Flight Test Validation of the RQ-4 Block 20 Global Hawk Aerodynamic and Propulsive Models

Christopher Eaton, United States Air Force Flight Test Center, Edwards AFB, CA
Reagan Woolf, United States Air Force Flight Test Center, Edwards AFB, CA

Abstract

The RQ-4 Block 20 Global Hawk is the newest version of the high altitude, long endurance unmanned aerial vehicle (UAV) produced for the United States Air Force (USAF). The nature of a fully autonomous UAV necessitated the need for high fidelity modeling as well as significant flight test model validation. Primary envelope expansion was performed over the course of 40 flights between October 2007 and March 2009 at Edwards AFB, California. The objective of the envelope expansion program was to evaluate the performance, stability, and control of the RQ-4 Block 20 and compare the results to model predictions. These flight test validated models were utilized by the USAF to support airworthiness certification as well as verification of Key Performance Parameters.

The autonomous nature of the UAV provided both benefits and challenges to the test program. Classic flight test techniques such as fixed-throttle cruise points and thrust-limited turns were not possible because of the nature of the autonomous flight control system. However, the autonomous UAV was always “on conditions,” allowing for significant aircraft performance data collection during normal flying. Automated flight test maneuvers provided consistent and repeatable maneuvers throughout the envelope, but also limited adaptability during the test program.

The envelope expansion test program evaluated several models including takeoff and landing simulations, three and six degree of freedom up-and-away flight simulations, and their associated aerodynamic and propulsive models. The ground and flight test program included installed thrust measurements, air data system calibrations, takeoff and landing tests, maximum thrust climbs, partial thrust cruise points, idle thrust descents, control surface pulses, bank-to-bank rolls, and steady sideslips. The expansion process methodically built up in altitude, gross weight, and Mach number, performing a standard maneuver block at each condition. The maneuver block included a trim shot, three axis control surface pulses, sideslips, bank-to-bank rolls, roll reversals, and engine transients. The maneuver blocks were simulated prior to the flight and the results were reviewed to identify maneuvers that approached or exceeded flight limitations. The simulation results were then used in the control room and post flight to compare with the observed flight responses.

The aircraft performance data were standardized to reference conditions using a modeling and simulation approach. The iterative process used the standardized data to update the lift, drag, and propulsive models. The flight test-adjusted models were then used to update the takeoff and landing simulations, up-and-away flight simulations, flight manual performance charts, and automated flight planning system.

The stability and control data were analyzed in two different ways. First, the test-day flight characteristics were compared to simulated results. Second, parameter estimation techniques were used to evaluate the various components of the aerodynamic model. The flight test-adjusted models were then used to update the six degree of freedom simulation.

Aircraft envelope expansion presents unique flight test challenges and the RQ-4 Block 20 Global Hawk was no exception. The autonomous nature of the aircraft allowed for consistent and repeatable maneuvers but was not easily adaptable to new requirements or maneuvers, ultimately resulting in increased reliance on models and simulation. Early envelope expansion results identified several differences from initial predictions, some resulting in critical software updates and future planned upgrades. Detailed flight test model validation of multiple aircraft models allowed the USAF to verify the performance and fully expand the RQ-4 Block 20 envelope. Ultimately, the aircraft was certified airworthy and all aerodynamic related key performance parameters were met. Without significant use of modeling and simulation during the RQ-4 Block 20 envelope expansion, the ability to fully evaluate the aircraft would not have been possible.