Subsonic scarf-inlet designs with inlet length transition angles of 180° and 67.5°.
A computational investigation is underway at the NASA Glenn Research Center to determine the aerodynamic performance of subsonic scarf inlets. These inlets are characterized as being longer over the lower portion of the inlet, as shown in the preceding figure. One of the key variables being investigated in the research is the circumferential extent of the longer portion of the inlet. The figure shows two specific geometries that are being examined: one in which the length of the inlet transitions from long-to-short over the full 180° from bottom to top, and a second in which the length transitions over 67.5°.
Subsonic scarf inlets are of interest for several reasons. First, extending the lower portion of the inlet has a positive effect on the directional characteristics of the inlet-radiated engine noise. Because of both the geometric barrier of the extension and the resulting internal flow-velocity gradients, the forward-radiating noise is directed upward and away from the ground, thereby reducing the inlet-radiated flyover noise. In addition, because a scarf inlet draws in more of its air from above than below, the aircraft engine is less likely to ingest any debris that may be present on the runway during takeoff and landing. Finally, the tendency to draw in more airflow from above than below leads to a higher angle-of-attack capability. This characteristic offers the potential for achieving a given angle-of-attack requirement by extending the lower portion of the inlet to form a scarf inlet, rather than by using the traditional approach of increasing inlet thickness.
There are challenges to scarf-inlet design and operation as well. These are most notably obtaining acceptable performance at static conditions, during engine-out climb, and at cruise. These challenges are related to the skewed nature of the incoming airflow and to the “corner” that forms when the length transition angle is less than 180°. This is apparent in the figure for the 67.5° scarf inlet.
Thus far, Glenn researchers have used the three-dimensional WIND computational code described in reference 1 to examine the aerodynamic performance at static, takeoff, and cruise conditions for a whole range of scarf-inlet designs. Scarf-inlet geometric variables that have been investigated include the length of the extension of the lower portion of the inlet, the circumferential extent over which the extension transitions back to the shorter length, and the inlet thickness. Results of the research are given in references 2 and 3 in the form of scarf-inlet performance maps. Additional work is planned for assessing the limitations of engine-out climb performance and expanding the range of scarf-inlet geometric variables. Computing the acoustic performance of scarf inlets and assessing the tradeoffs between acoustic and aerodynamic performance are long-term goals.
Glenn contact: John.M. Abbott, 216–433–3607, John.M.Abbott@nasa.gov
Author: John M. Abbott
Headquarters program office: Aeronautics Research
Last updated: July 19, 2005 9:44 AM
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