A typical engine inner-loop control architecture uses fuel flow to control fan speed, which is assumed to be highly correlated to engine thrust. However, as the engine ages the relationship between fan speed and thrust changes, thus changing the thrust response to throttle input. If all engines on a multiengine aircraft do not have the same throttle-to-thrust relationship, a thrust imbalance can result, producing unwanted yaw, which requires pilot intervention to correct. To overcome this problem, researchers at the NASA Glenn Research Center developed and demonstrated a retrofit architecture for intelligent turbofan engine control and diagnostics that changes the outer-loop fan-speed reference signal to account for changes in the fan speed-to-thrust relationship. This architecture includes a thrust estimator, the output of which is compared with the thrust response of a “nominal” engine to generate the corrected fan speed reference.
Hierarchical intelligent control architecture consisting of the direct control level (the two small rectangles with yellow shading including engine control and engines 1 and 2), which controls the engine fan speed to a setpoint; the outer-loop control level (the two large green rectangles), which adjusts the fan-speed reference signal to the direct control level to set thrust; and the intelligent control level (the orange rectangle), which performs diagnostics and health management and sets the thrust reference for the outer-loop control.
Long description of figure 1.
For a two-engine aircraft, as in the example, the intelligent control system evaluates the safety, performance, and capabilities of the engines. It takes input from both engines (control signals, sensed variables, thrust estimate, etc.) to determine each engine’s current health and fitness for the mission. It also generates the outer-loop thrust command that both engines follow. If the intelligent control determines that the condition of an engine is such that corrective action is beyond the scope of the propulsion control (for example, a problem that might compromise the mission), it communicates this information to the mission manager. The intelligent retrofit architecture was demonstrated to compensate for thrust imbalance in a fixed-base piloted flight simulator for a commercial aircraft/engine simulation with one of the two engines severely degraded. With the intelligent control adjustment on, the pilot did not have to retrim the plane or adjust the throttles individually to maintain heading, thus resulting in reduced pilot workload. The thrust estimator tracked the actual thrust of the degraded engine and enabled direct thrust control. The work was performed in-house at Glenn.
Plots showing pilot workload in three cases: (1) both engines nominal, (2) one engine degraded with no outer-loop control, and (3) one engine degraded with outer-loop control on. The variables are the four pilot input devices: pitch stick, lateral stick, rudder pedals, and throttle or power lever angle. It is clear that the nominal and degraded cases under intelligent control were similar in terms of pilot workload (the amount of control movement the pilot needed to use to maintain the flight path). The degraded case with no intelligent control (no outer loop control) required much more effort by the pilot in terms of both lateral stick (which affects roll) and rudder pedals (which affect yaw).
Litt, Jonathan S., et al.: A Demonstration of a Retrofit Architecture for Intelligent Control and Diagnostics of a Turbofan Engine. AIAA-2005-6905, 2005 (NASA/TM-2005-214019. ARL-TR-3667). http://gltrs.grc.nasa.gov/cgi-bin/GLTRS/browse.pl?2005/TM-2005-214019.html
U.S. Army Research Laboratory at Glenn contact: Jonathan S. Litt, 216-433-3748, Jonathan.S.Litt@nasa.govLast updated: October 6, 2006
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