To maintain lift at a higher altitude (lower air density), for a given angle of attack, the aircraft must fly at a greater true airspeed.

• A. True
• B. False
• C. Only at high Mach
• D. Only for certain wings

Answer: (A)

Lift is proportional to ρ V^2 times Cl, where ρ is air density, V is true airspeed, and Cl is the lift coefficient set by the angle of attack. If you hold the angle of attack fixed, Cl stays essentially the same. At higher altitude the air is thinner, so ρ decreases. To keep the same lift (equal to the aircraft’s weight) you must raise the dynamic pressure ½ρV^2, which means increasing V. Since dynamic pressure scales with V^2, a higher true airspeed is needed to compensate for the lower density. In short, with lower air density, maintaining the same lift at a fixed angle of attack requires a greater true airspeed. Note that at very high Mach or different wing designs other effects can come into play, but the fundamental relationship holds in subsonic conditions.