To generate the same amount of lift as altitude is increased, an airplane must be flown at a higher true airspeed for any given angle of attack.

To generate the same amount of lift as altitude is increased, an airplane must be flown at a higher true airspeed for any given angle of attack.

• A. To generate the same lift at higher altitude, reduce angle of attack.
• B. To generate the same amount of lift as altitude is increased, an airplane must be flown at a higher true airspeed for any given angle of attack.
• C. Lift is independent of airspeed.
• D. To generate the same lift at higher altitude, fly at a lower true airspeed.

Answer: (B)

Lift is proportional to dynamic pressure, which is ½ times air density times true airspeed squared, multiplied by wing area and the lift coefficient. At a given angle of attack, the lift coefficient remains fixed. As altitude increases, air density drops, so to keep the same lift you must raise the dynamic pressure by increasing true airspeed. Mathematically, if lift stays constant and density decreases, the required V must increase (roughly with the square root of 1/ρ). That’s why, for the same angle of attack, you fly faster at higher altitude to generate the same lift. The other ideas don’t fit this condition: reducing the angle of attack would change the lift coefficient, lowering lift; flying at a lower true airspeed reduces dynamic pressure and lift; and claiming lift is independent of airspeed contradicts the fundamental lift equation.

Lift is proportional to dynamic pressure, which is ½ times air density times true airspeed squared, multiplied by wing area and the lift coefficient. At a given angle of attack, the lift coefficient remains fixed. As altitude increases, air density drops, so to keep the same lift you must raise the dynamic pressure by increasing true airspeed. Mathematically, if lift stays constant and density decreases, the required V must increase (roughly with the square root of 1/ρ). That’s why, for the same angle of attack, you fly faster at higher altitude to generate the same lift.

The other ideas don’t fit this condition: reducing the angle of attack would change the lift coefficient, lowering lift; flying at a lower true airspeed reduces dynamic pressure and lift; and claiming lift is independent of airspeed contradicts the fundamental lift equation.