WAKE INGESTION PROPULSION BENEFIT

It is well-known that the efficiency of propulsion is improved if part or all of the propulsive fluid comes from the wake of the craft being propelled. In this article this propulsion benefit is quantified in terms of wake parameters and propulsor properties. The formulations apply directly to unducted fans or propellers, but the conclusions are also relevant to ducted propulsors. It is found that the power saving is greatest when the propulsor disk loading is high, when the wake form factor is high (flow near separation), and when the propulsor design is such that the wake profile tends to be flattened as it passes through the propulsor (high wake recovery). Examples are given showing that the benefit can be in the 20% range in some cases. Propeller design parameters that lead to high wake recovery are also given.