Objective  The development of long-wave infrared radiation imaging technology is more and more threatening to helicopter. The sources of helicopter long-wave infrared radiation include the infrared radiation of the helicopter's own power system and its energy transfer with the engine compartment as well as the local heating of the fuselage by solar radiation. A large number of experiments and numerical analyses have been carried out on the highly efficient ejection blending system and infrared suppressor, which can effectively reduce the infrared radiation of the helicopter itself. Solar radiation has a heating effect on the local skin of the helicopter in flight, thus changing the infrared radiation distribution characteristics of the whole aircraft, but it is often ignored in numerical simulation calculation, and there are relatively few studies on the characteristics of solar radiation and infrared radiation of the whole aircraft considering various factors. Therefore, it is necessary to carry out the research on the effects of solar radiation on the fuselage infrared radiation characteristic.

  Methods  A physical model including helicopter fuselage skin, main rotor, engine casing and exhaust system was constructed to establish a structured and unstructured hybrid grid (Fig.6). The heat transfer of engine casing (Tab.1), exhaust system (Tab.2) and engine compartment skin is comprehensively considered, coupled with the helicopter forward incoming flow, rotor downwash flow and tail rotor flow (Fig.5). The solar radiation is simulated by the equation of normal direct irradiation applying the fair weather conditions method. A forward-backward ray-tracing method is used to calculate the helicopter infrared radiation.

  Results and Discussions  The whole helicopter model including the engine casing and the exhaust infrared suppressor is simulated and calculated (Fig.3). In the calculation of the flow field, the mixed flow field including the forward incoming flow, the main rotor downwash flow, the exhaust jet flow and the tail rotor flow are considered (Fig.4). With time, season and helicopter flight direction as variables, different solar radiation loading conditions are set (Fig.8-10). The detection points are evenly arranged on the horizontal, transverse and longitudinal detection planes (Fig.7), to calculate and analyze the effect of solar radiation on the infrared radiation characteristics of helicopter in 8-14 μm band.

  Conclusions  The calculation results show that the direct sunlight at noon in summer can increase the overall temperature of the fuselage to the sun side by more than 20 K, and the local maximum temperature can be increased by 25 K. The infrared radiation intensity of 8-14 μm band on the sun side of the helicopter fuselage showed a peak-like trend throughout the day, and its peak appeared around 12 o 'clock. The closer to the top side of the fuselage is, the more significant the enhancement effect of solar radiation on the infrared radiation intensity of 8-14 μm band is, up to 25%. Taking winter as the benchmark, the infrared radiation of the whole aircraft at the autumn equinox, spring equinox and summer solstice increases by about 7%, 11% and 21% respectively. Except summer, the infrared radiation intensity distribution of 8-14 μm band on both sides of the fuselage in other seasons presents asymmetry, and the difference between the two sides is about 5%. On the whole, the solar radiation at 10 am in summer has little effect on the infrared radiation intensity distribution of 8-14 μm band of helicopters in different flight directions.