2021

Sheikh Shahriar Ahmed (sheikhsh@buffalo.edu), University at Buffalo, SUNY
Grigorios Fountas, Edinburgh Napier University
Ugur Eker, Turk Hava Yollari
Stephen Still, University at Buffalo, SUNY
Panagiotis Anastasopoulos, University at Buffalo, SUNY

A new transportation mode that can simultaneously operate on land and in the air, namely the flying cars, is anticipated to penetrate the automobile fleet between 2020 and 2025.  Due to their flexible mobility patterns and automated operational characteristics, flying taxi and shared flying car services are expected to expand the existing shared mobility services (such as Uber, Lyft and similar services) of the urban transportation network.  Despite their forthcoming introduction in the shared mobility market, public perceptions and expectations about these services have not been investigated in travel demand literature. This study aims at providing an exploratory analysis of public perceptions and expectations towards the introduction of flying taxis and shared flying car services and at identifying the determinants of such expectations.  Using data collected from an online survey, individuals’ willingness to hire and willingness to pay for flying taxi and shared flying car services are statistically modeled within a correlated grouped random parameters bivariate probit framework. The analysis shows that various socio-demographic characteristics and individuals’ opinions towards the perceived benefits and challenges of flying cars affect public willingness to hire and pay.  Even though the awareness about the operation of flying taxis and shared services is very limited in public sphere, the findings of this study can provide insights on the challenges that policymakers, manufacturing companies and shared mobility providers will face with the introduction of such flying car services in the transportation networks.

Pavan Yedavalli, University of California, Berkeley

Increased congestion and emissions due to a growing rate of urbanization has crippled mobility in metropolitan areas across the world, as seen in times both before the COVID-19 pandemic and as societies begin to live with COVID-19 as they return to normalcy. With distributed electric propulsion technology and battery energy densities improving, urban air mobility (UAM) has emerged as a potential alternative to the automobile for longer distance trips. However, the actual deployment of UAM remains uncertain. This paper provides increased clarity of practical UAM deployment by (1) developing a tool to design vertiport networks under specific land use constraints, and (2) exploring the travel time impacts of these networks using regional-scale traffic microsimulation. Using millions of trips in the San Francisco Bay Area as a case study, a k-medians clustering algorithm constrained to specific land use characteristics, including area of the parcel and zoning/building type, is developed. Further simulation and sensitivity analysis across these parameters reveal that as the number of vertiports increases, the average travel times of trips taken by UAM improve, but there is only benefit compared to driving above a certain number of vertiports. In addition, area constraints of the parcels deeply impact the travel times of UAM trips, suggesting that the research agenda must include efficient vertiport designs. By using real-world land use constraints, concrete UAM networks can be recommended to metropolitan planning organizations, local city officials, and regulatory bodies for future infrastructure provision, pushing UAM from pie-in-the-sky to boots-on-the-ground.

Sai Mudumba, Purdue University
Hsun Chao, Purdue University
Apoorv Maheshwari, Purdue University
Daniel DeLaurentis, Purdue University
William Crossley, Purdue University

Urban Air Mobility (UAM) operations provide the potential for more, or more attractive, trips in a metropolitan area relative to wholly surface-based transportation. But the emissions produced by a UAM mode must be studied in relation to these benefits. In this paper, an emissions model for the UAM context using electric vertical takeoff and landing (eVTOL) aircraft is developed that incorporates CO 2  gases emitted from electricity production required for charging of the vehicle batteries. The model quantifies trip emissions utilizing UAM for part or all of the trip and compares these with automobile-based trips. The estimations consider using gasoline and electric automobiles, with the impact of autonomy and average ground travel speeds. Trip case studies in Chicago and Dallas metropolitan areas showcase the regional differences when using UAM and different automobile technology scenarios. In particular, differences stemming from how electricity generation from power grids (i.e. grid emission index) contributes to CO 2 emissions of eVTOL trips and electric automobile trips in Chicago and Dallas metropolitan areas are computed. This paper introduces the Surface-to-Air Distance Ratio and Detour Ratio to understand how trip properties influence the emissions of a trip. Results from the simulation on identified trip cases in Chicago and Dallas illustrate the significant impact of the grid emission index of a region’s power grid on emissions of electric vehicles.

Li Zhang (lzhang@cee.msstate.edu), Mississippi State University
Amin Aghalari, Mississippi State University
jun Wang, Mississippi State University
Junfeng Ma, Mississippi State University

As the unmanned aerial system (UAS) gains considerable attentions for its use in avoiding ground traffic congestion, considerations must be made to mitigate the risk involved with collisions, both with other UAS and unmanned flights, which could cause damages to both property and personnel. We consider NASA’s Highway-in-the-Sky (HITS) concept to propose a model that minimizes a HITS’ exposure in the airspace, and therefore reduces collision risk. The area in which HITS will be designed is divided into grids. The exposure is defined as a convex function of the weighted number of grids and the weighted network length. The HITS is subjected to FAA’s no-flight zones and other FAA restrictions. A binary decision variable is assigned to each grid. Three performance measures: number of utilized grids, total network length, and total travel distance are used to evaluate the HITS’ safety, efficiency, and effectiveness compared to direct flight operations. The results of case studies indicate that the proposed HITS significantly reduces airspace exposure with a minor increase in total distance traveled. Sensitivity analysis has been performed to provide HITS designing, planning, and operation guidance. A numeric experiment in a large area indicates the model and solution could be practically used in regional HITS planning and operations. This research on the optimum HITS in reducing risk is the first to implement NASA’s HITS in the UAS network. The low-cost implementation of the proposed HITS can be immediately applied to mitigate the UAS risk.

Chenxuan Yang, University of Alabama
Jun Liu, University of Alabama
Weike Lu, University of Alabama
Ismael Jones, University of Alabama

The development of small-size vertical take-off and landing (VTOL) air vehicles enables urban planners to consider a new mode of transporting goods and people within a metropolitan area using the air space. Urban Air Mobility (UAM) relying on small-size vertical take-off and landing (VTOL) air vehicles has been trending as a possible future intra-region mobility service. Studies have been largely focused on large cities and there is limited discussion about implementing such a mobility system in small cities where the travel demand exhibits different spatiotemporal patterns. Using a case study of Tuscaloosa, Alabama area, this study provides some initial thoughts on planning airways for UAM in small cities. The goal of this study is to compare two strategies for operating a UAM system in small cities: 1) Vertiport-To-Vertiport Operation (VVO) that allows VTOL vehicles to fly beeline paths between origin and destination vertiports and 2) Hub-Based Operation (HBO) that all VTOL vehicles need to fly within airways between virtual hubs in the air. The VVO operation has the shortest flight distance and time; but it may result in difficulty in air traffic control (due to disorganized flight conflicts in the air) and environment issues (such as the noise). The HBO operation appears to outperform the VVO in terms of air traffic controls; but it will cause delays and detours (increasing energy consumption and emissions) due to potential congestions in airways. This study simulated a hypothetical UAM system, serving travelers in Tuscaloosa, Alabama. The simulation varied the demand levels (number of trips), cruising speed, and the number of hubs (for the HBO operation) to examine their impacts on the UAM system performance. Results show that compared with the VVO operation, the HBO operation could significantly reduce the noise impact area (by 50%) and eliminate almost all air traffic conflicts found in the VVO operation; however, the HBO operation could increase the flight distance by up to 27%. More results are discussed in the paper. The findings are expected to be valuable to stakeholders or investors who will consider deploying a UAM system in small cities in the future.

Khashayar Khavarian, Sharif University of Technology
Kara M. Kockelman (kkockelm@mail.utexas.edu), University of Texas, Austin

Traffic congestion is challenging most of the world’s cities, and one way to avoid traffic delays is to take to the sky, using vertical take-off and landing craft or “VTOL”. This study examines opportunities, costs, and energy impacts for eVTOL (electrically-powered VTOL) supply and demand across the Austin, Texas region. Using different demand levels and VTOL sizes (4 and 8 seaters - separately and in combination), we estimate minimum costs averaging $23.00 per person-trip/-flight and $0.92 per person-kilometer using 4-seaters, which is less than current ride-hailing costs in U.S. cities, but ride-hailing is door to door, while eVTOL relies on stations, with non-negligible access and egress costs.  We find 4-seaters offer the greatest energy and greenhouse gas savings (CO2equivalent), based on the Texas power grid’s current feedstocks. The rate of emission using eVTOLs in operational phase only is found to be similar to electric vehicles’ which is about 70 grams per kilometer on average, but the total emission through the lifetime of eVTOLs is almost twice that.

Rajesh Ahluwalia, Argonne National Laboratory
Jui-Kun Peng, Argonne National Laboratory
Dionissios Papadias, Argonne National Laboratory
John Kopasz, Argonne National Laboratory

Urban air mobility (UAM) is as an emerging concept of passenger and cargo transportation with piloted and increasingly autonomous operations. Several companies are developing enabling elements of UAM for air taxis, including prototypes of vertical take-off landing (VTOL) vehicles, operational concepts, and potential business models. The VTOL prototypes incorporate electric and hybrid powertrains for the UAM market, including multi-rotor and tilt-rotor crafts. Electric propulsion systems are especially attractive for air taxis because of potentially lower environmental and noise pollution. Many eVTOLS are using batteries for propulsion and charging them rapidly between the flights or swapping them for slow charging overnight. Rapid charging degrades the battery cycle life while swapping requires multiple batteries and charging stations. This study has conducted a technoeconomic evaluation of the eVTOL air taxis with alternate powertrains using hydrogen fuel cell systems being developed for light-duty and heavy-duty vehicles. We consider performance metrics such as fuel cell engine power, weight, and durability; hydrogen consumption and weight of storage system; and maximum take-off weight. The metrics for economic evaluation are capital cost, operating and maintenance cost, fuel cost, and the total cost of ownership (TCO). We compare the performance and TCO of battery, fuel cell and fuel cell – battery hybrid powertrains for multi-rotor and tilt-rotor crafts. We show that fuel cells are the only viable concept for powering multi-rotor eVTOLs on an urban scenario that requires 60-mile range, and hybrid fuel cells are superior to batteries as powertrains for tiltrotor eVTOLs.