Prediction and Analysis of Inlet Pressure and Temperature Distortion on Engine Operability From a Recent T-38 Flight Test Program

Since the initial development of the T-38 Talon trainer, there have been upgrades to both the aircraft and to the J85-GE-5 afterburning turbojet engine to improve takeoff performance, reduce maintenance time and cost, and to decrease fuel consumption. The latest upgrades, referred to as the Propulsion Modernization Program (PMP), focused on improved performance of the T-38’s inlets, twin J85-GE-5 afterburning turbojet engines, and improved exhaust nozzle design. The T-38’s inlet includes bleed holes upstream of the engine face to provide cooling air flow from the inlet to the engine bay. However, at various locations in the flight envelope, the bay air is pressurized relative to the inlet resulting in reverse flow of hot engine bay air into the inlet. This reverse flow along with inlet heat transfer effects can cause total temperature distortion and reduce engine stability margin. During any flight maneuvers, there will be an associated level of total pressure distortion. When pressure distortion is combined with the temperature distortion due to engine bay flow reversal and inlet heat transfer, losses in stability pressure ratio (or stability margin) may further be increased. This analysis effort reported herein uses a modeling and simulation technique known as the parallel compressor theory (model) to investigate the effects of total pressure and temperature inlet distortion on system operability and makes predictions of the loss in stability pressure ratio associated with those combined pressure and temperature distortions during flight maneuvers.