Performance charts typically use which airspeed reference for lift and drag calculations?

• A. Indicated airspeed (IAS)
• B. True airspeed (TAS)
• C. Calibrated airspeed (CAS)
• D. Ground speed

Answer: (A)

Lift and drag scale with dynamic pressure, q = 1/2 ρ V^2, so performance numbers are tied to the aerodynamic state rather than raw speed in space. Indicated airspeed reflects that dynamic pressure directly, as the airspeed indicator measures impact pressure and presents a value that corresponds to the actual lift and drag the wing is producing at standard conditions. Because IAS links you to the same aerodynamic condition across different altitudes and temperatures, the performance charts—stalls, climb, takeoff distances, and the like—remain consistent and meaningful. True airspeed would change with air density and hide those consistent aerodynamic states, making the chart values less reliable for planning. Ground speed, being a measure of motion over the ground, doesn’t describe the forces on the wing. Calibrated airspeed is the IAS corrected for instrument error, but the practical, consistently applicable reference for lift and drag on charts is the indicated airspeed.