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Radar/infrared integrated stealth optimization design of helicopter engine intake and exhaust system

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Abstract

Along with the diversification and sophisticated development of detection methods in modern warfare, helicopters are increasingly subject to unilateral or simultaneous threats from radar and infrared detectors. In order to improve the survivability and operational effectiveness of the helicopter, a comprehensive stealth approach based on Pareto solution is presented. Considering the geometric constraints and aerodynamic characteristics of the engine intake and exhaust system, the model of the system is established by the full factorial design, the internal, central and external flow fields are constructed, then the high-precision computational fluid dynamics method is used to simulate the total flow field under the rotor downwash airflow in hovering state. The radar cross section of the system is evaluated by the physical optics and physical theory of diffraction. Based on the Monte Carlo and ray tracking method, the infrared signature of the system is calculated and analyzed in detail. Under the comprehensive evaluation and selection of comprehensive stealth approach, the optimization model of the system is continuously established and updated. The ultimate design has achieved good results in both radar cross section reduction and infrared radiation suppression and the proposed method is effective and efficient for radar/infrared integrated stealth of helicopter engine intake and exhaust systems.

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A numerical investigation is performed in the current study to illustrate the influences of exhaust angle of the integrated infrared suppressor on plume flow and helicopter infrared radiation, under the hover and cruise statuses. Three exhaust models are concerned, including the exhaust direction at 60°, 45°, and 18° angles to the horizontal plane. The results show that the heating effect of the exhaust plume on the fuselage is modified with the decrease of the exhaust angle. However, the decrease in exhaust angle will reduce the pumping coefficient of the mixing duct to some extent, resulting in an increase of exhaust temperature. For the model with an exhaust angle of 18°, three sets of comparative tests were designed. The results show that when there is no airflow outlet at the bottom of the fuselage, the surface temperature of the fuselage will increase significantly, and the position of the airflow outlet at the bottom of the fuselage also has a certain influence on the temperature field of the fuselage. On the horizontal detection plane, the infrared radiation intensity of the model with an exhaust angle of 60° and 45° is significantly lower than that with an exhaust angle of 18°. In the vertical detection plane, especially in the direction below the fuselage, the infrared radiation intensity of the model with an exhaust angle of 18° is the smallest. When compared to the hover status, the cruise status reduces the infrared radiation intensity somewhat.
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Serpentine nozzles are widely used in stealth bombers and unmanned aerial vehicles to significantly suppress the infrared radiation signatures and to acquire excellent stealth performance with the goal to improve the survivability on the battlefield. The flow characteristics of serpentine nozzles are complex and noticeably different from those of axisymmetric nozzles due to the geometric configuration. This paper aims to obtain the flow characteristics of both serpentine nozzles and axisymmetric nozzles and to compare them under various working conditions. We obtained the flow characteristics of serpentine nozzles and axisymmetric nozzles with the employment of a schlieren system, PSI electronic pressure scanning valves, a six-component force balance system and flowmeters. The results show that the static pressure distributions on the upper and down walls of serpentine nozzles are completely different from those of axisymmetric nozzles, which are mainly affected by the flow tube near the walls. The flow velocity increases and the static pressure drops when the flow tube contracts. The outcome is opposite once the flow tube expands. The nozzle configuration and the bypass flow pressure ratio have little influence on the schlieren photographs of the flow fields downstream of the nozzle exit, which are composed of expansion waves and shock waves alternately. The values of the flow coefficient and the thrust coefficient both decrease when the difference between the core flow pressure ratio and the bypass flow pressure ratio gets large. The configuration of the serpentine nozzle, especially the wall curvature, affects the values of its flow coefficient and thrust coefficient. The value of the flow coefficient for serpentine nozzle 2 is about 1.5% lower than that for serpentine nozzle 1, and the value of the thrust coefficient for serpentine nozzle 2 is about 0.5% lower than that for serpentine nozzle 1.
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With the continuous development and enhancement of anti-stealth technology, fighters will be increasingly threatened by joint detection of radar and infrared detectors. To improve the overall stealth performance of the aircraft's intake and exhaust systems, a mixed design approach (MDA) is presented to reduce the aircraft's radar cross-section (RCS) and suppress infrared signature. Full factorial design method is used to establish the aircraft model, taking into account the influencing factors of the air intake model, nozzle stages, exhaust port form, lower baffle, center baffle, lateral baffle and nozzle opening width. The physical optics and physical theory of diffraction are used to solve the electromagnetic scattering characteristics of the target, and the Monte Carlo and ray tracing method are used to solve the infrared radiation signature of the system. After comprehensive evaluation and selection of MDA, the RCS mean index and tail RCS index of the optimized aircraft model have been effectively reduced, and the infrared radiation index has also been greatly improved. MDA is effective and efficient in terms of radar/infrared integrated stealth design for the intake and exhaust systems of advanced fighters.
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With the advancement of helicopter stealth technology and the development of compound helicopters, dynamic electromagnetic scattering of rotor or multi-rotors has become a hot topic of research. In order to deal with the radar stealth problem of compound helicopter target non-coaxial multi-rotor, a multi-rotor dynamic scattering (MDS) calculation method based on dynamic simulation principle (DSP) and multi-axis rotation transformation (MRT) is presented. Single rotor, azimuth angle, elevation angle, rotor disc tilt and attitude angle are all calculated and analyzed in detail. The results show that the electromagnetic scattering of the rotor-like targets exhibits significant periodicity, and the disc tilt or attitude angle affects this periodic and full-machine dynamic scattering. The proposed MDS method is effective and efficient for the dynamic electromagnetic scattering of aircraft operating on individual rotor or multi-axis rotor-type targets.
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Experimentally simulating the infrared signature of an aircraft with subscale models can significantly reduce the difficulties and costs. The governing equations of fluid flow, heat transfer, and radiation transfer were rendered in only length dimensionless form so that the relationship between infrared signature and scale factor was revealed. Based on the relationship, a method using subscale models to simulate the infrared signature of aircraft was studied. The infrared integrated radiation intensities of four geometrically similar turbofan engine exhaust systems were calculated to validate the simulation measurement method. The results indicate that the parameters of flow field such as velocity, pressure, temperature and mass fraction, as well as the temperatures of walls, are equivalent in the same dimensionless locations in case that the boundary conditions of the geometrically similar models are the same. The relationship between integrated radiation intensity and scale factor is not constant and varies with the aircraft structure and boundary conditions.
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Journal of Computations & Modelling, vol.9, no.1, 2019, pp.33-53 http://www.scienpress.com/download.asp?ID=940703 For more than half a century, the radar has been indisputably the most important sensor in the battlefield, especially in the air domain. Radars have always been competing with electronic warfare systems, which are trying to hinder detection and tracking with the use of various jamming techniques. However, the apparition of stealth or low observable technology since the late '80s has been the game changer which has really contested the radar dominance. Therefore, other parts of the electromagnetic spectrum have been revisited, in an effort to substitute or complement the radar. In this way, infrared seems to be a viable approach. Even if significant efforts have been exerted in order to minimise the IR signature of fighter aircraft, it is impossible to make a fast flying jet, propelled by hot exhaust gases, completely disappear, in the IR spectrum. InfraRed Search & Track or IRST systems offer significant advantages with respect to traditional radar systems, such as passive operation, resistance to jamming, and long detection ranges (under certain conditions). On the other hand, there is no direct range measurement, as in the radar case. This paper begins with a brief presentation of various military applications of IR, followed by an update on current IRST systems. An approach to the estimation of the detection distance of a jet fighter by an IRST system is then proposed. This approach is based on the modelling of a typical turbofan engine and of a modern IRST system. In the simulation, various weather conditions and different fields of view are taken into account. It is shown that, under favourable conditions, the detection range of a non-afterburning engine, observed from behind, is at the order of 100 km or more, outperforming the typical fighter radar in terms of detection against stealth threats.
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Differential infrared thermography (DIT) is a method of analyzing infrared images to measure the unsteady motion of the laminar–turbulent transition of a boundary layer. It uses the subtraction of two infrared images taken with a short-time delay. DIT is a new technique which already demonstrated its validity in applications related to the unsteady aerodynamics of helicopter rotors in forward flight. The current study investigates a pitch-oscillating airfoil and proposes several optimizations of the original concept. These include the extension of DIT to steady test cases, a temperature compensation for long-term measurements, and a discussion of the proper infrared image separation distance. The current results also provide a deeper insight into the working principles of the technique. The results compare well to reference data acquired by unsteady pressure transducers, but at least for the current setup DIT results in an additional measurement-related lag for relevant pitching frequencies. Graphical abstract Open image in new window
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The large, EU Supported ESPOSA (Efficient Systems and propulsion for Small Aircraft) project has developed new small gas turbines for small aircraft. One of the important tasks was the engine - airframe aero-thermal radiation integration that included task of minimizing the infrared radiation of the small aircraft, too. This paper discusses the factors influencing on the aircraft infrared radiation, its possible simulation and measurements and introduces the results of small aircraft infrared radiation measurements. The temperature of aircraft hot parts heated by engines were determined for validation of methodology developed and applied to engine - aircraft thermal integration.
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We propose a dual-band metamaterial perfect absorber with a metal–insulator–metal structure (MIM) for use in infrared (IR) stealth technology. We designed the MIM structure to have surface plasmon polariton (SPP) and magnetic polariton (MP) resonance peaks at 1.54 μm and 6.2 μm, respectively. One peak suppresses the scattering signals used by laser-guided missiles, and the other matches the atmospheric absorption band, thereby enabling the suppression of long-wavelength IR (LWIR) and mid-wavelength IR (MWIR) signals from objects as they propagate through the air. We analysed the spectral properties of the resonance peaks by comparing the wavelength of the MP peak calculated using the finite-difference time-domain method with that obtained by utilizing an inductor–capacitor circuit model. We evaluated the dependence of the performance of the dual-band metamaterial perfect absorber on the incident angle of light at the surface. The proposed absorber was able to reduce the scattering of 1.54 μm IR laser light by more than 90% and suppress the MWIR and LWIR signatures by more than 92%, as well as maintain MWIR and LWIR signal reduction rates greater than 90% across a wide temperature range from room temperature to 500 °C.
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An analysis scheme and a mission system model were applied to the evaluation of the military utility of efforts to reduce infrared signature in the conceptual design of survivable aircraft. The purpose is twofold: Firstly, to contribute to the development of a methodological framework for assessing the military utility of spectral design, and secondly to assess the threat from advances in LWIR sensors and their use in surface-to-air-missile systems. The modeling was specifically applied to the problem of linking the emissivity of aircraft coatings to mission accomplishment. The overall results indicate that the analysis scheme and mission system model applied are feasible for assessing the military utility of spectral design and for supporting decision-making in the concept phase. The analysis of different strike options suggests that LWIR sensors will enhance the military utility of low emissive paint, at least for missions executed in clear weather conditions. Furthermore, results corroborate and further clarify the importance of including earthshine when modeling.
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Incorporation of an infrared suppressor is generally accompanied by a compromise in engine performance, which indirectly reduces the effectiveness of infrared signature suppression. This investigation illustrates the percentage increase in the infrared signature level in the 1.9–2.9 μm and 3–5 μm bands resulting from an increase in engine backpressure in a jet engine due to a reduction in the exit area of a choked converging nozzle. The effectiveness of optically blocking the hot engine parts by reducing the choked nozzle-exit area is estimated. Thermodynamic offdesign point analysis of the jet engine is done using GasTurb software to evaluate the percentage reduction in thrust and the net change in infrared signature level for the reduced choked converging nozzle-exit area relative to that for the design point.
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Human knowledge of infrared (IR) radiation is about 200 years old. However it was in the late 20th century that we developed a wide range of smart technologies for detection and started to take advantage for our benefit. Today IR detector technology is in its 3rd generation and comes with challenging demands. Based on the propagation of IR radiation through free space it is divided into different transmission windows. The most interesting for thermal imaging are the mid-wave IR (MWIR) and the long-wave IR (LW IR). Infrared detectors for thermal imaging have a number of applications in industry, security, search & rescue, surveillance, medicine, research, meteorology, climatology and astronomy. Currently high-performance IR imaging technology is mainly based on epitaxially grown structures of the small-bandgap bulk alloy mercury-cadmium-telluride (MCT), indium antimonide (InSb) and GaAs based quantum-well infrared photodetectors (QWIPs), depending on the application and wavelength range. However, they operate at low temperatures requiring costly and bulky cryogenic systems. In addition there is always a need for better performance, which generates possibilities for developing new technologies. Some emerging technologies are quantum dot infrared photodetectors (QDIPs), type-II strained layer super-lattice, and QDIPs with type-II band alignment. In this report a brief review of the current and new technologies for high performance IR detectors, will be presented.
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The infrared (IR) seekers have exploited techniques to passively acquire and inter-cept airborne targets, by detecting their IR energy [1]. The basic principle of IR detection is the discrimination of target's IR radiance in the detector's wavelength band with the background IR radiance (atmospheric emission/solar radiation) [2]. In an aircraft, the internal sources include plume and surface emissions; and power-plant is the major and reliable source. The aircraft rear fuselage skin of a typical military aircraft is heated by the flow of hot combustion products in the embedded engine [12]. The solid angle subtended by the rear fuselage skin is an order of magnitude larger than that of the tailpipe [9]. Therefore, its contribution is significant especially in the 8–12 micron band; in which, IR-detection is possible also due to external sources, e.g., earthshine and skyshine reflection [19]. Unlike surfaces of solids, gases emit and absorb radiation only at discrete wavelengths associated with specific rotational and vibrational frequencies. These frequencies depend on the particular type of molecule, temperature, pressure, and molecular concentration of radiation participating species [13]. The atmosphere limits the use of the IR spectrum to specific bands called as at-mospheric windows; and has a crucial role, which includes that of transmission and background radiance [8]. For reducing detection by IR-guided missiles, aircraft and helicopters use IR Signa-ture Suppression (IRSS) techniques. A well-designed IRSS system can drastically reduce IRSL by restricting the visibility of hot parts and by matching the visible radiance with the back-ground [22]. Their effect in reducing target's susceptibility (P H) can be gauged by models that relate the two. Due to significant advancements in the performance of IR-detectors, modern mis-siles are generally constrained by their burnout range rather than their lock-on range. The 'lethal range' is a function of target's lock-on range, target's velocity, missile velocity, missile burnout range, missile's guidance logic and blast kill radius; and it is a superior estimate of P H [23].
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Due to their low-attitude and relatively low-speed fight profiles, helicopters are subjected to serious threats from radio, infrared, visual, and aural detection and tracking. Among these threats, infrared detection and tracking are regarded as more crucial for the survivability of helicopters. In order to meet the requirements of infrared stealth, several different types of infrared suppressor (IRS) for helicopters have been developed. This paper reviews contemporary developments in this discipline, with particular emphasis on infrared signature suppression, advances in mixer-ejectors and prediction for helicopters. In addition, several remaining challenges, such as advanced infrared suppressor, emissivity optimization technique, helicopter infrared characterization, etc., are proposed, as an initial guide and stimulation for future research. In the future, the comprehensive infrared suppression in the 3-5μm and 8-14μm bands will doubtfully become the emphasis of helicopter stealth. Multidisciplinary optimization of a complete infrared suppression system deserves further investigation.
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This paper assesses aircraft susceptibility from the first principles, with respect to the threat posed by passively guided infrared homing missiles; with an objective of gaining insight into the comprehensiveness of the relationship between aircraft susceptibility and aircraft infrared signature level. The conventional criterion of aircraft susceptibility assessment based on its lock-on envelop is found to be inadequate, and a new criterion termed here as the Lethal Envelop is presented. The proposed susceptibility assessment criterion is more relevant for coming generation of infrared-guided missiles, because of advancements in the infrared detection technology. A threshold infrared signature level is also proposed as benchmark to be satisfied by all infrared signature suppression systems; if aircraft susceptibility to infrared guided missiles is to be reduced. This analysis is vital for gauging the effectiveness of infrared signature suppression systems. A typical air-to-air combat situation is simulated, and the results that lead to aircraft susceptibility assessment are obtained from this model, which illustrates the comprehensiveness of the redefined aircraft susceptibility assessment approach.
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1) An infrared signature suppression system is conceptualized for a helicopter engine exhaust duct, based on low observables principles of conceal and camouflage. 2) The exposed surfaces of the IRSS system are camouflaged with the background by thermal design, which considers multimode heat transfer, including surface radiation interchange. 3) The IRSS system completely blocks the visibility of the exhaust-duct outer surface and the inner surface of its module 1. It reduces the solid angle subtended by the inlet disk surface and the inner surface of the hot exhaust duct, and also the range of the viewing aspect angle (ø) over which they are visible. 4) The penalties associated with the installation of the IRSS system are restricted to a minimum in the conceptual design. The weight penalty is reduced by using lightweight composite and glass wool; both have low thermal conductivity, which reduces their thicknesses. The engine backpressure penalty is reduced by minimizing the disturbance to the exhaust flow and by avoiding excessive cooling of the exhaust gases within the flow path. 5) Because the engine exhaust flow is not disturbed, surfaces that are wetted by the flow are not completely blocked, but their visibility is restricted to a narrow range of φ.
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Infrared (IR) emissions from aircraft are used to detect, track, and lock-on to the target. MAN Portable Air Defence Systems (MANPADS) have emerged as a major cause of aircraft and helicopter loss. Therefore, IR signature studies are important to counter this threat for survivability enhancement, and are an important aspect of stealth technology. This paper reviews contemporary developments in this discipline, with particular emphasis on IR signature prediction from aerospace vehicles. The role of atmosphere in IR signature analysis, and relation between IR signature level and target susceptibility are illustrated. Also, IR signature suppression systems and countermeasure techniques are discussed, to highlight their effectiveness and implications in terms of penalties.
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For the integrated stealth issue of ducted tail rotor noise radiation and radar scattering, a comprehensive optimization method based on high frequency electromagnetic calculation theory and boundary element method is introduced. The radar cross section of the rotor, radiated noise and radar cross section of the ducted tail rotor are designed as optimization goals under the constraints of geometric parameters and aerodynamic force. The model of the ducted tail rotor is established by using the full factorial design and the flow field is constructed by high-precision unstructured grid technology. The aerodynamic characteristics of the ducted tail rotor is simulated by the computational fluid dynamics method based on Navier–Stokes equations and k–ε standard viscous model. The noise radiation is solved by boundary element method and the radar cross section value is calculated by physical optics method and physical theory of diffraction. On the basis of these calculation results, the optimization model of the ducted tail rotor is obtained and generated by comprehensive optimization method based on Pareto solution. The final scheme has been satisfactorily improved in terms of noise suppression, radar cross section reduction and aerodynamic lifting. The proposed approach is very effective and efficient for the acoustic/radar comprehensive stealth design of the ducted tail rotor.
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In this paper, a fast estimating method of thermal infrared radiation signatures for low-altitude exhaust plumes was proposed. In engineering application, it is essential to predict the thermal radiation effect of the plume as soon as possible. A simple numerical model was established considering thermal, species formation, entrainment and radiating effects. In this methodology, the mixing region was treated as a hot, under-expanded, reacting and isotonic flow. The reacting flows were simulated by solving the governing equations with finite rate kinetics and conservation matching relations. A single-line-group (SLG) model with Curtis-Godson approximation was utilized to evaluate radiative properties of radiating species. A line-of-sight (LOS) method was used to compute the spectral radiation intensity. This computational model was verified against the Atlas-II’s reference data within the wavelengths of 2–6 μm. The simulation analyzed combustion flows and infrared thermal effects of a typical exhaust plume along flight trajectory points. Results show that the current model can dramatically improve the computational efficiency by 100–1000 times by comparing with the commonly used method. According to this model, the plume’s range and an appropriable cutoff temperature for thermal radiation calculations are easy to obtain. Infrared radiation phenomena accord with the experimental observations. As a main outcome, this simple model can provide a time-saving and range-unlimited method for the infrared radiation signature prediction of a low-altitude plume in engineering application.
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Aimed at diversity of hypersonic flight mission, multi-objective coordinated control of regeneratively-cooled scramjet engine with two-stage kerosene injection is discussed for guaranteeing a safe, steady and high-efficient operation. Firstly, characteristics analyses and system identification are conducted under the situation of flight Mach number 5 using unsteady 1-D model for regeneratively-cooled scramjet engine. Based on transfer functions obtained from system identification, corresponding proportional integral (PI)controllers are designed for different control objectives such as thrust, steady margin and kerosene temperature at cooling channels outlet. Secondly, different control strategies where flow rates of two stage kerosene injections are adjusted are created in different single control loops, including thrust regulation, steady margin protection, restart recovery, temperature protection and overtemperature recovery control loop. Optimal control strategy is determined ultimately in respective single control loop by analyzing merits and demerits from the perspectives of response speed and engine performance. Lastly, multi-loop coordinated control system is designed on the basis of switch rules and algorithm. Simulation results indicate that rapid responses of control system, smooth and steady switch between different loops are realized. Engine can operate steadily with high performance and recover to a safe operation state rapidly during unstart and overtemperature.
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A summary is given of the validation of an integrated aircraft environmental simulation software, with focus on acoustic and engine exhaust emissions. First, an experimental campaign was carried out to gather noise data of several commercial aircraft on departure from Manchester airport (ICAO code: EGCC). Field measurements were taken with microphones placed at a point 8,500 m (4.6 n-miles) from the estimated brake-release point. The departure measuring point was chosen due to the variability in flight trajectories. Comparisons between measurements and numerical predictions are shown for 12 commercial airplanes: A319-111, A320-214, A321-211, A330-243, A330-343, A350-900, A380-861, B737-800, B757-200, B787-800 and -900 versions, and finally the ERJ-195. The validated airplane models are used with a multi-objective, multi-parameter trajectory optimisation at the same airport to determine optimal departure profiles for airplanes in three weight classes: commuter jet, medium range transport, and long-range transport. It is demonstrated how different or concurrent cost functions (noise, fuel consumption, environmental price) lead to considerably different flight profiles and environmental emissions.
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This paper presents an integrated approach for the aeroacoustic assessment of four-dimensional rotorcraft operations. A comprehensive rotorcraft code is utilized to model aircraft flight dynamics across complete missions. A free-wake aero-elastic rotor model is employed to predict high-resolution unsteady airloads, including blade-vortex interactions, at each mission element. A rotor aeroacoustics code is developed to calculate source noise and far-field ground acoustic impact. Time-domain acoustic formulations are used to evaluate near-field noise generation across designated acoustic spherical surfaces surrounding the helicopter main rotor. A numerical procedure is developed for the derivation of acoustic spheres on-the-fly, coupled with trajectory-adaptive ground observer grids. The individual analytical models are incorporated into a mission analysis numerical procedure. The applicability of the integrated method on “real-world” rotorcraft operations is demonstrated for two generic, four-dimensional missions, without the need of pre-stored noise data. The proposed approach provides insight into helicopter noise prediction at mission level, elaborating on the coupling of aeroelastic rotor response with rotorcraft flight dynamics and aeroacoustics.
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Without careful consideration of aerodynamic installation effects on exhaust system performance the projected benefits of high bypass ratio engines may not be achievable. This work presents a computational study of propulsion system integration in order to quantify the effect that aircraft installation has on the aerodynamic performance of separate-jet aero-engine exhaust systems. Within this study the sensitivity of exhaust nozzle performance metrics to aircraft incidence and under wing position were investigated for two engines of different specific thrust. Upon installation, thrust generation was found to be beneficial or detrimental relative to an isolated engine depending on the position of the engine relative to the wing leading edge. The dominant installation effect was observed on the exhaust afterbodies and, over the range of engine positions investigated at cruise conditions, the installed modified velocity coefficient was shown to vary up to 1% relative to an isolated engine. Furthermore, due to variations in the core nozzle mass flow rate by up to 10% relative to an isolated engine, it is concluded that aerodynamic installation effects need to be taken into consideration when sizing the core nozzle in order to ensure engine operability.
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An experimental investigation was conducted to study the impact of geometrical surface modification on the thermal performance of a spray cooling system. All experiments were performed using a closed loop spray cooling system. Deionized water was used as the working fluid. Three different modified surfaces were examined and compared with a plain copper surface under the same operating conditions. The first surface (M1) was modified with four circular grooves each having a width and depth of 0.5 mm and a pitch of 1.5 mm. The second and third surfaces (M2) and (M3) were modified with four circular grooves each overlaid with four and eight radial grooves, respectively. Each radial groove had width and depth of 0.5 mm. All surfaces were tested at three nozzle differential pressures: 80 kPa, 140 kPa, and 185 kPa. The nozzle-to-surface distance, coolant inlet temperature, surface temperature, and chamber pressure were maintained at 10 mm, $22 °C, <100 °C, and atmospheric pressure, respectively. The results indicated that the nozzle differential pressure had a significant effect on the spray cooling thermal performance of all surfaces. Furthermore, surface (M3) had the highest heat transfer enhancement ratio at all operating conditions , followed by surfaces (M2), and (M1), where the maximum heat transfer enhancements were 80%, 36.3%, and 28.7%, respectively. Thus, signifying that using surfaces modified with a combination of circular and radial grooves can enhance spray cooling heat transfer performance.
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A number of experiments is performed to evaluate the infrared radiation characteristics of two-dimensional convergent–divergent (2-D/CD) vectoring nozzles with and without infrared suppression measures. Film cooling and low-emissivity coating, as two general infrared suppression measures, are adopted on the centerbody and divergent flaps of the nozzle. The wall temperature and infrared radiation characteristics are measured, and the infrared suppression effectiveness of film cooling and the low-emissivity coating are obtained by analyzing the experimental results. The investigated results indicate that the infrared radiation of 2-D/CD vectoring nozzle increases with the increase of the vectoring angle. The synthetic infrared suppression of film cooling and the low-emissivity coating enable a remarkable decrease of the infrared radiation of 2-D/CD vectoring nozzles, and the synthetic infrared suppression measure adopted on different components has different suppression effects. For the cases studied in this paper, the integral radiation intensities of the nozzle at 0, 10, and 20 deg vectoring angles are depressed by 38.1, 33.5, and 32.4% at a 0 deg measurement angle, respectively, with a synthetic infrared suppression measure on the centerbody. Although the synthetic infrared suppression measure is adopted on the divergent flaps of the nozzle, the corresponding descents are 17.2, 31.8, and 35.2%.
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Effective emissivity of the conical blackbody cavity is calculated by Monte-Carlo method, the method is compared with the finite element method.The influence of some factor, such as the geometrical parameters, the emissivity of the material and the distance from the cavity, on the effective emissivity of the blackbody cavity is analyzed. It indicates that the results obtained by the two methods are essentially same, that is, the ratio of cavity length to cavity radius is 6, the opening radius to cavity radius is 0.5, the cone angle is greater than or equal to 90°, the material emissivity is greater than equal to 0.5, the distance from the detector to the cavity is 15 times as large as the cavity radius. At the same time, when all the conditions above are satisfied, the effective emissivity of the conical blackbody cavity is large.
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With rapid advancements in Infra-Red (IR) detection techniques, the range from where the IR-guided missiles are able to lock the target aircraft has increased. To avoid the detection and tracking by modern IR-guided missiles, the aircraft and helicopters also demand progress in its stealth techniques. Hence, study of Infra-Red Signature Suppression (IRSS) systems in aircraft and helicopters has become vital even in design stage. Optical blocking (masking) is one of the effective IRSS techniques used to block the Line-Of-Sight (LOS) of the hot engine parts of the exhaust geometry. This paper reviews the various patents on IR signature suppression systems based on the optical blocking method or a combination of IRSS techniques. The performance penalties generated due to installation of various IRSS methods in aircraft and helicopters are also discussed.
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To reduce the aerodynamic noise and radar cross section (RCS) of the helicopter rotor without sacrificing its aerodynamic characteristic, a comprehensive optimization method (COM) based on Pareto solutions is presented. An initial model of the rotor is created by full factorial design (FFD) and meshed by unstructured grid techniques to participate in the calculation and analysis of flow field and electromagnetic scattering field. The aerodynamic characteristics of the rotor flow field are simulated by computational fluid dynamics (CFD) method based on Navier–Stokes (N–S) equations and k–ε standard viscous model. According to the aerodynamic performance constraints of not reducing rotor lift, the thickness noise and the loading noise solved by Farassat 1A formula and the RCS value calculated by physical optics (PO) method and physical theory of diffraction (PTD) are designed as the comprehensive optimization goals. With the progress of the comprehensive balance analysis of these stealth indicators for the rotor models to be optimized, an excellent model with high comprehensive stealth performance and aerodynamic characteristic is generated by the proposed optimization method based on Pareto solutions. In addition, the effects of the models and parameters of each part of the rotor on these characteristics including aerodynamic lift, RCS, thickness noise and loading noise are analyzed in detail. It is effective and impactful of the comprehensive optimization method to deal with the multidisciplinary optimization problems of aerodynamic noise and radar stealth for helicopter rotor.
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The liquid nitrogen (LN2) fracturing technology is a promising and effective means for coalbed methane stimulation. The flow and heat transfers in coal with LN2 injections directly influence the characteristics of fracture initiation and propagation which are being considered as determining factors of methane recovery efficiency of coal seams. The aim of this study is to probe the heat and mass transfer behaviors and fracturing mechanisms of LN2 stimulation. Using combined infrared thermal imaging, ultrasonic detection, and acoustic emission techniques, experimental work was carried out on simulated coal samples to study the heat and mass transfer behaviors under triaxial stress condition. Compared with a single injection, cyclic LN2 injections resulted in denser fracture pattern in the samples. And it was also found that temperature decreased and rose more quickly during the freezing and thawing processes for cyclic injections. With the same injection duration, the cooling range of the cyclic injection was much larger than that of the single injection. In the single injection for 10,000 s, the whole sample was mainly shrunk due to freezing, whereas “freezing shrinkage–frost heave–freezing shrinkage” occurs in the whole sample after 6 times of the cyclic LN2 injections for 7000 s. The maximum acoustic emission energy signal appeared in the 6th cycle, proving that fractures in the samples connected to form a network, which coincided with a decrease in the temperature gradient in the infrared thermal image. Based on analysis of the infrared temperature results, this study established a model for the temperature of the samples with LN2 injection time and demonstrated the changes in the cooling radius and temperature gradient during LN2 injections.
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This research investigates the infrared signature of the exhaust plume ejected from a microturbine engine. Circular and square nozzles are designed and tested to study their effects on the resultant infrared signature of the plume. A microturbine engine is operated under steady conditions with a kerosene added lubricant oil as a fuel. The measurements of the infrared signature are conducted using a spectroradiometer. Blackbody radiance is also measured at an arbitrary temperature and compared to theoretical values to validate the reference and to calibrate the raw spectrum. The infrared signatures emitted from the plume are measured at three measurement locations along the plume for two nozzle configurations. The results are grouped into sub-bands to examine and discuss their specific spectral characteristics. The infrared signatures are shown to decrease as the distance from the nozzle exit increases, which is attributed to the hot exhaust plume mixing with ambient air. The degree to which the signature is reduced at the different the measurement locations was dependent on the sub-band. Comparison of the results shows that the infrared signature of the square nozzle is lower than that of the circular nozzle in specific bands.
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The general design has a key influence on the successful development of a helicopter. With the development of the relevant disciplines of helicopter and IT technique, the helicopter general design has developed from traditional prototype design method, parameter statistics method to modern “multidisciplinary design optimization method”. Especially the application of the ideas of system engineering, concurrent engineering and modern program management in the development of aviation products produces great influence on the helicopter general design idea, pushing helicopter general design to develop towards intelligence, synthesis and systematization. Firstly, the development of helicopter general design methods is reviewed briefly, and the application of the traditional and modern design methods in the helicopter development and the differences between those methods are introduced. Then, the influences on the ideas and methods of helicopter general design produced by such aspects as the ideas of system engineering, concurrent engineering and modern program management, the helicopter new configuration and new digital technology are analyzed. Finally the development trends of helicopter general design methods are prospected. © 2016, Press of Chinese Journal of Aeronautics. All right reserved.
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An experimental study of RCS reduction for a helicopter inlet syncretized with engine cabin was completed in vertical and horizontal polarization ways. Mean while, the RCS reduction effects were analyzed for various reduction schemes by comparing with RCS of pure metal model. The results show that RCS of this inlet is lower than that of the pure metal model. Sticking radar absorbing materials on front lip, upper lip, inferior lip, output shaft, and engine cabin has best RCS reduction effect. This method can achieve RCS reduction of about 8 dB. Besides, the effect of RCS reduction in vertical polarization way is better than that of in horizontal polarization way.
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Physical theory of diffraction (PTD) is an important method in calculating wedge's diffraction whose advantage is eliminating the singularity of geometric theory of diffraction (GTD). Another expression of the diffraction factors is introduced that may clearly imply the singularity. The diffraction factor is transformed into an arc tangent function whose singularity is zero and the singularity happens to be in incident bounds and reflection bounds. The PTD eliminates most of the GTD's singularities, but they still appear under a very special situation. This situation is that scattering wave incidents on the wedge and the bi-static angle is 180°. The singularity may induce a very huge error in bi-static scattering calculation. But in mono-static, there is no sigularity. The examples of 2-dimensional square and triangle's scattering problems validate the analytical result.
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Based on the battlefield environment for performing operational mission, emphasis should be placed on the threat from infrared guided missile when conducting infrared stealth for armed helicopter. Modes and developing tendency of infrared guidance were introduced, calculation formulas of performance range of point-source and imaging guided missile were derived, and two approaches, theoretical analysis computation and flight measurement, of obtaining infrared radiation characteristic of armed helicopter were described. Effects of radiation intensity and difference in temperature on missile performance range were studied through samples. Infrared suppression effects and approaches were analyzed.
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