Which factor primarily affects lift at a given speed as altitude changes?

• A. Thrust
• B. Drag
• C. Air density
• D. Temperature

Answer: (C)

Lift at a given speed is determined largely by air density. The lift produced by a wing or rotor depends on how many air molecules are being pushed downward per unit time, and at a fixed velocity, that amount scales with rho, the air density. As altitude increases, air becomes thinner, so rho decreases and lift drops. Temperature can influence density, but the direct reason lift changes with altitude is the change in density itself. Thrust and drag don’t set lift directly at a constant speed; they affect speed and flight dynamics, but the primary factor that varies with altitude to alter lift is air density. To maintain lift at higher altitude, you’d need to increase airspeed or angle of attack (or otherwise boost the effective lift coefficient or wing/rotor area).