How does high density altitude affect propeller efficiency compared to low density altitude?

At high density altitudes, the air is less dense compared to low density altitudes. This reduction in air density affects the efficiency of the propeller in a significant way. The propeller depends on the surrounding air to generate thrust; the less dense air at high altitudes means that the propeller blade encounters fewer air molecules.

As a result, the propeller exerts less force because it is effectively "pushing" against a medium that offers less resistance and lift. This leads to a decrease in the overall efficiency of the propeller, as it cannot convert engine power into useful thrust as effectively as it can at lower density altitudes, where the air is thicker and more molecules are available to generate lift. Therefore, the correct understanding is that efficiency is reduced because of this decrease in thrust generation capability at high density altitudes.