What method is used by the APU to develop power?

• A. Combustion within the fuel cells
• B. Compression of ambient air by a centrifugal compressor
• C. Electromagnetic induction
• D. Piston-driven mechanical expansion

Answer: (B)

The correct answer involves the principle of compressing ambient air by a centrifugal compressor for power development in an Auxiliary Power Unit (APU). An APU is designed to provide energy for starting the main engines, generating electrical power, and supplying air for aircraft systems when the main engines are not running.

In this context, during operation, the APU draws in outside air and utilizes a centrifugal compressor to increase the pressure of that air. The compressed air is then used for various functions, including starting the jet engines and supplying bleed air for cabin heating and cooling. This method is efficient and allows the APU to operate independently of the aircraft's engines, making it essential during ground operations.

Other methods, such as combustion within fuel cells or electromagnetic induction, are not applicable to the typical operation of an APU. While some alternatives like piston-driven mechanical expansion could theoretically provide power, they are not part of how APUs are generally designed to function in aviation.