If density altitude increases, what changes can be expected in engine power, lift, and prop efficiency?

• A. Reduced engine power, reduced lift, and lower prop efficiency.
• B. Increased engine power, increased lift, and higher prop efficiency.
• C. No changes.
• D. Only lift is affected.

Answer: (C)

Density altitude rising means the air is thinner. That thinner air reduces the amount of air and oxygen available for the engine, so the power you can develop drops, especially for normally aspirated engines. Lift depends on air density, so with less dense air, the wing generates less lift at the same speed and angle of attack. Propeller efficiency also falls because a propeller converts shaft power into thrust by accelerating a mass of air; when the air is less dense, there’s less air mass to accelerate and the thrust produced for a given blade angle and RPM decreases, lowering overall efficiency. So, increasing density altitude leads to reduced engine power, reduced lift, and lower propeller efficiency. The option describing reductions in all three is the best choice.