frequently incorporate washout—a geometric twist that gives the wing root a higher angle of incidence than the tip—so the root stalls first. This preserves aileron effectiveness through the stall and provides aerodynamic warning (buffet) before full wing stall. Some wings use different airfoil sections at the root and tip (aerodynamic twist) to achieve the same goal.
High-Lift and Drag-Modifying Devices
Fixed airfoil shape is a compromise, so most airplanes use movable surfaces to reshape the wing for different phases of flight:
- Flaps (plain, split, slotted, Fowler) increase camber, area, or both to raise CLmax for takeoff and landing.
- Slats and slots energize boundary-layer flow over the upper surface, delaying separation and increasing critical AOA.
- Spoilers disrupt lift and increase drag for descent and roll control.
- Vortex generators re-energize the boundary layer to delay separation on swept or laminar-flow wings.
Practical Takeaway for Pilots
Understanding airfoil design helps explain real-world handling: why a clean laminar wing loses substantial lift with frost, why extending flaps lowers stall speed but increases drag, why washout keeps the ailerons working in a stall, and why a symmetrical-airfoil aerobatic airplane flies as well inverted as upright. The airfoil is the foundation of every aerodynamic discussion that follows.