Originally answered on Quora on June 19 2018

As other answers have pointed out the propeller is a rotating wing. Like a wing, the lift it produces is proportional to the square of the speed it is traveling through the air, so if you compare a section of the propeller near the hub with a section twice as far away from the hub, the section further out will be traveling twice as fast through the air, and produce four times as much lift as the inner section.

The upshot of this summed over all the sections of the propeller blade is that the inner 25% or so of the propeller radius is contributing almost nothing to the thrust (i.e. forward pointing lift) of the propeller. The outer 75% radius is 15/16 ~ 94% of the area of the disc swept by the propeller and is largely unobstructed by the engine cowling, although the standard cowling in the picture above still contributes a fair bit of drag. A popular modification to many light aircraft is a Lopresti cowl, which apart from adding a few knots of airspeed is a beautifully sculpted thing to look at: