AEROSPACE Modelling and simulation for urban air mobility

Simulating the urban air mobility future

GORDON WOOLLEY FRAeS from the RAeS Flight Simulation Group considers what modelling and simulation is needed to de-risk the futuristic vision of ‘aerial taxis’ navigating the skyscrapers of tomorrow’s megacities.

The VoloCity Volocopter personal air vehicle is hoping to be the future of urban travel. Volocopter

Recent RAeS activities, including conferences and articles in AEROSPACE, have examined many aspects of the development of electric air vehicles (EAVs), particularly for urban air mobility (UAM). The Flight Simulation Group, for its part, has begun to look at how modelling and simulation can be used to support those developments. This has been given new impetus by the release, within the past few months, of important papers by EASA – Opinion 01/2020 High level framework for the U-space, – and the FAA – Concept of operations (Conops) v1.0 for UAM and Concept of operations v2.0 for Unmanned air systems traffic management (UTM) and a proposal by a major Chinese company, EHang – The Future of Transportation: White Paper on Urban Air Mobility Systems.

These documents set the stage for the first ventures into bringing EAVs into service and sketch out the operating environments, constraints and standards and compliances needed to get the process off to a sound start.

All acknowledge the potential, at some point in the future, of a full range of platforms from passenger-carrying air taxis at the top end and large freight-carrying air vehicles to small drones weighing a couple of kilos.

  • These will operate between large, well-resourced heliports, or simpler bases from parcel distribution centres, from hospitals and emergency services bases and from ad hoc sites.
  • Their roles will include passenger carrying, the delivery of freight and packages, survey work of various kinds, event and media coverage, recreational and sporting activities and some purposes likely to emerge which we cannnot yet imagine.
  • They will be operated in close proximity to the ground, in the most hazardous and sensitive blocks of airspace.
  • They will perform far more varied flight profiles between a far greater range of sites, at far higher tempo, than established aviation services.
  • They will use technologies which have yet to be fully developed.
  • Their operation will raise public concerns over issues such as noise, nuisance, interference with other everyday activities and, not least, privacy.

CFD modelling of an eVTOL but aerodynamics simulation is just one part needed. NASA​The EASA document Opinion 01/2020 centres on the concept of a ‘U-Space’ and its Executive Summary includes the statements:

The objective of this opinion is to create and harmonise the necessary conditions for manned and unmanned aircraft to operate safely in the U-space airspace, to prevent collisions between aircraft and to mitigate the air and ground risks.

Therefore, the U-space regulatory framework, supported by clear and simple rules, should permit safe aircraft operations in all areas and for all types of unmanned operations. This Opinion proposes an effective and enforceable regulatory framework to support and enable operational, technical and business developments, and provide fair access to all airspace users, so that the market can drive the delivery of the U-space services to cater for airspace users’ needs. This Opinion is, therefore, a first regulatory step to allow immediate implementation of the U-space after the entry into force of the regulation and to let the unmanned aircraft systems and U-space technologies evolve.

EASA concludes that the U-space regulation should be performance- and risk-based, while ensuring consistency and interoperability with other associated EU regulations, with a combination of flexibility for member states or even local implementation.

FAA Conops V2 Unmanned aircraft systems traffic management – (UTM) ​

The Executive Summary defines UTM as the manner in which the FAA will support operations for UAS operating in low altitude airspace.

  • It is a community-based traffic management system, where the operators and entities providing operation support services are responsible for the co-ordination, execution, and management of operations, with rules of the road established by the FAA.
  • This federated set of services enables cooperative management of operations between UAS operators, facilitated by third-party support providers through networked information exchanges.
  • The services provided are interoperable to allow the UTM ecosystem to scale to meet the needs of the UAS operator community.

Flight intent will be submitted and shared among operators for situation awareness in the form of an operation plan, which will be developed prior to the operation and indicate the four-dimensional (4D) volume of airspace within which the operation is expected to occur, the times and locations of the key events associated with the operation, including launch, recovery, and any other information deemed important.

UTM operators will ultimately be responsible for maintaining separation from other aircraft, airspace, weather, terrain and hazards, and avoiding unsafe conditions throughout an operation.

Remote identification (RID) will provide a means to address public concerns and protect public safety vulnerabilities associated with low altitude UAS operations, including privacy and security threats. The FAA, in co-ordination with NASA, industry and the greater UTM community, is implementing a spiral development of UTM, starting with low-complexity operations and building in modules, higher complexity operational concepts and requirements.

Stages of development are based upon three risk-oriented metrics: (1) the number of people and amount of property on the ground, (2) the number of manned aircraft in close proximity to the UAS operations, and (3) the density of UAS operations.

NASA’s vision of future crowded skies. NASA

FAA Conops V1 urban air mobility (UAM) ​

UAM is a subset of UAT and will be developed to be consistent with its principles. UAM development focuses on the transition from the traditional management of air traffic operations to the future passenger or cargo-carrying air transportation services within an urban environment. The mature state operations will be achieved at scale through a crawl-walk-run approach, wherein:

  1. Initial UAM operations are conducted using new vehicle types that have been certified to fly within the current regulatory and operational environment.
  2. Higher tempo UAM operations are supported through regulatory evolution and UAM corridors that leverage collaborative separation methodologies.
  3. New operational rules and infrastructure facilitate highly automated traffic management, enabling remotely piloted and autonomous vehicles to safely operate at increased operational tempos.

Aircraft automation level: UAM operations will evolve from a pilot-in-command (PIC) onboard the UAM aircraft to remote UAM PICs. The following categories describe the evolution of aircraft automation:

  1. Human-within-the-loop (HWTL)
    • Human is always in direct control of the automation (systems).
  2. Human-on-the-loop (HOTL)
    • Human has supervisory control of the automation (systems).
    • Human actively monitors the systems and can take full control when required or desired.
  3. Human-over-the-loop (HOVTL)
    •  Human is informed, or engaged, by the automation (systems) to take action.
    •  Human passively monitors the systems and is informed by automation if, and what, action is required.
    •  Human is engaged by the automation either for exceptions that are not reconcilable or as part of rule set escalation.

Both EASA and the FAA take a prescriptive, cautious step-by-step but open-ended approach to a very challenging set of issues. In setting out their ideas for this new and rapidly but unpredictably growing, aerospace sector they recognise that:

  • The documents are very much ‘foundational’. As developments proceed, technologies evolve and information and experience are gathered, major regulatory revisions will be needed.
  • Initial operations of passenger-carrying craft will be piloted, with automation introduced incrementally.
  • Developments in platforms, operating bases, technologies, systems, services and so on will be market-led.
  • Operations will depend on effective co-operation and communication between stakeholders.
  • Their documents raise many questions and will not satisfy many of those keen to get the market moving.
  • Not least, the authors note that the complexity of flightpaths, mission profiles and the growing traffic density will raise issues which cannot yet be identified and pose problems which current technologies cannot yet resolve.

By contrast, the EHang proposal proposes a ‘full on’ approach, under centralised control, with predetermined flight paths and aircraft under totally automated flight control. The White Paper states that:

‘Based on our experience with AAVs and our research, we believe UAM, as a revolutionary idea, can be implemented now, yet in a more innovative way.’

‘Safety would be the first priority, so any UAM vehicle needs to be outfitted with power redundancy provided by multiple motors and propellers and backup systems.’

‘Smart’ UAM vehicles to us mean that they are piloted autonomously, which not only obviates the need for an in-vehicle pilot and the associated costs, but also enhances safety and makes the vehicle more controllable from the ground.

‘Finally, cluster management techniques centralised at a ground-based command-and-control centre would allow UAM operators to control a multitude of vehicles simultaneously in an orderly and safe manner. All flight routes could be pre-registered and predetermined so that UAM vehicles can travel only between certified ‘base points’’.

However, the concept also includes some statements which may not be acceptable in Western jurisdictions:

  • ‘It is difficult to imagine that the safety of an autonomous UAM system can be guaranteed if each vehicle is allowed to fly freely in cities’.
  • New technological advancements, especially the development of centralised command-and-control platforms, have made UAM vehicle manufacturers natural UAM operators.
  • A centralised command-and-control platform ensures all air vehicles are registered and controlled to fly on specific routes set by computers.
  • The elimination of unnecessary competitors will lead to sufficient pricing power for the remaining operator(s) to ensure reasonable investment returns.

M&S research There is clearly a significant difference between the way the primary Western regulators see the evolving UAS sector and that postulated by EHang. To bring either, or some other form, into reality will require a great deal of study and research into the enormous variety of issues and challenges the platforms, systems, activities, capabilities, limitations, and so on, that the documents allude to.

Testing and resolving these operational and regulatory issues will be heavily dependent on modelling and simulation-based research (M&S). The uses of simulator-based training for, for instance, flight crews and maintenance personnel, are well understood and these will be expanded into training UAS operators and controllers. Less well known are a wide variety of research areas, such as ship air-wake modelling for establishing helicopter power and manoeuvre margins, air traffic flow patterns to optimise approach paths and separation and human factors research into training transfer, crew communication and co-operation. These indicate the breadth of M&S research and noted below are some examples to illustrate where M&S can help resolve some of the problems and challenges.

The EHang Command Centre for its passenger eVTOLs. eHang

M&S research project Example 1

Modelling the UTM architecture

One of the biggest challenges is putting together a control and co-ordination architecture which can ensure the optimum use of the available airspace, and be flexible and responsive enough to cope with changes in traffic demand and priority patterns, weather restrictions, sudden events and incidents, aborted flights, and so on. The sorts of questions M&S can address include:

  • Is the idea of a segregated U-space feasible?
  • What should be its vertical extent, and its overlap with conventional ATM?
  • What would be the best traffic management scheme – a combination of trajectory allocation and management, defined corridors, free flight operation area allocations, sanctuary levels and deconfliction heights, and so on.
  • Could it be co-ordinated into the trajectory-based operations (TBO) international model and linked with regulatory plans, such as the Eurocontrol 4D TM projects in support of SESAR, and the real time simulation investigation supporting it?
  • Is the complex multi-agency model envisaged by the FAA likely to be responsive and flexible enough to be scalable to the extent anticipated, or is EHang’s centralised command and control model more promising?
  • Would the restriction posed by the centralised registered and approved flight model be acceptable to Western societies?
  • How is deconfliction and avoidance to be achieved?
  • Who allocates priorities for airspace use, such as emergency services, the movement of human organs or the media desire to cover emerging situations?
  • What impact is acceptable to ongoing operations of changing priorities?
  • How are ‘rogue’ operators to be identified and controlled?
  • How is resilience to be guaranteed?

Example 2 UAS ‘Volumes’ ​

To assure safe procedural separation and the manoeuvre room for avoiding action, UAVs will require volumes, or bubbles, of protected airspace around them.

Smaller ‘bubbles’ would permit greater levels of activity, ie ‘tempo’, and M&S can help optimise the size and shape of any required ‘bubble’ by evaluating UAS platform and system parameters, such as:

  • Size of the UAS
  • Speed
  • Take-off and climb-out, and descent and landing parameters
  • Its agility and power and manoeuvre margins
  • Precision of navigation and path-following capability
  • ‘See and avoid’ or ‘detect and avoid’ systems effectiveness
  • Its ‘decision/action’ cycle; how quickly and effectively can the platform recognise and react to the need to respond to a potential hazard?

Example 3 development of flight control automation​

Flight simulation is best known for flight crew training and much of this experience may be used to inform the development of UAS flight control automation. At whatever level automation is used, from assistance to the pilot-in-the-loop, to the human oversight of multiple vehicles, it will be essential for the human operators to understand the principles of the automation, the decision-making processes and why, when, and how to intervene when necessary. Flight crew training principles and practices, such as competency-based training, and threat and error management, flight path selection and control strategies, and experience with a diversity of genders, cultures, and so on can be used to develop the programmes and algorithms used in UAS control systems.

Example 4 Privacy, nuisance and security ​

Privacy, the minimising of nuisance, and the assurance of safety and security, must be accurately assessed before UAVs are used. Many of the threats and sensitivities can be modelled and assessed by a combination of simulation and practical tests.

How will eVTOLs or hybrid-electric designs like the Q-Starling overcome fears about noise pollution by residents?

Privacy is a particular concern because many UAVs will be equipped with ultra-high-definition cameras, face recognition technology and the ability to interface with personal smart devices. In busy areas, such as proximity to parcel depots, there may be dozens of drones each day flying at very low level over homes and gardens, schools, parks, businesses, etc.

Public concern about increased annoyance, discomfort, mental and emotional distress and the nuisance from aircraft noise and visual pollution must be addressed and allayed.

Security is another major concern. UTM introduces new security challenges due to reliance on interconnectivity, integration and network complexity, which provide opportunities for cyber incidents and attacks – including threats to system security and unintended or malicious degradation of system performance.

Furthermore, UAS in the hands of malicious actors pose a security threat to public spaces, critical infrastructure, sensitive sites (eg prisons, police facilities, military installations) and both high-profile and private persons. UAS operations in such environments must comply with strict security requirements.

Samad Aerospace

Example 5 Wind and turbulence modelling

A great deal of research effort is devoted to ship wake modelling to support effective simulation-based training for helicopter and fixed wing pilots. Low mass UAS, especially those with low rotor loading, will be greatly affected by local wind velocity and turbulence as well as individual and clusters of large buildings and other structures, which proliferate in UAM airspace which will generate their own air mass characteristics. Additionally, small UAS will be vulnerable to weather effects, such as icing and heavy precipitation and performance affected by extremes of temperature.

M&S can be used to research UAS power and manoeuvre limitations, the parameters and frequency of local weather reports and the measuring systems needed to support them, and guiding the operators who make Go/NoGo decisions.

Conclusion​

Regulators are grappling with a range of novel platforms, airspace requirements and safety challenges against the background of rapidly expanding activities, the prospect of levels of activity far beyond conventional aviation operations and with much more public exposure and immediacy than traditional activities.

The companies developing the platforms are already investing considerable money and effort into performance, reliability, operating sites, and market viability, seeing the potential in this new sector. Many other stakeholders may be less well served unless their needs are identified and addressed.

Not the least are the public, who need not only to be informed of the ‘Gee Whiz’ aspects of ‘flying cars’ but the everyday practicalities, limitations, and risks of UAS and be reassured that these have been considered and that everything possible is being done to address privacy, security and safety concerns.

Thus, there are many aspects of the vision which need to be addressed if much of it is to be realised and there is a great deal that researchers, modellers and those involved in simulation can do to assist.

There is plenty for established aerospace professionals, from a number of disciplines, to get their teeth into, plenty for researchers and academia to study, and plenty in this novel sector of aerospace to attract and excite new and emerging generations. The M&S community should take an active interest in meeting these challenges and providing this ‘frame of reference’, filling in the blanks and answering emerging questions, as it progresses.  

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