Propellers
In reciprocating and propeller engines, disk loading can be defined as the ratio between propeller-induced velocity and freestream velocity. Lower disk loading will increase efficiency, so it is generally desirable to have larger propellers from an efficiency standpoint. Maximum efficiency is reduced as disk loading is increased due to the rotating slipstream; using contra-rotating propellers can alleviate this problem allowing high maximum efficiency even at relatively high disc loadings.
The Airbus A400M fixed-wing aircraft will have a very high disk loading on its propellers, and this had to be factored into the design.
Read more about this topic: Disk Loading