Understanding the Challenges

Most, if not all, of these emerging design ideas in eVTOL aircraft face similar challenges when it comes to proving qualified flight and ensuring an understanding, in fine detail, of those issues which can mean the difference between flight success and failure.  This is very evident when attempting to scale up designs and evaluate core stability and control, flight control system designs and their evaluation, and the development of high fidelity simulation model standards for pilot training and subsequent certification.

eVTOL aircraft are generally unconventional in their design, operation and performance and these facts present specific challenges in their on going development, analysis, test flying and simulation. These challenges break down into a number of  areas:

• The distribution of mass and its impact on inertias throughout all phases of flight.
• The impact of multi rotor vortices and their interactions especially in downwash and upwash dynamics and downwash ingestion.
• The flight control system logic that is able to model pilot control inputs and the corresponding response of the motors, rotors and the aircraft.
• Changes in the torque required from electric motors and the resulting aircraft response to increases and decreases in power.
• Better understanding of the aircraft responses and rates of change and their impact on pilot initiated oscillations (PIO’s), resulting in potential loss of control.
• Failure modes and their outcomes
• Being able to demonstrate tangible and measurable progress and improvement to stake holders and investors.

High Fidelity Simulation

The j2 Universal Tool-Kit is already able to develop high fidelity digital twin models of any aircraft. This built-in capability comes from a background of aircraft design and engineering, working with both conventional and unconventional designs to ensure safe and stable flight.   This ensures that the engineers and designers have all the information necessary to maintain and deliver controlled and stable responses, especially in the transitions from hover to forward flight and back.  This information can then be used in controller scheduling to enable smooth transitions. The pilot-in-the-loop simulation capabilities enables the design to be fully tested off-line and evaluated against any criteria or standard prior to first flight.

The aero engineering origins of the j2 Universal Tool-Kit ensures that the focus is always on developing accurate results.  This experience is built into the software so there is no need for engineers to write code or develop block diagrams to build up the model.  j2 Builder provides a comprehensive hierarchical structure where detailed data tables can be created and structures located to provide the correct aerodynamic characteristics, with motors positioned to give the appropriate moment arms.  Using j2 Freedom, it is possible to create manoeuvres that can be run over a range of known, fixed, initial conditions.  The manoeuvres can be repeated as the design evolves to provide a consistent testing, simulation and evaluation environment.

As further information is developed so high fidelity simulations of subsystems such as electrical, hydraulic, flight controls (FCS) can be integrated with the model through the j2 Developer / j2 Matlab ToolBox API’s to produce a complete Digital Twin of the whole aircraft.

Aircraft Certification

Understanding the challenges in the area of Stability & Control analysis and Aircraft Certification processes when using an existing High Fidelity Modelling and Simulation environment has put j2 Aircraft Dynamics in the ideal position to investigate the certification of eVTOL aircraft.  j2 Aircraft Dynamics already work with organisations globally to support the development of standards for handling qualities and ultimately certification.  Whilst the optimum approach to piloting is still to be determined, what has become paramount is that any move to Certification will require much more than some preliminary gaming simulation and a comment that “it flies great”.  The certification process will require detailed modelling and simulation of the aircraft across the complete flight regime.  This will need to be further supported and cross referenced to flight test data that can then be used to demonstrate the validity of the modelling environment. Using j2 Flight, flight test data can be collated and compared to the model through the sue of Reprediction and the model can be refined using the automatic Regression techniques.

Due to the complex nature of the systems and the almost total reliance on flight control laws, a lot of the failure model analysis and safety cases will need to be thoroughly tested in an off-line simulation environment.  These tests need to be repeatable and should only be conducted on a full Digital Twin model that covers all elements of the system under test as opposed to the use of a generic gaming solution.