What happens to propeller efficiency at maximum speed?

Propeller efficiency is generally observed to increase with maximum speed due to a combination of factors related to the design and operational characteristics of the propeller. As the aircraft or vessel approaches maximum speed, the thrust produced by the propeller becomes increasingly effective because the airflow over the blades is optimized for generating lift. At higher speeds, propellers are able to convert more of the engine's power into useful thrust rather than being lost to drag or turbulence.

At maximum speed, the angle of attack of the blades also plays a significant role. When the design speed is aligned with the propeller's optimum operating conditions, airflow remains smooth and attached to the blades, minimizing drag and maximizing lift. This efficient performance at high speeds is critical in applications where speed is essential, such as in racing or military operations.

The other options do not hold true as strongly at maximum speeds. While it is true that propeller efficiency does have limits and may decrease if pushed beyond optimal speed, at maximum design speed, it generally performs better compared to lower speeds. Additionally, though factors such as variations in altitude can affect efficiency due to changes in air density, these factors are not the primary considerations when looking at maximum speed scenarios.