Aerodynamics: Basics & Definitions

081-01-01Pablo Asensio Martínez2026-04-011 min

The Atmosphere

  • Composition: 78% Nitrogen, 21% Oxygen, 1% Other (Argon, CO2).
  • ISA (International Standard Atmosphere): Used for calibration and comparison.
    • Temperature: 15°C (288.15 K) at Mean Sea Level (MSL). Lapse rate -1.98°C/1000 ft up to 11km (Tropopause).
    • Pressure: 1013.25 hPa (29.92 inHg) at MSL.
    • Density: 1.225 kg/m³ at MSL.

Continuity Equation

For a steady flow of an incompressible fluid (subsonic flow is treated as incompressible): $ A_1 V_1 = A_2 V_2 $

  • $A$: Area.
  • $V$: Velocity.
  • If the cross-sectional area of a tube decreases, the velocity of the fluid increases.

Bernoulli's Principle

Subject to the flow being steady, incompressible, and inviscid (no friction). The sum of static pressure and dynamic pressure is constant along a streamline. $ P_{static} + \frac{1}{2}\rho V^2 = Constant $ $ P_{static} + P_{dynamic} = P_{total} $

  • As velocity ($V$) increases (due to converging tube), dynamic pressure increases, and static pressure ($P_{static}$) decreases.
  • This explains lift generation: air accelerating over the curved upper surface of a wing has lower static pressure than the air below.

Airspeeds

  • IAS (Indicated Airspeed): Read from the instrument. Driven by dynamic pressure ($q = \frac{1}{2}\rho V^2$).
  • CAS (Calibrated Airspeed): IAS corrected for instrument and position error.
  • EAS (Equivalent Airspeed): CAS corrected for compressibility (significant > 300kt / Mach 0.6).
  • TAS (True Airspeed): The actual speed of the aircraft through the air mass. Corrected for density error.
    • $TAS = EAS \times \frac{1}{\sqrt{\sigma}}$ (where $\sigma$ is relative density).
    • At constant IAS, TAS increases with altitude (approx 2% per 1,000 ft).