Why do propellers have an 'avoid' range of rpm?

Propellers have an 'avoid' range of rpm primarily to mitigate the risk of resonance peaks that can lead to fatigue damage in the propeller. When a propeller operates within certain rpm ranges, specifically those close to its natural frequency, it can experience vibrations that amplify and may cause structural stress. Over time, these resonant vibrations can result in material fatigue, ultimately compromising the integrity and safety of the propeller. By avoiding these critical rpm ranges, operators can enhance the longevity and reliability of the propeller.

The other provided options relate to different aspects of engine and propeller performance, but they do not address the specific hazards associated with resonance in the same manner. Excessive noise, inefficiencies in engine operation, and detonation concerns are relevant to overall system performance but do not directly explain the unique considerations associated with the resonance phenomena affecting propeller durability.