Objective  The wide use of small rotorcraft brings new challenges to the safety of high-value targets. At present, there are many researches on the infrared radiation characteristics of fixed-wing aircraft. The optical characteristics of rotorcraft are mainly reflected in infrared/visible light fusion and infrared small target detection, etc. However, there are few theoretical studies on the characteristics of temperature distribution, infrared radiation spectrum and space distribution of rotorcraft. In order to detect and defend small rotorcraft effectively, it is of great significance to study the spatial distribution characteristics of infrared radiation of small rotorcraft.

Methods  In order to accurately grasp the infrared radiation characteristics of rotorcraft, firstly the geometric model of a small rotorcraft is established (Fig.1), and the model is divided into unstructured grids to generate surface grids (Fig.2), and the area and direction vector of each surface grid are determined. Secondly, temperature measurement and infrared radiation image acquisition are carried out for the rotorcraft at stationary, no-load flight for 10 min and 100 g load flight for 10 min respectively (Fig.3). Finally, based on the Monte Carlo method (Fig.5), and the absorption and reflection of light beams by radiation grid elements are considered, the distribution and law of the middle and far infrared radiation intensity of the rotorcraft under different working conditions and different detection directions are calculated.

Results and Discussions  According to three different working conditions, the spatial distribution of infrared radiation intensity of the rotorcraft in the band of 3-5 μm and 8-14 μm is calculated (Fig.6). At the zenith angles θ of 78°, 101°, and 160°, the radiation intensity of the rotorcraft presents the peak value. The peak value is the highest at about 78°, and the maximum values are 1.13, 1.28, 1.35 W/sr respectively. When the zenith angle θ is 0°, 90° and 180, the radiation intensity is small. In the horizontal direction, when the circumferential angle φ is about 90° and 270°, the radiation intensity of the rotorcraft is relatively strong. The infrared radiation intensity distribution curve of the rotorcraft at stationary and 100 g load flight for 10 min is compared and analyzed (Fig.7-8). When φ is 90°, the radiation intensity of the band 3-5 μm increased by about 56.1%, and the radiation intensity of the band 8-14 μm increased by about 19.5%. When θ is 78.3°, the radiation intensity of 3-5 μm increases by about 53.7%, and the radiation intensity of 8-14 μm band increases by about 19.5%. It can be seen that a longer flight time and a heavier load will significantly enhance the infrared radiation of the rotorcraft.

Conclusions  The geometric model of a small rotorcraft is established, and the flight and data collection of the rotorcraft are carried out to obtain the temperature field distribution data of the surface of the rotorcraft. On this basis, Monte Carlo method is used to simulate the radiation transfer process of rays, and the middle and far infrared radiation intensity distribution of the rotorcraft in different detection directions under different working conditions is obtained, and the following conclusions are drawn. The rotor motor and battery is the main infrared radiation source of rotor vehicle; The longer the flight time of the rotorcraft, the larger the load, the larger the output power of the motor and battery, the more waste heat will be released, and the stronger the infrared radiation of the rotorcraft; When the zenith angle θ is 78°, 101° and 160°, the radiation intensity of the rotorcraft shows the peak value, while when the zenith angle θ is 0°, 90° and 180 °, the radiation intensity is small. The radiation intensity of small rotorcraft in 3-5 μm band is about 0.05 W/Sr, and that in 8-14 μm band is about 0.8 W/sr. The radiation intensity of rotorcraft is mainly concentrated in long-wave infrared band.