Aircraft low observables’ features are crucial in the long-wave infrared (LW-IR) band, due to imaging sensors used in IR search and track and in the latest generation of IR-guided missiles. Earthshine irradiance on the aircraft bottom surface is an important source; hence, it is derived using data for atmospheric transmission. Emission due to skin-friction heating (important at high ε b o t ) and earthshine reflection (important at low ε b o t ) are compared by a dimensionless ratio for different bottom surface emissivities ( ε b o t ). The infrared cross section of aircraft in direct view from below is obtained in the LW-IR band, which shows that aircraft is seen also due to negative contrast.

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Wind-milling occurs when air flowing through the flamed out engine results in increasing spool rotation. Aircraft forward speed and dive angles play an important role in achieving sustainable spool rotation for relight. Aircraft fuel pump connected to power take-off shaft of Engine shuts when engine rpm falls below design speed and cannot deliver pressurized fuel to Engine during wind-milling. Under this condition, engine has to suck the fuel from aircraft using its own fuel pump. The atmospheric pressure available on the aircraft fuel tank assists the engine to operate in suction mode. The datum height between engine inlet and fuel tank outlet changes with the dive angle of the aircraft. A test set up was established in the engine test bed to vary the datum between aircraft fuel tank and Engine inlet. The datum was varied to simulate various dive angles of the aircraft. The negative gravity head (Engine fuel inlet above fuel tank outlet) between engine and fuel tank was varied in steps. Total four test cases were carried out in an engine test bed located at an altitude of 920 m above sea level. The engine was successfully started in suction mode without external assistance of fuel pump. This paper presents the test setup, comparison of engine start cycle under various dive angles of the aircraft to evaluate optimum flight conditions to attempt windmill relight. These tests show that as the negative gravity head is increased, the time taken to start the engine increases. The slope of N2 build up becomes shallower with increase in gravity head.