Flow Distortion in a Small Unmanned Aircraft Boundary Layer Ingestion Inlet

Abstract This paper presents a flow distortion study for a small, unmanned aircraft engine inlet that ingests fuselage boundary layer flow at low subsonic speeds. Recent efforts to reduce commercial transport aircraft drag at high subsonic speeds include embedding an aft-mounted engine to reduce momentum deficit in the fuselage boundary layer and wake, consequently reducing fuel consumption. Embedded engines at low subsonic speeds may also provide substantial performance improvements by generating thrust with low-momentum boundary layer flow and filling the aircraft wake with engine exhaust; however, the propulsion system must operate effectively in a distorted flow field. To characterize expected distortion, a wind tunnel test article was designed and fabricated with a notional boundary layer ingestion inlet having a 1.25:1 area contraction ratio, 10-inch exit diameter and duct geometry based on the National Advisory Committee for Aeronautics (NACA) 0012 airfoil. The exit diameter is characteristic of typical electric ducted fans used with unmanned aircraft applications. Experiments were conducted in a low-subsonic wind tunnel with a traverse-mounted pitot-static probe to measure dynamic pressure downstream from the inlet entrance at local Reynolds numbers from 1 to 3 million. Results show up to 1.67% circumferential distortion intensity, and velocity contour plots reveal over 15% velocity deficit for a significant portion of the inlet lower surface that could have crucial implications for fan operability. Observations from this study provide new knowledge for designers of emerging unmanned aircraft with inlets embedded in the aft section of a fuselage, motivating methods to reduce operability concerns associated with the propulsor.