What is the best way to reduce propeller and engine noise after takeoff?

Reducing RPM when reaching a safe altitude is the best method for minimizing propeller and engine noise after takeoff. When the engine operates at lower RPMs, it produces less noise, as the vibrations and sound generated by the propeller are directly related to its rotational speed. After takeoff, once a safe altitude is achieved, pilots can often reduce power settings while still maintaining an adequate rate of climb. This action not only helps in reducing noise but also contributes to fuel efficiency.

Other options, while they may have their merits, do not directly address the task of minimizing noise. Increasing altitude and speed can enhance performance and may gather some noise reduction due to higher distance from ground observers, but it doesn’t materially decrease the noise produced by the engine and propeller directly. Extending flaps during climb is not typically beneficial and could lead to increased noise and drag. Changing to a more efficient cruise setting refers generally to the optimization of flight performance rather than specifically targeting noise reduction.