Which item is listed as a major disadvantage of axial-flow compressor blades?

• A. Good efficiencies over only narrow rotational speed range
• B. Difficulty of manufacture and high cost
• C. Relatively high weight
• D. High starting power requirements

Answer: (A)

Axial-flow compressor blades are optimized for peak efficiency at a specific rotor speed and flow condition. When the engine speeds up or slow downs, the flow through the stages changes, so the blade angles and the incidence of the flow aren’t at their design values anymore. That causes the aerodynamic performance to fall off, so you get high efficiency only in a relatively narrow band of rotational speeds. In other words, the major drawback of axial-flow blades is their limited efficient speed range, which makes the compressor less forgiving to throttle changes and operating off-design.

Other factors like fabrication cost, weight, or starting power are considerations in design and operation, but they don’t define the principal limitation of axial-flow blades the way the narrow efficient speed range does.