Which statement is true?

• A. Unducted Fan engines are expected to be twice as large as current turboprops.
• B. They will exceed Mach 0.9.
• C. They are expected to be two to three times the size of current turboprop engines.
• D. They use only aluminum blades.

Answer: (C)

Unducted fan engines rely on a very large-diameter open rotor to achieve high efficiency, trading a big rotor for good fuel economy rather than relying on a ducted flow. Because the rotor is unsheltered, the overall installation ends up much larger than a conventional turboprop, typically about two to three times the size. This larger diameter is what delivers the propulsion performance with high efficiency at cruise speeds.

The statement about exceeding Mach 0.9 isn’t aligned with how these engines are typically used; open-rotor concepts aim for efficiency at subsonic cruise, not breaking speed records, and their design constraints keep them below that speed range. Regarding materials, modern open-rotor blades aren’t made only of aluminum; they use advanced composites and other materials to handle high loads and reduce weight, so stating aluminum-only blades isn’t accurate.