Which regulation must the reciprocating engine for a helicopter be approved under?

The correct regulation for the approval of reciprocating engines used in helicopters is 14 CFR Part 33. This part specifically covers the airworthiness standards for aircraft engines and applies to the design, testing, and performance criteria required to ensure that engines are safe and reliable for their intended use. This includes reciprocal engines that power various types of aircraft, including helicopters.

In both aviation and regulatory contexts, ensuring that the engine meets specific standards set forth in Part 33 guarantees that it performs effectively under the operational conditions encountered in flight. This regulation is crucial for ensuring that all engines, including those in rotorcraft, are subject to rigorous evaluation to maintain safety and reliability.

Other parts listed, such as Part 27, Part 29, and Part 23, regulate different categories or types of aircraft. Part 27 pertains to the airworthiness standards for small rotorcraft, while Part 29 is concerned with large rotorcraft. Part 23 covers smaller airplanes. Each of these parts focuses on the specific operational characteristics and requirements distinct to their respective types of aircraft. Hence, while other parts address aircraft categories, Part 33 specifically pertains to engine standards, making it the correct choice for the approval of reciprocating engines in helicopters.