Which factor primarily affects lift at a given airspeed and angle of attack as altitude changes?

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

Answer: (D)

Lift at a given airspeed and angle of attack is driven by air density, because lift is proportional to ρ times V^2 times S times Cl. As altitude increases, the air becomes thinner, so density (ρ) drops and lift decreases even though speed and angle of attack are unchanged. Temperature affects density too, since hotter air is less dense, but the primary change with altitude is the reduction in density. To maintain the same lift at higher altitude, you’d need to increase airspeed (to raise dynamic pressure) or increase the angle of attack to raise the lift coefficient, within the limits before stall. Thrust and drag influence other aspects of flight, but they don’t set lift directly under these fixed conditions.