Ian W. Hunter

Stop-rotor aircraft are a class of vertical takeoff and landing (VTOL) vehicle that offer improved efficiency across flight modes through the usage of a single central lifting surface. In VTOL, the central lifting surface rotates like a helicopter blade to achieve an upward force. In forward flight, the central lifting surface locks in place like a conventional fixed-wing aircraft and achieves lift from airflow over the surface. The improved efficiency across flight modes enables more complex mission profiles that balance flight time in VTOL and forward flight, such as package delivery and inspection over a large area. Despite the promise of stop-rotor aircraft, challenges in modeling and control, particularly due to the nonlinear rotor dynamics across flight modes, have limited practical implementation. To this end, this paper presents two types of models: 1. Analytical models, derived from first principles physics, provide insight into the stability and control of the vehicle and demonstrate closed-loop stability of yaw and altitude using classical PID control, 2. Computational models, based on numerical integration of the system's ordinary differential equations, provide full-state dynamics of the vehicle. Validation against bench-top constrained flight tests shows that the analytical models capture over 97% of the variance in the computational results, while the computational models account for up to 40% of the variance observed in experimental data.

Reversible morphing wings, capable of exchanging the direction of the leading and trailing edge, improve the feasibility of stopped-rotor aerial vehicles and thereby expand aerial robotics capabilities. Despite the potential benefits of reversible morphing wings, few designs exist and most do not consider the coupled aerodynamic and structural effects of the introduced morphing mechanisms. We report a six-bar linkage-inspired reversible morphing wing design that is compliant, yet structurally rigid against aerodynamic loading. Compared to alternative methods for reversing airfoil direction, the developed wing increases the maximum aerodynamic performance by 50%, while also yielding a non-zero efficiency at 0° angle of attack. The proposed linkage system derives rigidity by strategically eliminating degrees of freedom upon actuation. To validate the linkage theory of rigidity, the full wing is studied under one-way fluid-structural interaction simulations. Furthermore, we demonstrate how constrained optimization can be applied to improve the aerodynamic efficiency of these wings subject to the constraints of a six-bar linkage system.

Stop-rotor aircraft have long been proposed as the ideal vertical takeoff and landing (VTOL) aircraft for missions with equal time spent in both flight regimes, such as agricultural monitoring, search and rescue, and last-mile delivery. Featuring a central lifting surface that rotates in VTOL to generate vertical thrust and locks in forward flight to generate passive lift, the stop-rotor offers the potential for high efficiency across both modes. However, practical implementation has remained infeasible due to aerodynamic and stability conflicts between flight modes. In this work, we present SPERO (Stopped-Penta Rotor), a stop-rotor uncrewed aerial vehicle (UAV) featuring a flipping and latching wing, an active center of pressure mechanism, thrust vectored counterbalances, a five-rotor architecture, and an eleven-state machine flight controller coordinating geometric and controller reconfiguration. Furthermore, SPERO establishes a generalizable design and control framework for stopped-rotor UAVs. Together, these innovations overcome longstanding challenges in stop-rotor flight and enable the first stable, bidirectional transition between VTOL and forward flight.

Stimulated Raman scattering (SRS) is advantageous for in vivo diagnostic imaging due to its non-destructive, label-free, and chemically selective nature. It can significantly enhance the signal-to-noise ratio compared to the conventional spontaneous Raman scattering process, enabling fast acquisition of Raman signals for hyperspectral imaging. While it is used in various medical fields, such as cancer diagnostics, stain-free histopathology, and pharmaceutical research, its application outside clinical settings or laboratories is limited due to the complexity of the required light source. This study focuses on the development of a portable SRS system based on a tunable dual-output fiber-based light source which can be used not only for medical imaging but also for proximal standoff detection of chemicals and explosives. In order to determine the fiber laser design requirements, we set up a benchtop SRS system using a commercial free-space tunable dual-wavelength laser and experimentally analyze the laser-related factors influencing SRS signal generation, such as wavelength tunability, output power, power ratio of the two incident beams, spectral bandwidth, and pulse duration. Additionally, we evaluate the factors affecting the sensitivity and reproducibility of SRS detection, including the ratio of pump-Stokes beams and the distance between the sample and the detector. Based on the parametric study of SRS detection with the benchtop SRS system, we have determined the design parameters of the new fiber-based SRS source where the broadband pump beam is produced through supercontinuum generation for fast hyperspectral SRS imaging.

Reversible morphing wings, which can exchange the leading and trailing edges, expand architectural possibilities for aerial robotics (e.g., stopped-rotor configurations). However, few designs are scaled for uncrewed aerial vehicle (UAV) applications or effectively address the coupled aerodynamic and structural challenges of morphing. As such, we present a novel reversible wing design that uses rigid parallelogram slats mounted on a flexible substrate, creating a compliant yet aerodynamically robust structure. One-way fluid-structure interaction simulations validate the wing's structural performance under airflow. Compared to other reversed-flow wings, the proposed configuration doubles reverse-flow performance relative to a Clark-Y wing and improves upon the 0° angle of attack performance compared to other reversible morphing wings.

High Resolution Image Download MS PowerPoint Slide In electrocatalysis, mechanistic analysis of reaction rate data often relies on the linearization of relatively simple rate equations; this is the basis for typical Tafel and reactant order dependence analyses. However, for more complex reaction phenomena, such as surface coverage effects or mixed control, these common linearization strategies will yield incomplete or uninterpretable results. Cohesive kinetic analysis, which is often used in thermocatalysis and involves quantitative model fitting for data collected over a wide range of reaction conditions, requires more data but also provides a more robust strategy for interrogating reaction mechanisms. In this work, we report a robotic system that improves the experimental workflow for collecting electrochemical rate data by automating sequential testing of up to 10 electrochemical cells, where each cell can have a different electrode, electrolyte, gas-phase reactant composition, and applied voltage. We used this system to investigate the mechanism of carbon dioxide electroreduction to carbon monoxide at several immobilized metal tetrapyrroles. Specifically, at cobalt phthalocyanine (CoPc), cobalt tetraphenylporphyrin (CoTPP), and iron phthalocyanine (FePc), we see signatures of complex reaction mechanisms, where observed bicarbonate and CO 2 order dependences change with applied potential. We illustrate how phenomena such as electrolyte poisoning and potential-dependent degrees of rate control can explain the observed kinetic behaviors. Our mechanistic analysis suggests that CoPc and CoTPP share a similar reaction mechanism, akin to one previously proposed, whereas the mechanism for FePc likely involves a species later in the catalytic cycle as the most abundant reactive intermediate. Our study illustrates that complex reaction mechanisms that are not amenable to common Tafel and order dependence analyses may be quite prevalent across this class of immobilized metal tetrapyrrole electrocatalysts.

In electrocatalysis, mechanistic analysis of reaction rate data often relies on linearization of relatively simple rate equations; this is the basis for typical Tafel and reactant order dependence analyses. However, for more complex reaction phenomena, such as surface coverage effects or mixed control, these common linearization strategies will yield incomplete or uninterpretable results. Cohesive kinetic analysis, which is often used in thermocatalysis and involves quantitative model fitting for data collected over a wide range of reaction conditions, requires more data but also provides a more robust strategy for interrogating reaction mechanisms. In this work, we report a robotic system that improves the experimental workflow for collecting electrochemical rate data by automating sequential testing of up to ten electrochemical cells that can each have a different electrode, electrolyte, gas-phase reactant composition, and applied voltage. We use this system to investigate the mechanism of carbon dioxide electroreduction to carbon monoxide at several immobilized metal tetrapyrroles. Specifically, at cobalt phthalocyanine (CoPc), cobalt tetraphenylporphyrin (CoTPP), and iron phthalocyanine (FePc), we see signatures of complex reaction mechanisms, where observed bicarbonate and CO2 order dependences change with applied potential. We illustrate how phenomena such as electrolyte poisoning and potential-dependent degrees of rate control can explain the observed kinetic behaviors. Our mechanistic analysis suggests that CoPc and CoTPP share a similar reaction mechanism, akin to one that has been previously proposed, whereas the mechanism for FePc likely involves a species later in the catalytic cycle as the most abundant reactive intermediate. Our study illustrates that complex reaction mechanisms which are not amenable to common Tafel and order dependence analyses may be quite prevalent across this class of immobilized metal tetrapyrrole electrocatalysts.

This work presents an evaluation of reversible water electrolysis as a method for pneumatic actuation via the electrochemical decomposition of water into hydrogen and oxygen gas. In addition to a theoretical evaluation of the performance of electrochemically-driven pressurized hydrogen generation, pneumatic generation methods were experimentally tested across two axes: performance and compatibility. Through these experiments, the achievable pressure was shown to be at least 0.75 MPa gauge with a flow rate of 0.06 [L/min]. It was also determined that negligible losses were incurred due to switching from air to hydrogen. Despite low experimental round-trip efficiencies of 0.005%, the reversible electrolysis of water reaction was shown to be a feasible method for pneumatic actuation of soft robots, especially under scenarios where actuator bandwidth is not of concern.

Stop-rotor aircraft, which utilize a central lifting surface capable of transitioning between rotation and fixed configurations, offer a promising option for improving the efficiency of vertical take-off and landing aircraft. However, across the transition between rotation and fixed configurations, the necessary airfoil directionality for efficient aerodynamic performance changes. This work presents the design and experimental analysis of a novel reversible morphing airfoil capable of altering the airfoil directionality necessitated for stop-rotor aircraft. Inspired by roll-up chair design by Uros Vitas, the proposed morphing reversible airfoil, henceforth referred to as the folding slat method, takes shape primarily through a flexible strip that is symmetric about the center and consists of a series of rigid trapezoidal extrusions, forming slats, that are mounted on a flexible substrate. The directionality of the airfoil is switched by altering the compression and tension of each side of the strip, thereby pressing and releasing the contact points between the trapezoidal sections. While the proposed folding slat successfully morphed the leading and trailing edge, the experimental evaluation of the morphing airfoil was critical to justify its usage in a real system. As such, a suite of tests evaluating the compression, bending, and fatigue behaviors were performed. From this testing, it was shown that the folding slat could transfer up to a 150 N load without any added electrical energy usage and was able to switch the orientation of the leading and trailing edge at least 1,000 times, while still maintaining adequate stiffness.