Techniques for Integrating Ground and Flight Test and Modeling and Simulation to Improve Aircraft Propulsion System Acquisition

David S. Kidman, Technical Expert, Propulsion Integration Flight
Donald J. Malloy, Technical Specialist, Test and Evaluation Branch, Aerospace Testing Alliance
Andrew Hughes, Aerospace Testing Alliance

Abstract

The U.S. Air Force has a long history of using modeling and simulation (M&S) in the test and evaluation (T&E) process. Advancing technology and complex integration requirements are increasing M&S use across a broad spectrum of technical disciplines including aircraft propulsion T&E. High-fidelity aero-thermodynamic models that predict component operating characteristics and overall system performance are initially developed using physics based models, computational fluid dynamics and empirical data derived from component tests. These models are then used to predict installed steady-state and transient engine performance, support model-based acquisition processes and support pre-test planning, test execution, and post-test evaluations in conjunction with flight testing.

Modeling and simulation holds the promise of being able to identify invalid data and anomalous engine behavior and make quick, accurate predictions of in-flight propulsion system characteristics (performance, operability and durability). Therefore M&S can lead to a significant reduction in propulsion system test matrices and reduced re-fly requirements, thus shortening propulsion test time and enhancing test safety. Modeling and simulation cannot replace testing. However, M&S can be used to significantly improve knowledge gained during test and evaluation.

In order for models to be used effectively in model-based acquisition processes, they must accurately predict operation of the fully integrated propulsion system at conditions encountered by the warfighter. Differences between high-fidelity simulations and ground- and flight-test data can occur due to improper characterization of unmodeled dynamics, inlet distortion and recovery, horsepower extraction, customer bleed, engine-to-engine variation and degradation. Differences between model predictions and ground- and flight-test results must be understood before the models can be effectively used in model-based acquisition processes.

The U.S. Air Force’s Arnold Engineering Development Center (AEDC) and Air Force Flight Test Center (AFFTC) employ a comprehensive suite of modeling and simulation tools for validating and updating aero-thermodynamic models. The modeling and simulation tools incorporate automated data validation, system identification, parameter estimation, model validation and model calibration. As the models are validated, they are used for more detailed post-test analysis.

This paper highlights efforts by the U.S. Air Force’s test centers responsible for installed and uninstalled propulsion system test and evaluation to improve model-based acquisition through the use of ground and flight test. This paper explains why modeling and simulation is critical to advancing the state of the art in propulsion system T&E and provides examples in which model-based techniques were applied in all phases of the acquisition process up to and including operational test and evaluation. Lessons learned and knowledge gained during four recent propulsion integration flight test programs are also presented.