Which statements are true regarding aircraft engine propulsion?

• A. Turbojet and turbofan impart a relatively large amount of acceleration to a smaller mass of air
• B. In modern turboprop engines, nearly 50 percent of the exhaust gas energy is extracted by turbines to drive the propeller and compressor with the rest providing exhaust thrust
• C. An engine driven propeller imparts a relatively small amount of acceleration to a large mass of air
• D. Modern engines produce equal thrust from both propulsive and exhaust

Answer: (C)

Propulsion works by changing the momentum of the air. A propeller-driven engine moves a large volume of air and increases its velocity only a little, so the air experiences a small acceleration but in a very large mass flow. That combination—large air mass, small velocity change—produces the thrust from the propeller.

In contrast, turbojets and turbofans mainly produce thrust by accelerating a smaller mass of air to a very high speed in the exhaust, not by imparting a large acceleration to a large air mass. The energy split inside a turboprop (how much power goes to driving the propeller versus what ends up in the exhaust) isn’t a fixed 50/50 and varies with design and operating conditions. Modern engines do not produce equal thrust from propulsive and exhaust components; one usually dominates depending on the engine type and operating point.

So the statement that best describes this propulsion behavior is that an engine driven propeller imparts a relatively small acceleration to a large mass of air.