Research On Characterization And Suppression Of Helicopter Infrared Radiation

Helicopters are platforms of battlefield force transferring and anti-tank missions. They also playimportant roles in air to ground fire covering and short distance air to air fights. Due to their highmaneuverability, helicopters are of increasing importance in local conflicts and counter terrorismmilitary actions in recent decades. Owing to their low-flying height and hovering-attitude, thehelicopters are subjected to serious threats from radio, infrared, visual, and aural detection andtracking. Among these threats, infrared detection and tracking is regarded as more crucial for thesurvivability of helicopter. Firstly, passive detection and tracking by infrared signature seekingmissiles is tactically superior to the active, for comparable detection range. The infrared seekers haveexploited techniques to passively acquire and intercept airborne targets, by detecting their infraredemitting energy. The rapid advances in processor and detector array technology have led to enhancedsensitivity, low noise, multi-spectral, and smart detection capabilities. On the other hand, the ratio ofpower to weight for the turbo-shaft engines mainly been equipped in the helicopters increasestremendously, the maximum temperature under the thermodynamic cycle of aero-engine boosts,resulting in the helicopter infrared signature augment intensively. Consequently, infrared signaturesuppression and analysis is an important issue associated with helicopter susceptibility.The present paper focusing on three aspects:It is known that the temperature distributions on the helicopter fuselage skin and in the exhaustplume have a direct impact on the modeling of helicopter infrared signature. Because the temperaturedistribution on fuselage skin is governed by heat transfer between skin and inner hot elements as wellas skin and outer surrounding, there are many factors affecting the temperature distribution, such asthe rotor downwash, heat radiation from engine casting, convective heat transfer between skin andcold air, solar irradiance on the skin, etc. On the other hand, the exhaust plume temperaturedistribution is seriously affected by the rotor downwash flow owing to the mixing action. To preciselysimulate temperature distribution on the helicopter airframe and in the exhaust plume, the effects ofrotor downwash and solar irradiance must be taken into consideration considered in three-dimensionalflow and heat transfer calculation in a coupled mode. In the present paper, rotor propelleraerodynamics is introduced to model rotor downwash velocity distribution. The modeled downwash isthen added into numerical process through user defined function (UDF) as the boundary condition for rotor plane. Solar radiation including direct and diffusive radiative heat flux is also modeled in thenumerical simulation process. Based on the present modeling method, the exhaust flowfield andhelicopter skin temperature are analyzed. The roles of exhaust temperature, skin emissivity, solarradiation heat flux in helicopter infrared radiation characteristics are revealed.Mixer-ejector is the key part of an infrared suppressor. Firstly, to minimize the engine weight andenhance mixing between primary and secondary flow, short lobed nozzle with a large lobed angle is apromising setup in future application. But once the lobed angle is too large, there will be boundarylayer detachment near the lobed wall thus produces flow separation in the lobed mixer-ejector andbrings in more pressure loss. In the present paper, a short lobed mixer-ejector with a large lobed angleis researched. The relationship between lobed angle and flow separation is revealed. Ventilation andscarfying treatments are then introduced on the lobed nozzle to suppress separation bubble, enhancemixing and pressure recovery. The difference of those treatments on lobed wall in both ejected andforced secondary flow boundary conditions are revealed. Secondary, considering that double-stagedmixer-ejector is a promising configuration the ejector development, a double-staged mixer-ejector isstudied in this thesis. The aero-dynamical difference between confined secondary flow and opensecondary flow inlet modes on double-staged mixer-ejector is illustrated.As the infrared detection and guidance technology development, infrared stealth and anti-stealthin both3~5m and8~14m wavebands are confronting each other for a helicopter in real battlefield.Detectors with both3~5m and8~14m wavebands detection and guidance capabilities have madehelicopter’s stealth requirements even higher. In the present paper, a numerical and experimental studyon the infrared suppressor integrating the exhaust system with the tail part of a helicopter has beenperformed to investigate the effects of ambient air pumping-mixing, radiative heat sheltering and rotordownwash on reducing the exhaust system temperature and diminishing the target infrared signature.And for the common engine exhaust system, some methods for suppressing infrared radiation areinvestigated, including nacelle ventilation, radiative shelter, heat resistance layer and mxing ductcovering. A preliminary optimum configuration for suppressing helicopter infrared signature ispresented.