Types of Airspeed

By /

Airspeed is a fundamental concept in flight mechanics and pilot operations. Different types of airspeed account for instrument readings, atmospheric conditions, and aircraft performance. Understanding these distinctions is crucial for safe and efficient flight.

1. True Airspeed (TAS)

1.1 Definition

  • The actual speed of the aircraft relative to the air mass in which it is flying.
  • Used in navigation and performance calculations.

1.2 Formula

Related to Equivalent Airspeed (EAS):

TAS = EAS \sqrt{\frac{\rho_0}{\rho}}

Where:

  • \rho = Air density at flight altitude
  • \rho_0 = Standard sea-level density

1.3 Characteristics

  • Increases with altitude for the same EAS.
  • Used in flight planning for time, fuel, and distance calculations.

2. Calibrated Airspeed (CAS)

2.1 Definition

  • Indicated airspeed corrected for instrument and position errors.
  • Represents a more accurate measure than Indicated Airspeed (IAS).

2.2 Formula

No standard single formula—usually provided by manufacturer calibration tables:

CAS = IAS + \text{Corrections}

2.3 Uses

  • Used for flight operations and performance charts.
  • Aircraft limitations often given in CAS.

3. Indicated Airspeed (IAS)

3.1 Definition

  • Airspeed read directly from the airspeed indicator.
  • Based on dynamic pressure sensed by the pitot-static system.

q_c = \frac{1}{2} \rho V^2

3.2 Characteristics

  • Includes instrument and position errors.
  • Most commonly referenced by pilots during normal operations.

4. Equivalent Airspeed (EAS)

4.1 Definition

  • CAS corrected for compressibility effects at higher speeds and altitudes.
  • Represents the airspeed at sea-level density producing the same dynamic pressure.

4.2 Formula

At subsonic speeds:

EAS = TAS \sqrt{\frac{\rho}{\rho_0}}

4.3 Uses

  • Important for structural loads and aerodynamic forces.
  • Used in high-speed flight analysis.

5. Groundspeed (GS)

5.1 Definition

  • The actual speed of the aircraft relative to the ground.
  • Sum of true airspeed and wind components.

5.2 Calculation

GS = TAS \pm \text{Wind Component}

Where:

  • Add tailwind component.
  • Subtract headwind component.

5.3 Uses

  • Essential for navigation and estimating time en route.

6. Relationships Between Airspeeds

  • IAS → CAS: Correct for instrument/position errors.
  • CAS → EAS: Correct for compressibility effects.
  • EAS → TAS: Correct for air density changes.

Chain of corrections:

IAS \rightarrow CAS \rightarrow EAS \rightarrow TAS

7. Operational Significance

  • Pilots must know which airspeed to use for:
    • Takeoff and landing speeds (often IAS or CAS).
    • Structural limits (EAS for loads).
    • Navigation (TAS and GS).
  • Flight manuals specify speeds in IAS, CAS, or TAS depending on context.

8. Summary

Different airspeed types provide critical information for flight safety and performance. Understanding their definitions, corrections, and uses allows pilots and engineers to interpret instrument readings accurately and manage aircraft operation under varying atmospheric conditions.

Shopping Cart
Scroll to Top