In pursuit of perpetual flight through efficient flight testing of a fly-by-wire optionally piloted vehicle aircraft

Juan Sanz Rodríguez, Skydweller Aero (Spain)
Enrique Babio Fernández, Skydweller Aero (Spain)
Carlos Sanz Moreno, Skydweller Aero (Spain)
Andrés Cobos Usano, Skydweller Aero (Spain)
Alejandro Lorenzo Gallego, Skydweller Aero (Spain)
Travis K. Vetter, Skydweller Aero (Spain)

Abstract

Skydweller is an American-Spanish company in pursuit of perpetual flight. In 2019, it acquired Solar Impulse 2 as a flying testbed, a 72-meter wingspan solar-powered aircraft with reversible flight controls. Ever since, the aircraft has been continuously modified and flight tested in Albacete, Spain, with the ultimate goal of achieving a large, uncrewed, fully autonomous platform capable of extended endurance operations.

Throughout 2021, Skydweller completed a test campaign of 12 missions. Beyond confirming the aircraft’s airworthiness, initial auto-flight system (AFS) prototyping and investigations were carried out using the existing limited-authority actuation to determine optimal control strategies for the unique flying qualities of the air vehicle.

After reaching the limits of the existing actuators, the team transitioned the flight control actuation system (FCAS) to fly-by-wire (FBW) in 2022. This system was designed with an optionally piloted vehicle (OPV) functionality envisioned from its inception, but without mechanical reversion. The system achieves this objective by architecting a dual-redundant safety-critical FBW capability and a distinct non-safety-critical AFS input, with pilot (and automated) capacity to disengage the AFS with the FBW system.

The FBW and FCAS systems were designed and tested to a high degree of assurance. Strict processes and design practices were followed. Testing included laboratory, ground, functional, vibration, and taxi tests; flight test crew theoretical and practical training involved laboratory failure modes and effects (FMET) testing. This was completed in close coordination with the Spanish authorities (AESA) throughout the process. AFS was hosted in a dedicated vehicle management computer (VMC) outside the safety boundary and was tested and trained through simulation and involving engineering, the flight test team and the pilot during feature development.

Finally, the AFS flight test envelope, where the pilot would recover control in case of off-nominal behavior, was clearly defined allowing the pilot to act as the safety element. This approach allowed Skydweller to transition from the first FBW operation to a fully automatic flight in just three test events, totaling 3h 25 minutes of flight.