Ground Speed vs. True Airspeed

When discussing the speed of an aircraft or parachute, two key terms often come up: Ground Speed and True Airspeed. While they both describe motion, they serve different purposes and are measured in distinct ways. Understanding their differences is crucial, especially when calculating the performance of a wing or parachute.

1. Theoretical Explanation and Key Differences

Ground Speed

Ground speed is the speed of an aircraft or parachute relative to the ground. It represents how quickly you are covering distance over the Earth’s surface.

• Key Influences: Ground speed is affected by wind conditions. A tailwind increases ground speed, while a headwind decreases it.

• Example: If your true airspeed is 100 knots and you have a 20-knot tailwind, your ground speed will be 120 knots. Conversely, with a 20-knot headwind, your ground speed drops to 80 knots.

True Airspeed (TAS)

True airspeed is the speed of an aircraft relative to the surrounding air mass. It represents how fast the air is flowing over the wings or canopy.

• Key Influences: True airspeed depends on air density, which decreases with altitude and temperature. This means TAS is higher at higher altitudes for the same indicated airspeed.

• Example: At sea level, an indicated airspeed of 100 knots might match a true airspeed of 100 knots. At 10,000 feet, the same indicated airspeed could correspond to a true airspeed of 120 knots due to lower air density.

The Difference in Context:

• Ground speed tells you how fast you are moving over the Earth.

• True airspeed tells you how fast you are moving through the air, independent of wind.

2. Measuring Ground Speed and True Airspeed

Measuring Ground Speed

• GPS: The most common tool for measuring ground speed today is a GPS system. By tracking your position over time, GPS calculates your speed relative to the ground with high precision.

• Advantages: Accurate, independent of atmospheric conditions, and widely available.

Measuring True Airspeed

• Pitot-Static System: True airspeed is often derived using a pitot tube and static pressure system. These systems measure dynamic pressure (caused by air hitting the aircraft) and static pressure (ambient atmospheric pressure).

• Correction Factors: To calculate TAS, corrections for altitude and temperature are applied to the indicated airspeed (IAS).

Summary of Tools:

• Ground speed: Measured by GPS.

• True airspeed: Measured using a pitot-static system combined with environmental corrections.

3. Which Speed Matters for Wing or Parachute Performance?

When evaluating the performance of a wing or parachute, True Airspeed is the critical measure. Here’s why:

• Aerodynamic Behavior: Performance metrics like lift, drag, and glide ratio depend on the relative airflow over the surface of the wing or canopy. TAS reflects this airflow, whereas ground speed does not account for wind effects.

• Consistent Analysis: Ground speed can vary significantly with wind, making it unreliable for comparing performance under different conditions. TAS, on the other hand, isolates the performance of the wing or parachute from external factors like wind.

• Example for Parachutes: If you’re analyzing the glide ratio of a ram-air canopy, TAS tells you how efficiently the canopy converts forward speed into vertical descent, independent of wind conditions.

VIDeo

Understanding the distinction between ground speed and true airspeed is essential for anyone involved in aviation or parachuting. While GPS-based ground speed helps track your movement relative to the ground, true airspeed provides the accurate measure needed for evaluating aerodynamic performance. If you want to see the corresponding video on my youtube channel, to have some practical and visual examples you can find it here: https://youtu.be/wVz9tcpV0e4