Why is a propeller used?

• A. extremely efficient at low altitude and airspeed
• B. best at high altitude and speed
• C. produces no drag
• D. requires no maintenance

Answer: (A)

Propellers convert engine power into thrust by pushing air backward, and their efficiency is highest when the airplane is moving slowly in dense air. At low altitude the air is thick, so the blades can move a larger mass of air for each rotation, producing more thrust per horsepower. When the aircraft is at low airspeed, the propeller operates in a range where the blade’s angle of attack and loading give good thrust without hitting aerodynamic limits, keeping efficiency high. As you climb to higher altitudes, air becomes thinner and the propeller loses some of that efficiency; at higher speeds the blades approach their design limits and compressibility and drag increase, reducing thrust per unit power. So a propeller is best suited for efficient propulsion at low altitude and low airspeed. The statements that it creates no drag, requires no maintenance, or is best at high altitude and speed aren’t true, since propellers generate drag, require upkeep, and lose efficiency as speed and altitude rise.