Browse Topic: Energy storage systems
The current work presents a methodology to estimate the mission and performance capabilities of a generic rotorcraft configuration, to satisfy the need of evaluating the integration of a full electric powertrain in the aircraft design. To include all the design steps, two different approaches are proposed. For the preliminary phase, the "Analytic Method" is considered, which exploits a purely resistive model. Conversely, a method based on look-up tables called "Table Method" is intended to be used in more advanced phase, when the battery pack is defined. Both approaches are tested by evaluating a reference mission and a hover chart. Finally, a verification of the presented methodology is carried out by comparing the mission results with a commercial software, specialized in the evaluation of the cell discharge when a given power spectrum is provided.
This study examines the ability of a large (1200 lb gross weight) hexacopter with collective pitch controlled rotors to tolerate single motor failure. The hexacopter is considered in various orientations, and the vehicle is trimmed with one motor inoperative (OMI). Unlike RPM-controlled hexacopters, which were trimmable but uncontrollable in hover, and were untrimmable in cruise with an aft-rotor failure; with pitch-control the hexacopter is controllable in hover as well as trimmable for failure of any rotor in cruise (including an aft rotor failure). The study examines how pitch controls, and thrust are redistributed amongst the operational rotors, post-failure, for the different hexacopter orientations. For each case, the maximum thrust and torque increases on any individual rotor, and the total power increase, post-failure is examined. It is found that the hardest to trim cases are those where the hub torque and the hub drag induced yaw moment of the failed rotor add, and fault compensation for these cases usually comes at a high cost in terms of torque penalty and power requirement (necessitating use of more powerful motors and batteries). The results from the study indicated that operating the pitch-controlled hexacopter in an edge-first configuration with a clockwise spinning rotor 1 is the best choice from a fault tolerance perspective.
Researchers at the National Aeronautics and Space Administration (NASA) have conducted a series of module-level 50-ft dynamic drop tests on electric Vertical Take-off and Landing (eVTOL) Energy Storage Systems (ESS) for the generation of dynamic impact data to support standards developments. The tests were conducted on zero-state-of-charge Electric Power Systems (EPS) Electric Propulsion Ion Core (EPIC) modules at the National Institute for Aviation Research (NIAR), utilizing the NIAR outdoor drop test setup and conducted by NIAR test personnel. Four total tests were conducted on modules oriented in four different orientations. During initial post-test inspections at the drop facility, it was observed that the modules experienced varying amounts of damage in various locations and forms. The damage was quantified to the maximum extent possible via photogrammetric methods such as digital image correlation and marker tracking. Post-test modules were then disassembled, and forensics were conducted, which involved inspections into various containment structures and the cells themselves. A scoring rubric was developed in order to utilize a common methodology to quantitatively assess the damage incurred into each module as way of determining the least and most amount of damage present. Results in the form of post-test inspections, digital image correlation deformations, and scoring rubric values will be all presented, along with proposed paths forward for future work.
Electrification could improve full-size rotorcraft performance by reducing peak turbine power demand, reducing transmission system weight and complexity, and reducing operating costs. Integrating electric machines with mechanical powertrains requires careful consideration of the system-level weight and efficiency impacts. This paper presents an optimization framework for evaluating parallel hybrid powertrain configurations using Geometric Programming (GP). Both retrofit and clean-sheet vehicle designs are considered. The results show that high-speed electric motors integrated into a parallel hybrid configuration using batteries can reduce the sized gas turbine power, enabling more efficient engine operation at lower power levels. For retrofit designs, with a fixed vehicle gross weight, adding batteries and motors reduces usable fuel, decreasing mission capability. Clean-sheet designs offer additional flexibility to re-size the vehicle and rotor, resulting in energy savings for an equivalent design mission.
A regulated hybrid-electric power sharing architecture was developed and tested for VTOL applications. In this architecture, there are two power supply branches and one load. The first branch draws power from an engine-generator, and it has additional components of an AC-DC rectifier, a DC-DC buck converter, and a power diode. The second branch draws power from a battery, and it has additional components of a solid-state relay, a DC-DC boost converter, and a power diode. Any specified ratio of battery-to-engine power can be achieved with this architecture. Testing on the full range of power share ratios was conducted at a low load power of 300W. The key conclusions are that: (1) regulated power sharing is feasible between an AC supply and a DC battery, including the extremes of all engine and no battery to all battery and no engine, (2) a specified power share ratio can be achieved both in steady-state and transient conditions, and (3) there is a delay in achieving a specified power share ratio, caused not by the power plant, but rather by the change in RPM of the rotor.
This paper presents handling qualities (HQs) research findings for electrical Vertical Take-off and Landing vehicles. Testing in the Vertical Motion Simulator (VMS) investigated handling qualities of vehicle configurations having a degraded powertrain. Powertrain components, including batteries and electric motors, can degrade as the vehicle is flown. This paper investigates the impact of low battery charge and high motor temperature degradations on the pilot's ability to execute precise maneuvers. Pilot comments and ratings that were collected from four rotorcraft test pilots in VMS testing are used to quantify the effects that powertrain degradations had on the HQs of the vehicle.
Researchers at the National Aeronautics and Space Administration (NASA) have conducted a series of module-level tests on electric Vertical Take-off and Landing (eVTOL) Energy Storage Systems (ESS) for the generation of dynamic impact data to support standards developments. The tests were conducted on zero-state-of-charge Electric Power Systems (EPS) Electric Propulsion Ion Core (EPIC) modules at the National Institute for Aviation Research (NIAR), utilizing the NIAR outdoor drop test setup and personnel. Four total tests were conducted. For each test, the module was dropped at a specific orientation from a height of 50 feet while connected to a guided trolley in order to assess the effects of a 50-foot drop test on the ESS. The test velocities ranged between 46.9 and 52.8 ft/s with impact angles ranging between a flat, zero-degree impact and 18 degrees. Data were recorded in the form of temperatures, cell-level voltage, module level acceleration and digital image correlation from the tests. Accelerations were in the range of 1,500 g for a few millisecond duration, which were indicative of a shock type loading condition. No modules entered thermal runaway, and post-test inspections revealed a variety of internal deformations and damage present in the various modules tested, with specific damage occurring for specific orientations. The modules were ranked according to a custom developed scoring rubric developed by utilizing the test and post-test inspection results. The results were compiled, reported, and will be used to guide future ESS testing. Part 1 discusses the loading environments in the modules, while Part 2 will discuss the deformation and damage in the modules.
This paper presents experimental research aimed at developing novel low lubrication methods for rotorcraft and jet engines, focusing on sustaining minimal lubrication to prevent catastrophic bearing failure during loss of lubrication (LoL) events or to increase fuel consumption performance on once-through, fuel-oil bearing lubrication engines. Utilizing two high-speed bearing test rigs simulating low and high thrust class engine conditions, the study establishes lower bounds for oil flow rates necessary to maintain thermal stability and prevent thermal runaway in hybrid ball bearings. These findings inform the design of the Zulu Pod (ZPod), a passively driven, self-contained oil delivery system that uses engine compressor bleed air to precisely meter lubricant flow. Engine test stand results demonstrate that replacing traditional fuel-oil lubrication with the ZPod system reduces thrust specific fuel consumption (TSFC) by an average of 7%, with up to 11% savings, without compromising engine thrust or bearing health. The ZPod offers a simplified, efficient alternative to fuel-lubricated systems by eliminating fuel diversion for lubrication, enhancing fuel efficiency, and maintaining bearing performance in attritable or single-use engines. Additionally, the study highlights the potential of minimal lubrication supplied by the ZPod to extend operational life during LoL scenarios, enabling safer aircraft recovery. Future work will focus on extending testing to higher thrust classes and optimizing ZPod designs for broader applications.
WHY DO WE NEED SIMULATIONS? This paper is intended to provide a broad presentation of the simulation techniques focusing on transmission testing touching a bit on power train testing. Often, we do not have the engine or vehicle to run live proving ground tests on the transmission. By simulating the vehicle and engine, we reduce the overall development time of a new transmission design. For HEV transmissions, the battery may not be available. However, the customer may want to run durability tests on the HEV motor and/or the electronic control module for the HEV motor. What-if scenarios that were created using software simulators can be verified on the test stand using the real transmission. NVH applications may prefer to use an electric motor for engine simulation to reduce the engine noise level in the test cell so transmission noise is more easily discernable.
Hybrid-electric propulsion could provide numerous benefits for full-size rotorcraft, including reduced peak turbine power demand, reduced transmission system weight and complexity, and reduced operating costs. Variable speed electric motors, furthermore, could be configured to enable continuously variable rotor speed. Achieving these benefits requires accounting for coupling between the hybrid-electric drivetrain and vehicle performance within a large, unexplored design space. This paper presents a framework for simultaneous optimization of vehicle and electrified powertrain conceptual design using Geometric Programming (GP) methods. Four hybrid-electric powertrain architectures are evaluated relative to a baseline non-electrified powertrain for single main rotor, compound coaxial-rotor, and tiltrotor configurations. For designs with an upper limit on turbine power, electrification increases the maximum cruise speed for the compound coaxial-rotor configuration. Variation of the rotor speed by 15% allows the vehicle to carry 8% more fuel, relative to the non-electrified baseline, and 1,246 lb of battery. Operating the rotor at optimal speeds across the mission results in increased off-design mission performance, most notably a 43% increase in transport radius relative to a baseline powertrain. The results demonstrate the utility of the design optimization framework for exploration of novel hybrid-electric concepts as well as the challenges associated with incorporating electrical components into the drivetrain.
Electric Vertical Takeoff Landing (eVTOL) aircraft feature heavy electric motors, battery packs, and rigid fixed-pitch rotors supported on flexible arms. Under substantial time-varying aerodynamic loads associated with variable rotor speeds and, with low intrinsic damping, such lightweight arms respond in bending and torsion at relatively high levels. In this paper, two methods of reducing vibration response in the operating frequency range are explored, one based on damping, the other on stiffness. A tailored particle impact damper system was evaluated experimentally to address near-periodic vibration over a range of frequencies. A forced torsional response test showed consistent 50% vibration reduction, with a 5% mass penalty. To stiffen the system, a cross-braced strut approach linked two arms such that the natural frequencies of their torsion modes would be increased beyond the rotor operating frequency range. A finite element model was developed and validated for a representative eVTOL configuration. Validation was conducted using a scale model aluminum beam set. The addition of a cross-braced strut efficiently stiffened the system, increasing its natural frequency by almost 120%, thus greatly reducing resonant torsional vibration within the operating range. Both approaches to vibration reduction for variable-speed eVTOL aircraft merit continued consideration and research.
Electric aviation represents a new arena for battery engineering and development. In contrast to automotive applications, the electrification of aviation and aerospace is both less mature and requires higher safety and performance regulations. This work addresses a first step towards the development of standards and algorithms for measuring remaining useful energy for the battery system. Battery pack flight test data from 134 tests and two different manufacturers was analyzed to determine the weakest cell blocks in the pack, defined as cell blocks having the lowest voltage at the end of the test. It was found that the maximum initial voltage and voltage integral were two features with predictive power. Using the first five minutes of flight test data, accurate predictions were made ~85% of the time, in contract to the status quo where ~30 minutes of flight test data may be required. Sources of error and pathways to improve upon this result are discussed, such as improving data logging, adding additional logic to handle well-balanced packs, and engineering new voltage features with greater predictive power. It is possible to increase the accuracy beyond 95% this way. Future work will build on this to develop algorithms and standards for on-line fuel gauging for electric aircraft.
This study addresses safety concerns within the rapidly evolving Electric Vertical Takeoff and Landing (eVTOL) aircraft domain, focusing on efficient tools to quantify uncertainties in lithium-ion battery behavior - a critical aspect of eVTOL. One major issue with quantifying uncertainty is the prohibitive computational cost associated with many queries of an expensive-to-evaluate computational model. This work employs three physics-based battery models models of varying fidelity and cost to estimate the mean and the variance of the selected quantities of interest through a multifidelity method to reduce the computation cost. By combining information from multiple cheaper, lower-fidelity models through the Multifidelity Monte Carlo method, we significantly reduce the number of high-fidelity samples required for a prescribed mean-squared error, consequently reducing computational costs down to a tractable level. The proposed methodology is applied to estimate the mean and the variance of the battery temperature and voltage, accounting for uncertainties in flight conditions and materials. The first example focuses on a 580-second flight and is benchmarked against a standard Monte Carlo sampling technique. Results indicate a notable fourfold speed-up using the Multifidelity Monte Carlo method compared to the standard Monte Carlo method for the same mean-squared error for the voltage estimate. To showcase the method's generality, the multifidelity method is then applied to a longer flight of 3580 seconds for estimating the mean and the variance and utilizing these statistics to approximately estimate the probability of the flight completion. This demonstrates the adaptability of the methodology to various power profiles and considered uncertainties, with potential extensions to any battery chemistry. In conclusion, the presented multifidelity method offers a robust approach to enhance eVTOL safety by efficiently estimating uncertainties in battery behavior.
Fundamental advancements in aircraft design over the past 50 years have enabled a range of Vertical Takeoff and Landing (VTOL) air vehicle configurations and have significantly enhanced aircraft performance, safety, and reliability. This summary paper chronicles the evolution of rotorcraft design from 1974-2024 as presented by the Vertical Flight Society (VFS) Aircraft Design Technical Committee (TC). It is segregated into three key pillars of aircraft design preceded by an aircraft design overview. The three pillars are: processes and tools, technology, and air vehicle configuration. The first pillar on design processes and tools describes advancements in technology, methodologies, and computational capabilities such as the transition from design solely by wind tunnel testing, physical models, and hand calculations to computer aided design/synthesis and simulation in a model-based engineering digital-twin environment. The second pillar on technology focuses on advances in disciplinary technologies and how they have been incorporated into aircraft design. Technologies discussed include composite materials; rotor systems; propulsion systems, ranging from advancements in turbine engines to all- and more-electric propulsion technologies utilizing various energy storage systems to convertible engines; fly-by-wire systems; avionics and cockpit architectures, including digital displays, navigation aids, and communication equipment; and autonomous and semi-autonomous systems. The third pillar on air vehicle configuration focuses on platform architectures. Major architectures discussed are high-speed VTOLs such as the thrust-vectored aircraft, tiltrotors, lift/thrust compounded helicopters and all- and more-electric aircraft.
ABSTRACT
ABSTRACT
Electric vertical take-off and landing (eVTOL) aircraft require sufficient electric energy to complete their missions and sufficient electric power for hover conditions (including standard and degraded modes of operation). When using current battery technology, the energy storage system constitutes a significant portion of the vehicle mass, thereby significantly limiting vehicle performance. Advanced cell technologies using solid lithium (Li) anode technology have the potential to increase energy density as compared to traditional cell-based on graphite anodes. Efforts to develop battery solutions using Li anode technology for an eVTOL application demonstrate reduced battery weight while achieving near-term performance objectives.
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