Method for the flight path optimization of the electric-powered reconfigurable rotor (EPR2) VTOL concept

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Rancourt, David
Mavris, Dimitri N.
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Vertical Takeoff and Landing aircraft (VTOL) have been essential to our society since their introduction in the mid-1940s.Today, helicopters are extensively used for military and civilian applications such as search and rescue, police surveillance, oil rig servicing, and cargo transport over short distances. Although helicopters have seen a continuous improvement in performance over the last decades, they still fall short of fixed-wing aircraft in terms of fuel burn. Their limited range is the consequence of the low equivalent lift-to-drag ratio (L/D) in cruise and high empty weight fraction. Also, given the large number of moving parts and the higher level of vibration, helicopters incur a higher maintenance cost. Recently, Demers Bouchard and Rancourt developed an advanced VTOL concept using three tethered fixed-wing aircraft: the Electric-Powered Reconfigurable Rotor (EPR2) VTOL Concept. This concept takes advantage of the scale-free characteristics of electric motors and their high power density. The rotor is considered “reconfigurable” since the quasi-circular flight path of the unmanned aerial vehicle (UAV) can be adapted as a function of the flight condition. The features of this concept can enable performance enhancement never seen before compared to conventional helicopters. Initial studies of this concept, performed in 2014 and 2015, showed that a payload of one metric ton could be lifted with 60 kWe in hover. However, the capability to evaluate the performance throughout the flight regime was not available. The main research objective of this thesis was to develop a multidisciplinary model for the analysis and performance optimization of the EPR2 VTOL Concept in hover and with positive flight speed. The novel methodology, based on a two-level of fidelity environment, was used to answer the overarching research question: How can the flexibility in the tethered aircraft flight path of the EPR2 VTOL Concept be used to minimize the power required to fly throughout the flight envelope? Four main contributions are highlighted in this dissertation. First, this work details the development of a multifidelity and multidisciplinary method for the flight path optimization of electric-powered, tethered aircraft. The method detailed in this work is based on a prescribed flight path obtained from a minimal set of parameters P. This approach removes any feedback loop, and therefore, is ideal for a design space exploration. The lower fidelity environment was used to explore the design space and perform direct optimization while the higher fidelity environment is mainly used for analysis. The test cases in this work use the Makani's Wing 7 tethered UAV, initially developed for wind energy harvesting. It was shown that for a total fuselage mass of 800 kg, the power requirement is as low as 42 kWe. At a travel speed of 20 m/s, the power requirement increases to 62 kWe and reaches 95 kWe at 27.5 m/s. The absolute maximum takeoff weight of the whole system was estimated at 1,835 kg, with an estimated total empty weight of only 400 kg. This leads to an impressive empty weight ratio of 22%. The maximum hovering time could be as high as 150 hours (no payload), or 24 hours with a 900 kg payload if powered by jet fuel. Second, a rigid tether model for the flight path optimization of tethered aircraft was developed. As an alternative to the full dynamic tether model for the design space exploration phase, a kinematic model was developed to evaluate the approximate tether forces. This model uses the same aerodynamic modeling method than the higher fidelity dynamic model, except that it assumes that the tethers are rigid. The computational time was shown to be on the order of seconds, compared to 20 minutes for the complete dynamic problem with a minimal difference in the accuracy of the results. Third, an improvement to the calculation of the aerodynamic forces on tether segments was developed to minimize the number of required tether segments for this application. An analytical approach was used to demonstrate the benefits of this method versus the standard approach used in the recent literature. Finally, this dissertation details the development of a wake consolidation model for application to fixed-wing aircraft aerodynamics and its integration in a custom-designed free-vortex wake model. This model bridges the gap between conventional helicopter aerodynamic methods and fixed-wing aircraft methods. It considers, the wake interaction between the aircraft and the effect of control surface deflection on the loads and wake. To the author's knowledge, this complete aerodynamic model, which captures all the relevant physics required to analyze tethered aircraft, is novel and provides a strong foundation for future studies.
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