The German Aerospace Center (DLR) is currently developing an unmanned experimental solar-powered fixed-wing high-altitude platform designed to be stationed in the stratosphere for several days and to carry payload for earth observation missions. This paper deals with a flight mechanical analysis of the aircraft within the preliminary design phase. For this purpose, it briefly describes all disciplines involved in the preliminary design and gives an insight into their methods used. Subsequently, it presents an assessment of the aircraft in terms of stability and control characteristics. Doing so, it first deals with a dynamic stability investigation using a non-linear 6-degrees-of-freedom flight dynamic model with a simple quasi-stationary approach to account for flexibility, in which the aerodynamic derivatives are given for different airspeed-dependent flight shapes. The investigations show that the aircraft is naturally stable over the complete flight envelope. It does not have a typical short period mode. Instead, the corresponding mode involves altitude and airspeed changes to a large extent. At low airspeeds, the Dutch roll and spiral modes couple and form two non-classical modes. Second, it presents a control surface design evaluation process for the aircraft based on a flight mechanical requirement. This requirement addresses the necessary control authority to counteract the aircraft’s responses due to gust encounters to not exceed afore-defined limits and to prevent the aircraft from entering a flight condition that it cannot be recovered from.

Future aircraft generations require improved performance and efficiency to enable a reduced environmental footprint. To acquire this goal, for example new material and wing concepts are perused at the moment by the aircraft industry. These developments, which include aspects such as over-actuation and lowly damped flexible modes, give rise to more complex, multi-objective control problems. One candidate method, which delivers a solution to these problems for the whole flight envelope, is linear parameter varying (LPV) control. It naturally incorporates the controller scheduling in the synthesis process, guarantees stability and robustness over the entire parameter envelope, and enables intuitive multi-objective, multiple-input multiple-output (MIMO) controller designs. This paper proves the concept of LPV control in practice: The paper presents and discusses the LPV controller design process, simulation results, motion simulator test and finally, the in-flight validation of the control system on a Cessna Citation II aircraft. The developed inner loop controller structures are inspired by classical flight controllers used on state-of-the-art fly-by-wire airliners. The longitudinal aircraft motion is augmented with load-factor command and the lateral motion controller features a roll rate command with attitude hold behavior. The control laws are validated in flight by automated and actual pilot inputs with respect to functionality, flying and handling qualities. Test results are encouraging with the provided key findings and lessons learned aiming to provide a simplification for future LPV flight controller development and testing campaigns.