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High speed test results of subsonic, turbofan scarf inlets
A major constraint imposed on modern civil aircraft is noise reduction. Aircraft and the engines designed for them, must meet current and future proposed FAR regulations and the noise "footprints" criteria. One method of reducing the noise footprint is by introducing negative scarf on the inlet highlight plane and the extended lower lip acts as an acoustic "barrier". It is needed to understand the design implications of inlet scarfing at low and transonic speeds. In the AIAA-2000-0354, the authors described a staged and iterative design process leading to the use of negative scarf. The focus was on low speed and subsonic performance with and without incidence and side-slip in the operating range. This led to lip geometry for the negative scarfed intake that essentially had the same attached flow capability as the datum "conventional" inlet with positive scarf. An additional advantage with negative scarf was the weakening of the ground vortex at "zero forward speed". This is important as intakes get bigger for higher by-pass ratios and need to be set closer to ground. This paper addresses the next logical aspect i.e. the consideration of transonic performance. Starting with a datum inlet (+6° scarf), this paper describes the effect of lip shaping on scarfed (-20°) intakes at high speed (Mach 0.8 & 0.85). An encouraging equivalence has been achieved between the datum and scarfed intake and further detailed work (internal surfaces) eventually leading to experiments has been recommended. Avenues for related future work are noted e.g. on scarf angle optimisation. NOMENCLATURE
Results of a low speed wind tunnel test program are presented which demonstrate the aerodynamic and acoustic performance of a scoop inlet. Engine noise that would normally propagate toward the ground is directed upward by the extended lower lip of the scoop inlet. In addition, more of the scoop airflow comes in from above the inlet than below, leading to relatively higher surface velocities on the upper lip and lower surface velocities on the lower lip. These lower velocities on the lower lip result in a higher attainable angle of attack before internal flow separation occurs.
A scarf inlet is characterized by having a longer lower lip than upper lip leading to both aerodynamic and acoustic advantages. Aerodynamically, a scarf inlet has higher angle of attack capability and is less likely to ingest foreign objects while the aircraft is on the ground. Acoustically, a scarf inlet provides for reduced inlet radiated noise levels below the engine as a result of upward reflection and refraction of inlet radiated noise. Results of a wind tunnel test program are presented which illustrate the aerodynamic performance of two different scarf inlet designs. Based on these results, scarf inlet performance is summarized in a way to illustrate the advantages and limitations of a scarf inlet compared to an axisymmetric inlet.
Aerodynamic andDirectionalAcousticPerformanceof aScoopInlet
- J M Abbott
- D A Dietrich