Why turboprops instead of turbojets — primary advantage A turboprop aircraft engine is generally preferred over a pure turbojet for subsonic flight primarily because it offers:

Introduction / Context:
Turboprops couple a gas-turbine core to a propeller via a power turbine and reduction gearbox. They excel in regional and utility aviation, where cruise speeds are typically below about 700 km/h and runway performance matters.

Given Data / Assumptions:

• Comparison at subsonic flight regimes.
• Similar core technologies and materials.
• Focus on propulsive (not thermal) efficiency.

Concept / Approach:
Propulsive efficiency improves when the exhaust jet velocity approaches the flight speed. Propellers accelerate a large mass of air by a small amount, yielding very high propulsive efficiency at low to moderate speeds, which is where turboprops operate best. Turbojets accelerate a smaller mass flow to much higher jet velocities, which is less efficient at these speeds.

Step-by-Step Solution:
Identify operating regime: subsonic and relatively low Mach.Recognize propellers provide large mass flow, small delta-V → high propulsive efficiency.Conclude the primary advantage is higher propulsive efficiency at these speeds.

Verification / Alternative check:
Performance charts show turboprops achieving lower specific fuel consumption than turbojets at typical regional cruise speeds (~0.3–0.6 Mach).

Why Other Options Are Wrong:
(b) Turboprops are not for supersonic flight; (c) core OPR is not the defining advantage; (d) frontal area claim is not generally true; (e) turboprops do require reduction gearboxes.

Common Pitfalls:
Confusing thermal efficiency of the core with propulsive efficiency of the propulsor.

Final Answer:
Higher propulsive efficiency at low to moderate subsonic speeds