The Effects of Supersonic Inlet Topology on the Installed Performance of Tubofan Engines

Abstract

This study explores the system level design impacts of differing supersonic inlet topology. Historically, for Mach 2 flight, variable ramp angle external compression inlets have been preferred over normal shock inlets on the basis of pressure recovery. However, from a system design perspective, is inlet pressure recovery everything? We compare a variety of low-bypass ratio turbofan engines, modelled with NASA's Numerical Propulsion System Integration (NPSS), that can propel a F-16-inspired tactical fighter aircraft. We compute installed performance by applying installation limits (buzz and distortion) and losses (bleed, bypass, spillage, and total pressure recovery) as suggested by the USAF Performance of Installed Propuslion Systems (PIPSI) protocol. Our design study matrix comprises three different engines (Bypass Ratio of 0.5, 1.0, and 1.5) matched to one of three inlet topologies (normal shock, fixed ramp external compression and variable ramp external compression) scaled to any of five possible capture areas. We found that the variable ramp external compression inlet was unforgiving of higher bypass ratio engines. Compared to the robust nature of the normal shock inlet leads the authors to believe that the normal shock inlet, not the complex variable geometry ramp inlet, may be the superior choice for many supersonic propulsion design applications. NomenclatureA ∞ = Actual Inlet Capture Area (Freestream conditions) representing the flow through the inlet, ft 2 A c = Reference Area of Flow that passes through the Inlet, the "highlight area," ft 2 AB = Afterburner C D = Total Drag Coefficient (normalized on Reference Capture Area, Ac) BPR = Bypass Ratio CD spill = Spill Drag Coefficient (normalized on Reference Capture Area, Ac) CD bleed = Bleed Drag Coefficient (normalized on Reference Capture Area, Ac) CD bypass = Bypass Drag Coefficient (normalized on Reference Capture Area, Ac) D = Total Drag Force, lbf FPR = Fan Pressure Ratio M = Mach Number MFR = Mass Flow Ratio (ratio of bypass duct mass flow to core mass flow) OPR = Overall Pressure Ratio PLA = Power Lever Angle p t = Total Pressure, lbf/ft 2 P s = Specfic Excess Power ROC = Rate of Climb, ft/min SR = Specific Range, nm/lbf T = Thrust, lbf TIT = Turbine Inlet Temperature, °R tsfc = Thrust-Specific-Fuel-Consumption, lbm/(lbf-hr) V ktas = Knots True Airspeed, nm/hr