Flight Demonstration of Novel Atmospheric Satellite Concept

The major focus of the Phase II effort described herein was to develop and demonstrate an aircraft capable of autonomously sailing (i.e., to cruise without propulsion or external assistance), and thereby prove that the dual-aircraft platform (DAP) atmospheric satellite concept is potentially viable. This sailing mode of flight was identified as the #1 enabling technology required for the DAP concept in the Phase I effort. No scientific demonstration of this technology has ever been done to our knowledge. Great strides were made towards development of the enabling technology. A specialized prototype aircraft was developed including a novel cable release mechanism, novel lateron control surfaces, and a highly-accurate onboard wind measurement system. All of these devices were developed in house and validated in flight testing. A specialized flight simulator was constructed and utilized to develop the autonomous flight controller required onboard the aircraft, as support training of pilots for flying aircraft while tethered to a ground vehicle. Software has been developed to provide look-up tables that give the flight condition targets (i.e., 3-D position relative to ground vehicle), speed, orientation, and cable length, based on current wind speed and direction. These tables have been successfully validated in flight simulation and used onboard the aircraft. The aerodynamics of this aircraft were characterized with high fidelity analysis, as needed to produce accurate target sailing flight conditions. The aerodynamics of the cable was also accurately characterized using a novel wind tunnel measurement technique. Finally, novel auto-tuning software has been developed to refine the sailing flight condition targets based on an optimization technique involving doublet maneuvers. Although a real-world demonstration of the sailing mode of flight was not achieved during the Phase II effort, the concept has been further validated using detailed flight simulations and in precursor real-world flight tests. Virtual flights using the auto-tuning software indicate that the prototype aircraft should be able to reach and hold sailing conditions despite moderate levels of turbulence provided there is sufficient mean wind available. Hundreds of flight tests using primarily a dead runway at Deland Municipal Airport, and the long runway at Space Florida's Shuttle Landing Facility, resulted in successful demonstration of the closed-loop autonomous formation flight capability. Future work should include flight testing (and refinement) of the auto-tune software to refine the aircraft orientation targets and achieve and hold the sailing mode of flight. It is also suggested that higher fidelity onboard avionics may be necessary to facilitate sailing by reducing sensor errors. Although the DAP flight simulator, developed within the Matlab-Simulink framework, includes detailed treatment of aircraft/cable aerodynamics, cable dynamics, experimentally-derived propeller-motor thrust curves, and realistic air turbulence, realistic emulation of various sensor errors, more realistic actuator responsiveness, and hardware-in-the-loop testing is highly desirable to improve the fidelity of flight simulation evaluations of the onboard flight software.