Actions for Power Minimization For Fixed-Pitch Coaxial Rotors In Hover

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Power Minimization For Fixed-Pitch Coaxial Rotors In Hover

Author
Opazo Toro, Tomas
Published
[University Park, Pennsylvania] : Pennsylvania State University, 2022.
Physical Description
1 electronic document
Additional Creators
Zhu, Minghui
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Open Access.
Summary
The concepts of Urban Air Mobility (UAM), Drone Package Delivery and Electric Vertical Take-Off and Landing (eVTOL) are all powerful visions that are expected to radically change the landscape of aviation in the coming years. In particular, UAM is the vision of a safe and efficient aviation transportation system that will use highly automated aircraft to move passengers and cargo at lower altitudes within urban and suburban areas. In order to achieve this, it is necessary to harmonically integrate air-traffic into our cities in a way that is clean, sustainable, safe and quiet. Multiple UAM prototypes have appeared in recent years, with some of them already undergoing certification by national authorities (Joby Aviation, Volocopter, KittyHawk, Alia, Ehang, etc). A common feature among all of these models is the high number of actuators in their designs. There is also considerably more rotor-to-rotor interaction and rotor-to-airframe interaction in these vehicles compared with traditional airliners. This makes the modeling of actuator effectiveness and performance a very hard problem to solve. This work focuses on a subclass of multirotors known as coaxial multirotors. A coaxial rotor is a special configuration where two rotors are stacked vertically. The upper and lower rotors are normally rotated in opposite directions. There is a relevant number of present and proposed heavy lift rotorcrafted Unmanned Air Vehicles (UAV) that feature coaxial rotors as part of their designs. Coaxial rotor are known to be noticeably less efficient than two isolated rotors (put side-by-side) due to the mutual interaction between upper and lower wakes. But they are preferred in many designs due to their ability to produce a larger thrust for a given footprint, thus reducing the overall vehicle size. In addition, smaller vehicles (having a rotor diameter under 1.5m) often use fixed-pitch blades driven by brushless DC motors; this allows for a much reduced mechanical complexity when compared to traditional coaxial helicopter's swashplates (e.g Kamov Ka-32). This work examines the minimization of total power at a given thrust for a coaxial rotor pair using an analytical approach based on a modified Momentum Theory and Blade Element Momentum Theory. Experiments are also conducted in a hover test stand as well as in a series of hover flight tests using a 15 kg vehicle with an X8 configuration multirotor (four arms, one coaxial rotor per arm). In a coaxial arm, the same total thrust can produced by different combinations of upper and lower rotor thrust levels. A re-distribution of total thrust is applied via differential rotor speeds to the upper and lower rotors. This results in approximately 5\% savings in coaxial power. For the tested rotors the operating point yielding minimum power involves spinning the lower rotor faster than the upper rotor. The maximum hover power savings is approximately 5\%. The rotor speed differential that minimizes total power is dependent on rotor geometry but is independent of total thrust. The last part of this work is framed in the context of control allocation for overactuated vehicles. Here, the development of an in-flight algorithm based on a Direct Search Method is presented. This algorithm aims to find the actuator combination that provides minimum power at hover. A dynamic control allocation is implemented in order to control the vehicle and at the same time enforce a given speed difference between upper and lower rotors. This is in turn used by the in-flight optimizer to find the point of minimum power. The results from a series of hover flights on the X8 multirotor are presented. They confirm reductions consistent with analytical models and previous experiments.
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Dissertation Note
Ph.D. Pennsylvania State University 2022.
Technical Details
The full text of the dissertation is available as an Adobe Acrobat .pdf file ; Adobe Acrobat Reader required to view the file.
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