Integrated Flight and Propulsion Control for Loss-of-Control Prevention

Gas turbine engines can produce substantial moments on their aircraft, which can be used for flight control. For situations where an aircraft faces adverse conditions (such as aero control surface damage or failures, runway incursions, or imminent loss of control), the engines offer the potential to augment aircraft controllability by providing differential thrust for steering and Dutch roll damping, modulated thrust for altitude and phugoid damping, and overthrust to shorten obstacle clearance distances. Engines have advantages in loss of control prevention which often requires urgent attitude changes despite uncertain aerodynamics and deteriorating control surface authority. Large correction moments can be applied via the engine mounts without violating airframe structural limits, and engine thrust performance can be maintained at higher angles of attack than can aero surface performance. Integrated flight and propulsion control to exploit this opportunity is relatively new and engine and flight controls need to be modified to accommodate it. First, the flight tests have shown that engine responses to small throttle motion were too slow to be effective in damping the underdamped aircraft phugoid and Dutch roll oscillations. The time constants of the engines and the oscillations were too close together. Second, the engine thrust response needs to be more precise than is typical. The amount of differential thrust needed for yaw control is only on the order of 1% of takeoff thrust. Finally, the engine must tolerate changing and asymmetric inlet conditions.