This paper presents a description of the physical principles of aerodynamic power savings from boundary layer ingestion (BLI) propulsion and a quantitative evaluation of the BLI benefit for advanced civil aircraft. Control volume and one-dimensional analyses are used to illustrate two major features of BLI: 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 BLI 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 BLI and non-BLI 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-BLI configuration with the same propulsors, in agreement with CFD calculations and wind tunnel

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Wind tunnel experiments were performed to quantify the aerodynamic benefit of boundary layer ingestion (BLI) for the D8 transport aircraft concept. Two powered 1:11 scale, 13.4 ft span models, in BLI and non-BLI versions, were tested at the NASA Langley 14×22 Foot Subsonic Wind Tunnel to directly compare their performance. The models share the same basic airframe and propulsor units for the most direct comparison. They are also fully tripped to make the measured BLI benefit results scalable to the full-size aircraft. The comparison metric is the propulsor power required to produce a given net stream-wise force on the entire aircraft. The results show that the model BLI propulsors require 6% less electrical power at the simulated cruise point. These experiments provide the first back-to-back assessment quantifying the aerodynamic benefits of BLI for a civil aircraft. The BLI benefit quoted is preliminary in nature because it is defined in terms of electrical power, but we are in the process of obtaining a value in terms of flow power and there is indication that the BLI saving will remain essentially the same.

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The flow field around the rotor in ground effect has been studied using flow visualization and hot-wire anemometry. Two distinctive flow phenomena, recirculation and ground vortex, are observed in the flow visualization. Hot-wire measurement results indicates high intensity of unsteadiness of the flow field with long time scale. The evolution of the ground vortex is better understood from the experimental results. Three different types of velocity fluctuations are observed around the ground vortex.