Understanding airfoil nomenclature is the essential first step in studying how wings generate lift, drag, and other aerodynamic forces. This section explains the standard terms and definitions used to describe the shape and geometry of airfoils.
Clear, consistent naming is crucial for communication in aerodynamics and for interpreting airfoil data such as lift and drag coefficients.
1️⃣ What is an Airfoil?
✅ An airfoil (or aerofoil) is a 2D cross-sectional shape of a wing, blade, or sail designed to produce lift when moving through air.
While real wings are 3D, studying airfoils helps isolate essential aerodynamic characteristics in 2D flow, simplifying analysis.
2️⃣ Basic Geometry Terms
Chord Line
- Straight line connecting the leading edge (front) to the trailing edge (back) of the airfoil.
- Chord length (c): distance along the chord line.
Camber Line
- Curve joining midpoints between upper and lower surfaces.
- Describes mean shape of the airfoil.
✅ A straight camber line = symmetric airfoil.
✅ A curved camber line = cambered airfoil.
Thickness Distribution
- Variation of airfoil thickness along the chord.
- Typically given as distance between upper and lower surfaces at each chordwise position.
Example:
Maximum thickness often expressed as percentage of chord (e.g. 12% for NACA 2412).
Leading Edge
- Front-most point of the airfoil.
- Typically rounded to reduce flow separation and stagnation pressure spikes.
Trailing Edge
- Rear-most point where upper and lower surfaces meet.
- Can be sharp (most common) or blunt (for some control surfaces).
3️⃣ Camber and Thickness
Maximum Camber (f)
- Greatest distance between chord line and camber line.
- Often given as percentage of chord.
- Affects lift characteristics.
Example: NACA 2412 → 2% camber.
Location of Maximum Camber (p)
- Distance from leading edge to point of maximum camber.
- Usually expressed as fraction of chord.
Example: NACA 2412 → 40% chord.
Maximum Thickness (t)
- Largest thickness between upper and lower surfaces.
- Also given as percentage of chord.
Example: NACA 2412 → 12% thickness.
4️⃣ Angle of Attack (α)
- Angle between incoming airflow and the chord line.
- Primary control variable for lift and stall.
5️⃣ Zero-Lift Angle of Attack (α₀)
- Angle of attack at which net lift = 0.
- Symmetric airfoils: α₀ ≈ 0°.
- Cambered airfoils: α₀ typically negative.
6️⃣ Aerodynamic Center (AC)
- Point along chord where moment coefficient is constant with angle of attack.
- For subsonic airfoils, typically at quarter-chord (25% of chord from leading edge).
✅ Critical for stability and control calculations.
7️⃣ Center of Pressure (CP)
- Point where resultant aerodynamic force acts on airfoil.
- Location moves with angle of attack.
- Unlike AC, it’s not fixed.
8️⃣ NACA Airfoil Naming
✅ Airfoils are often described by standardized systems:
NACA 4-Digit Series (e.g. NACA 2412)
- 1st digit: maximum camber (% of chord).
- 2nd digit: location of maximum camber (tenths of chord).
- Last two digits: maximum thickness (% of chord).
Example:
- NACA 2412 = 2% camber at 40% chord, 12% thickness.
✅ More complex NACA series exist (5-digit, 6-series) with detailed performance goals.
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NACA 5-Digit Series
The NACA 5-digit series was developed to provide better control over lift characteristics than the 4-digit series.
NACA LPXXT
- L: First digit—designed ideal lift coefficient in tenths.
- P: Second digit—location of maximum camber in tenths of chord.
- XX: Next two digits—camber shape modifier (reflects camber line shape; often relates to design lift and reflex).
- T: Last digit—maximum thickness as % of chord.
Example
NACA 23012
- 2 → Designed CL = 0.3 (first digit × 0.15 gives design lift coefficient).
- 3 → Max camber at 15% chord (second digit × 5%).
- 0 → Camber shape modifier (affects reflex for trim).
- 12 → 12% thickness.
✅ Special Feature
- Camber lines in 5-digit series can be reflexed to reduce pitching moment (helpful for tailless aircraft).
✅ Why Use It?
- Allows precise control of design lift coefficient.
- Includes reflexed camber lines for better stability/trim.
NACA 6-Digit Series
The NACA 6-series was developed for laminar flow airfoils with better drag characteristics over a wide speed range.
NACA 6X-XXYY
Typical form:
- 6 → Series designation (6-series).
- X → Location of minimum pressure (in tenths of chord).
- – → Hyphen separator.
- XX → Range of lift coefficient over which favorable pressure distribution is maintained.
- YY → Design lift coefficient in tenths.
Example
NACA 641-212
- 6 → 6-series laminar flow airfoil.
- 4 → Minimum pressure at 40% chord.
- 1 → Indicates 0.1 CL range with laminar flow.
- -2 → Design CL = 0.2.
- 12 → 12% thickness.
✅ Special Feature
- Designed to maximize laminar flow to reduce drag.
- Custom pressure distributions tailored for different design lift coefficients.
✅ Why Use It?
- Provides low-drag sections for high-speed aircraft.
- Allows aerodynamicists to control pressure gradients for laminar boundary layers.
9️⃣ Summary of Airfoil Nomenclature
| Term | Description |
|---|---|
| Chord Line | Straight line from leading to trailing edge |
| Camber Line | Curve joining midpoints between surfaces |
| Leading Edge | Front-most point |
| Trailing Edge | Rear-most point |
| Maximum Camber (f) | Peak camber as % of chord |
| Location of Camber (p) | Distance from LE to max camber (% chord) |
| Maximum Thickness (t) | Max thickness as % of chord |
| Angle of Attack (α) | Angle between chord line and freestream |
| Zero-Lift α₀ | AoA at which lift = 0 |
| Aerodynamic Center | Point of constant moment coefficient |
| Center of Pressure | Varies with AoA, location of resultant force |
Summary of NACA Series
| Series | Purpose | Key Feature |
|---|---|---|
| 4-digit | Basic, general-purpose airfoils | Simple camber/thickness specification |
| 5-digit | Improved camber control, design CL | Reflex options for trim, precise CL |
| 6-series | Laminar flow optimization | Pressure distribution tailored to CL |
In summary, airfoil nomenclature provides the standardized language for describing airfoil shapes and their geometric features. Mastery of these terms is essential for understanding lift, drag, pitching moments, and for interpreting performance data in both 2D and 3D wing analyses.
