Methodology for a Dynamic Assessment of Multiple Aircraft Tethered to a Shared Payload
Demers Bouchard, Etienne
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Vertical Takeoff and Landing (VTOL) aircraft play an important role in our society by accomplishing a wide range of missions. Many concepts have been introduced to increase the maximum speed of helicopters. This characteristic has usually been conflicting with hover endurance due to the high disk loading typical to high-speed vehicles. However, there has also been interest in long endurance flight, highlighted by the AHS International Igor I. Sikorsky 24 Hour Hover Challenge. The use of tethered fixed-wing aircraft to lift a shared payload constitutes a promising configuration to provide long endurance VTOL capabilities. Preliminary studies suggested that the system is expected to perform long endurance missions efficiently, due to the possible low empty weight fraction and the very low equivalent disk loading. Previous studies analyzed the use of manned aircraft to perform this operation. While they provide the core lifting capabilities, the performance is limited by the type of aircraft, and the flight path limitations imposed by having a pilot on board: limited centrifugal loading, long tethers and large flight path radius, as well as high pilot workload imposed by aircraft coordination. The recent advances in autonomy and electric propulsion are enablers to the use of unmanned aircraft to perform multiple aircraft load lifting. The present dissertation aims at presenting a methodology to select dynamic influenced design parameters for the system. Among them, the attachment of the tether on the aircraft and the nominal flight path parameters of the systems are identified as new design and operational degrees of freedom that have an important impact on the dynamics of the system. The effects of important variations in suspended mass throughout the mission due to fuel burn are also considered. In order to inform decisions about these parameters, a dynamics-influenced design framework is presented. First, a lower-complexity performance evaluation module is implemented as a means to rapidly differentiate between the various flight path combinations. This module is based on a quasi-steady formulation with only the wing used as lifting surface. Second, a dynamic simulation environment that incorporates the main characteristics of the system is developed, while consideration is given to complexity and runtime. This model is used to evaluate a trimmed-constrained tether attachment region. Linearization about the trim condition and a conversion to multi-blade coordinates are used to create a dynamic model with a minimal level of complexity, while representing the fundamental motion of the system during the circular hover flight phase. The open-loop characteristics of the system are then evaluated for the system in hover, and requirements on the open-loop further constrain the tether attachment point. An approximation of the states, control input and lift coefficient during an operation with a constant wind velocity allows to differentiate between feasible configurations by evaluating a performance criterion. This procedure is repeated for multiple flight path parameters and can be used to differentiate between the optimal configurations. Analogously, for configurations with variable suspended mass, different flight paths are compared, and an evaluation function is used to find an attachment point that represents an appropriate compromise between the different suspended masses while meeting all the constraints. Finally, a closed-loop formulation in multi-blade coordinates is proposed to demonstrate the time-marching simulation results.