Energetic assessment of an embedded aircraft propulsion: an analytic approach

This paper investigates the energetic advantage of the embedded propulsion compared to a state-of-the-art propulsion of an aircraft. Hereby, the integral method of boundary layer theory together with the potential theory is applied to analyze the boundary layer thickness and the impact of the flow acceleration due to the embedded propulsion. The aircraft body is treated as a flat plate and the engine as a momentum disk. For an embedded propulsion, there is a trade-off of the engine size, since the propulsion efficiency is affected by the boundary layer. On the one hand, the propulsion inlet momentum is noticeably reduced for a small engine size and the viscous friction is reduced due to boundary layer ingestion. On the other hand, a slow jet speed, i.e., a large engine size, increases the propulsion efficiency as known. The outcome of the energetic assessment is the following: the propulsion efficiency is increased and the drag of the aircraft body is reduced by the embedded propulsion compared to a conventional propulsion. The optimized aircraft engine size depending on Reynolds number is given as well as the dimensionless cost function.