Collaborative Payload Lifting using Tethered Unmanned Fixed-Wing Aircraft

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Helicopters have been essential to modern society as they have been the only solution for air-transporting substantial payloads with no need for complex mile-long runway infrastructures. However, helicopters intrinsically have a low efficiency and are limited to short missions, both in time and distance. Current alternative approaches, such as airships pose storage complexity and limited maneuverability, while novel concepts attempting to increase the rotor disk area to minimize power requirements have reduced capability to translate.******A disruptive concept to vertical lift uses tethered fixed-wing aircraft to lift a payload, where each aircraft mimics an individual blade with its own trajectory control. Although radically different from modern helicopters, load lifting using multiple fixed-wing aircraft was proposed as early as 1942. Manned tethered airplanes imposed km-long tethers and challenging coordination between multiple heavy aircraft limited flight path flexibility. As a result, the load capacity was limited and with the requirement for a conventional runway, the interest in the concept has faded.******The novel idea is to replace tethered manned aircraft (with onboard energy, fuel) with electric-powered fixed-wing aircraft with remote energy source, which shows similarity with some airborne wind energy concepts. By keeping the energy source (battery and/or generator) in the “payload” section and transmitting the energy through the tethers, the lightweight airplanes have an increased agility and thrust-to-weight ratio allowing for a true vertical takeoff. The use of electric motors on agile aircraft along with modern sensing and control is an enabler for the tethered aircraft to perform complex flight path above the payload, defined as non-circular with continuous variation in airspeed, aircraft attitude, and tether tension. The ability to perform complex flight paths was identified as the main enabler for efficient load lifting with tethered fixed wing in a non-hover condition. Initial numerical studies also demonstrated a 5 times power reduction compared to an equivalent helicopter for similar payload capacity, and preliminary flight tests on a single tethered aircraft support those predictions. ******The short term objective of this research program is to develop the first small-scale demonstrator of a three electric-powered tethered aircraft concept with complex flight path capability. The work will advance science in three ways, (1) by developing the fundamental knowledge in aerodynamics, dynamics, and controls needed to enable tethered flight systems; (2), by developing the engineering knowhow to design, build, and test such new flight systems, and (3), by providing a complete set of experimental data that is necessary to corroborate the theoretical understanding and to extrapolate on the capabilities of the approach. The proposed project will contribute in training 2 PhD and 3 MSc students in the domain of advanced aircraft design.**