A Universal Method for Comparing Open Rotors and Ducted Fans in Hover

For vertical take-off and landing capable aircraft, designers must choose between open rotors and ducted fans. Vertical take-off and hover is energy-intensive, so propulsor performance must be maximised in this mode of flight. Despite this, the best method of assessing hover performance in previous literature remains unclear, particularly for ducted fans and their comparison with open rotors. Ducted fan designers typically re-use hover performance metrics derived for open rotors, but this frequently causes confusion and can result in misleading conclusions, caused by a flawed choice of reference area in the normalisation of the size of the machine. This paper presents a novel performance metric which allows direct, fair comparison between open rotors and ducted fans and correctly takes into account their size impact on the aircraft. The cause of confusion with existing performance metrics is explained, and a case study from previous literature is used to demonstrate how a different conclusion is drawn using the new metric. The new metric is used to produce a method to compare different propulsors with weight effects included. Finally, an analysis framework is presented that allows engineers to investigate real flows and break down the causes of excess power consumption to make better-informed design decisions.