~~Ph.D. Thesis Defense

By

David Rancourt
(Advisor: Prof. Dimitri N. Mavris)
11:00 AM, Friday, October 7, 2016
Weber Space Science and Technology Building (SST-II)
Collaborative Visualization Environment (CoVE)

Methods for the Flight Path Optimization of the Electric-Powered Reconfigurable Rotor (EPR2) VTOL Concept

ABSTRACT:
Vertical Takeoff and Landing aircraft (VTOL) have been essential to our society since their introduction in the mid-1940s. Recently, Demers Bouchard and Rancourt developed an advanced VTOL concept based on three tethered fixed-wing aircraft: The Electric-Powered Reconfigurable Rotor (EPR2) VTOL Concept. The rotor is considered “reconfigurable” since the quasi-circular flight path of the unmanned aerial vehicles (UAVs) can be adapted as a function of the flight condition to minimize the power requirement. The salient characteristics of this concept can enable radical performance enhancement compared to conventional helicopters with a reduction in power requirement in hover of up to 80%.

Four main contributions are presented. First, this work details the development of a physics-based 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. This approach removes any feedback loop, and therefore, is ideal for a design space exploration. The test case in this work uses the Makani's Wing 7 tethered UAV, initially developed for wind energy harvesting. It is shown that a 400 kg system could lift a payload of more than 1,400 kg, which leads to an empty weight ratio of only 24%. 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. The radical performance over conventional helicopters is the result of the flexibility of the tethered aircraft flight path and the reduction of the induced velocity. It is shown that non-conventional flight paths of the UAVs lead to power reduction both in hover and with the presence of wind.

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.  It is shown that for this application the computational time was shown to be in the order of seconds, compared to ~20 minutes for the complete dynamic problem.

Third, an improvement of the calculation of the aerodynamic forces on tether segments was developed to minimize the number of required tether segments for this application.

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 both the wake interaction with a free-vortex wake implementation and the effect of control 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 shows potential for future studies.

Committee Members:
Pr. Dimitri N. Mavris, School of Aerospace Engineering
Pr. Brian German, School of Aerospace Engineering
Pr. Marilyn Smith, School of Aerospace Engineering
Pr. Dewey H. Hodges, School of Aerospace Engineering
Dr. Thomas L. Thompson, Chief Eng., Aeromechanics Division, U.S. Army Aviation Eng. Directorate
Dr. Keith Bergeron, Senior Research Aerospace Eng., U.S. Army Natick Soldier RDEC