University Leadership Initiative

Topic addressed: Safe, Quiet, and Affordable Vertical Lift Air Vehicles

Outcome addressed: High-fidelity models and multidisciplinary tools for design and development of full-scale electrically powered vertical lift vehicles for advanced air mobility application.

Name and organization of PI: Dr. Maj Dean Mirmirani, Embry-Riddle Aeronautical University (ERAU). 

Key Personnel: Dr. Katerina Aifantis, University of Florida Associate Professor and Mechanical and Aerospace Engineering Faculty Fellow; Dr. Pat Anderson, Embry-Riddle Professor of Aerospace Engineering and Director of the Eagle Flight Research Center; Dr. Soon-Jo Chung, California Institute of Technology Bren Professor of Aerospace, Graduate Aerospace Laboratories; Dr. Kyle Collins, Embry-Riddle Research Assistant Professor at Eagle Flight Research Center; Dr. Patrick Currier, Embry-Riddle Associate Professor & Associate Chair of Mechanical Engineering; Dr. Borja Martos, Embry-Riddle Research Engineer at Eagle Flight Research Center; Dr. Mndoye Ndoye, Tuskegee University Assistant Professor, Department of Electrical Engineering.

List of partners: Embry-Riddle Aeronautical University (Lead), University of Florida, California Institute of Technology, Tuskegee University, Boeing, VerdeGo Aero, and Flight Level Engineering.

Requested funding & Duration: $2,000,000 / year or $8,000,000 and 4-year duration.

Research Objectives     back to top

The objectives of the proposed study include a) performing a detailed assessment and gap analysis of the existing tools for conceptual design of distributed electric propulsion (DEP) for fixed-wing or VTOL vehicle configurations (for example the NASA NDARC, Argonne National Laboratory, Aeronomie, and ERAU HCDT), b) identification of the need for and development of new tools, c) developing an open source high-fidelity model for a full-scale generic eVTOL, which incorporates the complex dynamic interactions among its various subsystems, d) testing tools and validating models using the existing array of hardware the ERAU team possesses to generate data and provide clarity of the scaling laws and complex interaction of various systems to transition eVTOL to the size and speed required for commercial viability of a manned vehicle, e) developing airworthiness criteria and means of compliances (MOC) for certification of this class of vehicles.

Partially-defined Technical Challenges     back to top

TC-1 Statement: The dynamics of eVTOL are complex and involve strong couplings among its energy generation, propulsion, and control systems. For commercially viable cargo or passenger-carrying eVTOL, the interactions between these dynamics need to be studied and included in the modeling and design. The architecture of a distributed rotor system, central to the safe, efficient, and quiet operation of eVTOL, must provide thrust as well as sufficient control authority at high frequency for attitude and directional control. While this dual demand on a vehicle’s propulsion system is easily met for small hobby size vehicles, that same architecture cannot be scaled up for large cargo or passenger-carrying vehicles. The inertia of the vehicle combined with the demand for control input require helicopter-like variable pitch propellers at each rotor, thus introducing significant complexity. Forces and moments to quickly stabilize the vehicle in turbulence or failure of one or more motors require a large burst of energy supplied by the power source. This occasional demand for significant additional power in a DEP system creates intricate design tradeoffs between weight, efficiency, battery thermal management, and reliability, which must be studied. A robust fault detection, fault tolerant, control system architecture is critical to achieving the safety required for mature AAM operation. Furthermore, noise due to propulsion is directly related to the propeller tip Mach number. To the first order effect, a lower Mach number means a quieter vehicle. For an eVTOL such improvement comes at the cost of propulsive efficiency. eVTOL models and MDO tools must include these complicated tradeoffs. Duration: 4 years (Year 1-4).

TC-2-Statement: Electrical energy production and storage systems are central to the safe, efficient and quiet operation of eVTOL.  The use of electrified propulsion results in reduction of noise and emissions—both at the heart of public acceptance of increased advanced air mobility. All-electric systems possess high efficiency and present desirable simplicity. Unfortunately, the current battery specific energy limits the use of these systems for larger heavy eVTOL applications. Serial hybrid systems can produce electrical power at an equivalent specific energy, typically 3 to 6 times higher than a battery. However, more research is required to develop models and tools to fully optimize hybrid systems for aerospace and eVTOL applications. Our team is well positioned to do this. In the past 10 years, we have developed an array of hardware including a parallel hybrid electric powered general aviation aircraft as an entry to the NASA Green Flight Challenge, an all-electric general aviation aircraft, and most recently a serial hybrid gas-electric powerplant for a typical cargo vehicle on a one-year $2.5 million contract. Duration: 4 years (Year 1-4). 

TC-3-Statement: Hybrid systems require a battery pack to stabilize the bus voltage, for added power in vertical takeoff, and to assist providing high frequency control authority in turbulence or failure. Thus, battery research remains at the heart of eVTOL technology. Physics-based models at the cell and pack levels are needed for propulsive battery increased efficiency. The pack level design of battery systems is critical for all electric and hybrid systems and is mission profile dependent. The driver of pack level specific energy is, of course, the cell specific energy and the battery thermal management system. Our research will focus on the cell-level specific energy, thermal management, and life cycle optimization at the pack level. The latter is of particular interest for cases where high c-rates are used to minimize the pack mass. For aerospace applications, the challenge is to develop models that allow for optimization of battery life, while achieving minimum weight for both the cells and the thermal systems. Our team has a wealth of experience and expertise through designing highly optimized battery packs, safe and reliable battery management systems, thermal management and modeling at the cell-level. Duration: 4 years (Year 1-4).

TC-4-Statement: In the above three sections, we outlined the technical challenges related to the main subsystems of an eVTOL, namely the energy generation, propulsive system, and vehicle attitude control. The control of the distributed thrust production, attitude control, noise, electric power generation and battery pack management are tightly coupled and require a model-based systems engineering (MBSE) approach. For example, reduction of noise often requires an increase in power and energy; thus, vehicle-level control should strike an optimal trade-off between noise and minimum available energy for mission completion. Propulsive thrust for attitude control requires sudden demand for battery power at high frequency resulting in thermal dissipation. The challenge here is a systems level optimization that provides appropriate trade-offs between thermal management, energy management, and noise with the weight of onboard batteries for all mission profiles. Duration: 4 years (Year 1-4).

Summary of Technical Approach for the Effort     back to top

The underlying technology to develop a passenger-carrying certifiable eVTOL already exists. Yet, the efforts of over 200 well-funded startups in this space have so far failed to produce a practical solution. As described above, complicated dynamic interactions among the subsystems, design tradeoffs and scaling laws are not well understood or sufficiently validated. Additionally, there is no airworthiness criteria and means of compliances to certify these vehicles. Current FAR 23/25 and FAR 27/29 rules are inadequate to certify the new AAM vehicles with simplified vehicle operations features and vertical takeoff and landing (VTOL) capability. The proposed ULI will focus on removing these barriers. Our approach to achieve the objectives of the proposed research is:

  1. Through development of software tools and models
  2. Through using hardware testing and validation

Starting in year 1, and related to TC1, we will validate key concepts through targeted hardware experiments. Our existing hardware shown below will be leveraged to produce vast new data, through which we will assess and calibrate models and tools within the NDARC ecosystem including vehicle flight dynamics and control (rotors/props, motors and motor controllers), power distribution, thermal management, and power generation and storage modules. In parallel, we will develop new software tools to fill the gaps in existing ones. For the new tools, a design space that includes various propulsive electric motors, rotors, and controllers (air cooled, liquid cooled) will be considered.  We will include fixed pitch rotors, variable pitch rotor/propellers, collective-only configurations and collective and cyclic at a minimum of three different thrust ranges and various tradeoffs between noise and propulsive efficiency, safety and affordability. Once insight is gained, an open source Matlab Simulink model for a full-scale generic eVTOL incorporating salient dynamic interactions among various systems will be developed and its main characteristics in various mission profiles will be studied.

Existing Multi-Rotor Test Vehicles Hardware

Verdego 8 Rotor

VerdeGo 8-Rotor

Scale: 10 lbs constant rotor speed only
Status: In-flight testing
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Scale: 2-rotor 110 lbs collective and cyclic
Status: In-flight testing
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ERAU Paver

ERAU Paver

Scale: 8-rotor 500 lbs collective and cyclic
Status: In-ground testing
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Boeing CAV

Boeing CAV

Scale: 12-rotor 1,200 lbs fixed pitch rotors
Status: +100 flights

Related to TC2, we will leverage our decade-long experience and the array of hardware we have to develop better models and tools that fully optimize hybrid systems for aerospace and eVTOL applications. Our team will investigate battery packs and serial hybrids powered by reciprocating engines running on Avgas and Jet A for their superior noise, fuel consumption and affordability relative to turbine-hybrids.

Existing Hybrid and Fully Electric Powerplant Hardware

ECO Eagle Parallel

Eco Eagle Parallel

Scale: 30 kw battery/75 kw ICE AvGas
Status: Retired
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eSpirit Fully Electric

eSpirit Fully Electric

Scale: 75 kw battery
Status: Operating
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ERAU Boeing Serial

ERAU/Boeing Serial

Scale: 150 kw peak/100kw continuous AvGas ICE generator
Status: Operating
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Verdego Serial

VerdeGo Serial

Scale: 500kw peak/200 kw continuous Jet A diesel ICE generator
Status: Summer 2020

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Research into the interaction between the battery pack and the vehicle control system to maximize safety and minimize weight will be conducted in relation to TC3. To meet the needs for an eVTOL vehicle, state-of-the-art rechargeable lithium-ion batteries must be improved in specific energy, specific power, and safety. Our team will develop computational models for nano-Si/polymer that can be employed as the anode in next generation Li-ion batteries. The theoretical specific gravimetric capacity of silicon is about 10 times higher than that of commercially used graphitic anodes. Likewise, cobalt-free nickel-rich layered oxide is inexpensive, environmentally friendly and has a high reversible specific capacity. The combination of these as anodes and cathodes, could result in significant increase in specific energy at the cell level – research our team will pursue in addressing TC3. In parallel we will focus on high c-rate discharge, high specific power, thermal management and life cycle optimization at the pack level. Thermal management strategies are of particular interest to minimize the pack mass. Air cooling, channel cooling, phase change cooling and immersion cooling methods will be studied. For aerospace applications, the challenge is to develop battery models that optimize life (low cost) while achieving minimum weight for both cells and thermal systems.  

Existing Propulsive Battery Pack Hardware

Eco Eagle Gen I (Image not available)

Power: 10 kw-hr energy pack air cooled pouch cells
Status: Retired
eSpirit Gen 2

eSpirit Gen 2

Power: 24 kw-hr energy phase change cooled cylindrical
Status: Operating
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ERAU Gen 3

ERAU Gen 3

Power: 4.5 kw-hr power pack, immersion cooled cylindrical (high C-rate)
Status: Summer 2020
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Verdego Gen 1

VerdeGo Gen I

Power: 67 kw-hr energy pack, immersion cooled cylindrical
Status: Operating
Learn More

Finally, related to TC4, we will focus on vehicle-level performance optimization with the aim of reaching a safe level of autonomy to survive when unforeseen scenarios and failures are encountered. Our research will result in optimal design tools for multi-rotor placement and fault-tolerant adaptive flight control algorithms for superior flight control authority and agile and safe maneuvers in case of failures or wind disturbances. We consider various hardware and software limitations, such as uncertain hardware failure modes (e.g., faulty actuators, sensors, attrition of platforms), or limitations in platforms (e.g., endurance, mobility, power, and computational capacity) to develop and update vehicle design architectures and experimental testbeds. 

Assessment of what is innovative or novel in the proposed concept and how it will contribute to the chosen strategic thrust outcomes     back to top

Novel aspects of the proposed research include:

  1. Hardware validated scalable fault tolerant propulsive pods with an emphasis on safety (including engine out), efficiency, and noise
  2. Hardware validated optimal design and thermal management strategies to achieve battery pack high current-rate power production for emergency operation versus battery life cycle and weight
  3. Hardware validated vehicle-level optimal thermal management strategies
  4. Theoretical and experimental research to uncover and understand the tightly coupled dynamics between vehicle control, propulsion, power generation, and noise

Expected Research Products     back to top

  1. Hardware generated data that will allow calibration of the next generation of models and tools within the NDARC ecosystem related to safety, efficiency, quietness, performance and affordability
  2. Gap analysis and evaluation of the existing NASA and other public domain AAM design and analysis tools
  3. New multidisciplinary design, analysis and optimization tools incorporating dynamic couplings among various eVTOL systems and performance
  4. A high-fidelity model for a generic full-scale eVTOL and its MatLab/Simulink toolboxes
  5. Physics-based fault-tolerant control laws for safe increased autonomy and accounting for vehicle dynamics, aerodynamics, and practical hardware limitations
  6. Physics-based computational models for design of the next generation Li-ion batteries
  7. Draft methods and means of certification compliance

Anticipated Transition Opportunities of Research Products/Technologies to NASA or the U.S. Aviation Industry     back to top

The outcomes of the proposed research include public domain models, tools and data aimed at advancing the current state-of-the-art, improving the understanding of eVTOL and its application for AAM. Our team members and partners have an established track record of working with NASA, FAA and other government agencies and aerospace and aviation industry. As such, the proposed research, if funded, will easily transition and benefit this nascent industry through close interaction of our researchers with NASA scientists, the U.S. aviation industry and through public tools, models, data and algorithms.

Overall Teaming and Education Strategy     back to top

Our university industry partnership is strategic and brings significant, relevant, and complementary experience and expertise to achieve the objectives of the proposed ULI – bringing the development of a certifiable passenger-carrying eVTOL steps closer to reality. As an aviation and aerospace focused university, Embry-Riddle brings a wealth of experience in aircraft design, hybrid electric propulsion, hardware development and certification, while the University of Florida, Caltech, and Tuskegee University bring national level expertise in battery research, controls, autonomy, and multidisciplinary systems and optimization — all of which are vital to this ULI. Our industry partners include Boeing, a leader in developing AAM vehicles; VerdeGo Aero, a leader in eVTOL design and development; and Flight Level Engineering, an innovator with unique experience and ongoing contracts with the FAA and NASA for AAM concepts and certification.

Our team can rapidly advance the state-of-the-art in eVTOL while broadening the knowledge base across multiple U.S. educational institutions. Our education strategy includes sponsoring a large number of graduate and undergraduate students at the three participating universities and thus contributing to furnishing a pipeline of talent needed for full deployment of AAM. The aeronautics curriculum has largely remained traditional and built around conventional aircraft. We will develop curricula where design, aerodynamics, controls, electric power generation and propulsion, acoustics, and autonomy come together in cross-disciplinary content. Students at all three universities working under this ULI will regularly interact with each other and with our industry partners and have opportunities for internship and employment. These opportunities will be advertised through AIAA, VFS, IEEE, and SAE to attract and recruit talented students to our three institutions.

Contact Us

Questions on the NASA ULI research proposal submission may be referred to: 
Principal Investigator Dr. Maj Mirmirani 
Co-Principal Investigator Dr. Pat Anderson