Airfoil Geometry
An airfoil (or aerofoil) is the cross-sectional shape of a wing. Its geometric quantities determine the aerodynamic behaviour.
Main terms
| Term | Definition |
|---|---|
| Leading edge (LE) | front boundary of the airfoil, usually rounded |
| Trailing edge (TE) | rear boundary, sharp-wedge shaped |
| Chord line (c) | imaginary line connecting LE and TE |
| Chord length | length of the chord |
| Mean camber line | mid-line between upper and lower surface |
| Camber | maximum distance between chord and camber line |
| Thickness | maximum perpendicular distance upper to lower surface |
| LE radius | radius of curvature at the leading edge |
Angle of attack (α)
Definition: angle between the chord line and the relative wind (= incoming flow direction).
- Small (e.g. 2°): cruise AoA, low CL.
- Medium (8–12°): high lift, climb.
- Large (14–18°): near stall.
- Above α_stall: flow separation, stall.
Angle of incidence (i)
Definition: angle between the chord line and the aircraft longitudinal axis (set by design).
- Constant for a given aircraft — fixed by installation.
- Typical: 1°–3° (positive nose-up).
- Angle of attack α = i + pitch angle (simplified).
Airfoil types
Symmetric airfoil
- Camber line = chord line (no camber).
- At α = 0° no lift (CL = 0).
- Use: aerobatic aircraft, some high-performance aircraft.
Cambered airfoil
- Camber line curved upward.
- At α = 0° already positive lift.
- Use: standard for most aircraft (Cessna 172, PA-28).
Reflexed airfoil
- Trailing edge bent upward.
- Low or negative pitching moment about the chord point.
- Use: tailless aircraft, flying wings.
NACA designation
The NACA 4-digit series (e.g. NACA 2412) encodes:
- 1st digit: max camber in % of chord (e.g. 2 → 2 %).
- 2nd digit: location of max camber in tenths of chord (e.g. 4 → 40 %).
- 3rd + 4th digit: maximum thickness in % of chord (e.g. 12 → 12 %).
Standard airfoils:
- NACA 2412 → Cessna 172.
- NACA 0012 → symmetric, tail section of many aircraft.
- NACA 65A012 → high-performance series.
Source: NACA Technical Reports, public at ntrs.nasa.gov.
Reynolds number
Measure of the ratio of inertial to viscous forces:
Re = ρ·v·c / μ
with ρ=density, v=velocity, c=chord length, μ=dynamic viscosity.
- Typical PPL Re: 1–6 million.
- Affects CL_max and boundary-layer behaviour.