Digital Control Of Highly Augmented Combat Rotorcraft

Proposed concepts for the next generation of combat (scout/attack) helicopters are to be embodied in a complex, highly maneuverable, multiroled vehicle with avionics systems which are as important to mission success as the airframe itself. Single pilot and nap-of-the-Earth operations require handling qualities which minimize the involvement of the pilot in basic stabilization tasks. To meet these requirements will demand a full authority, high-gain, multimode, multiply-redundant, digital flight-control system. The gap between these requirements and current low-authority, low-bandwidth operational rotorcraft flight-control technology is considerable. this research aims at smoothing the transition between current technology and advanced concept requirements. The report: (a) extensively reviews the state of the art of high-bandwidth digital flight-control systems; (b) illustrates the important concepts in design and analysis of high-gain, digital systems with a detailed case study involving a current rotorcraft system. Frequency-domain methods for design and analysis are stressed in this report as essential for the practical implementation of high-gain control systems for rotorcraft. Approximate and exact methods are explained and illustrated for treating the important concerns which are unique to digital systems. Proposed specifications and flight-test data concerning the handling qualities of scout/attack rotorcraft imply restrictions on the maximum effective time delay which must be carefully considered in the design process.

Advanced flight control system design, analysis, and testing methodologies developed at the Ames Research Center are applied in an analytical and flight test evaluation of the Advanced Digital Optical Control System (ADOCS) demonstrator. The primary objectives are to describe the knowledge gained about the implications of digital flight control system design for rotorcraft, and to illustrate the analysis of the resulting handling-qualities in the context of the proposed new handling-qualities specification for rotorcraft. Topics covered in-depth are digital flight control design and analysis methods, flight testing techniques, ADOCS handling-qualities evaluation results, and correlation of flight test results with analytical models and the proposed handling-qualities specification. The evaluation of the ADOCS demonstrator indicates desirable response characteristics based on equivalent damping and frequency, but undersirably large effective time-delays (exceeding 240 m sec in all axes). Piloted handling-qualities are found to be desirable or adequate for all low, medium, and high pilot gain tasks; but handling-qualities are inadequate for ultra-high gain tasks such as slope and running landings. Tischler, Mark B. and Fletcher, Jay W. and Morris, Patrick M. and Tucker, George T. Ames Research Center...