No abstract

Flight testing of a military low bypass turbofan engine involves multitudes of tests to ensure the Engine - Aircraft compatibility across the flight envelope. One of the safety critical tests is to conduct In-Flight restart of the engine. Detailed planning and careful execution is mandated for a single engine aircraft. Accurate modelling of sub-idle performance characteristics of the engine during windmilling conditions enables better prediction of engine behavior during in-Flight shutdown and restart. Typically, Engine manufacturer provides a Performance Cycle Deck (PCD) to predict and assess the performance of the engine across the flight envelope for all throttle positions. However, the PCD does not include sub-idle behavior. The present work focusses on developing a torque based engine behavior model which enables prediction of time dependent fan and compressor characteristics during sub-idle operations. The proposed model is divided into two parts. The first part deals with deceleration characteristics during engine shut-off and spool down, and the second part deals with the acceleration characteristics during spooling up and engine restart. In-flight spool-down (a quick relight without windmilling) restart data obtained through flight tests was used to validate the present model. The model is intended to be used for future flight tests which include windmill restarts under various operating conditions. The model is expected to accurately predict the correlation between aircraft speed and engine windmilling rotor speeds for arriving at a windmill restart envelope for the aircraft.

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.

The challenges faced during starting of an aircraft gas turbine engine using a Jet Fuel Starter (JFS) at high altitude airbase are discussed in this paper. Autonomous ground starts at high altitude airbase in soaked sub-zero temperature condition without any external ground support assistance is a challenge. Generally, the start cycle (sub-idle speed) at sub-zero temperatures of a gas turbine engine at high altitudes is influenced by several factors. Drag loads are estimated due to change in lube oil viscosity of engine gearbox and accessory gear box that affects available torque margin of a starter. These estimated loads are superimposed on starter characteristics to identify the available margins for successful starts. The cold start is particularly severe, since it increases the tip clearance between rotor and casing of the engine due to difference in its thermal growth. Higher tip clearances significantly degrade compressor surge margin and results in rotating stall. Inconsistent engine starts were resolved by adopting alternative methods without any change in hardware. This paper presents set of methods used to overcome inconsistent engine starts at high altitude cold weather conditions.