Boundary layer ingestion propulsion benefit for transport aircraft

This paper presents a description of the physical principles of aerodynamic power savings from boundary layer ingestion propulsion and a quantitative evaluation of the boundary layer ingestion benefit for advanced civil aircraft. Control volume and one-dimensional analyses are used to illustrate two major features of boundary layer ingestion: reduction of jet mixing losses due to decreased jet kinetic energy from reduced velocity of flow entering the propulsor and, to a lesser extent, reduction of airframe wake mixing losses. Embedded boundary layer ingestion propulsion systems can also enable nacelles with reduced surface area and associated weight and drag, further decreasing the aircraft propulsive power requirement. The required propulsor flow power is shown to decrease with increases in both the amount of boundary layer ingested and the propulsor mass flow, and there is thus no unique way to compare boundary layer ingestion and non-boundary-layer-ingestion systems. Using the ideas presented, however, the benefit can be assessed for any given comparison. The analysis is applied to an advanced civil transport aircraft concept with 40% of the fuselage boundary layer ingested, yielding a reduction in required propulsor mechanical power of 9% relative to a non-boundary-layer-ingestion configuration with the same propulsors, in agreement with computational fluid dynamics calculations and wind tunnel experiments.