Digital Twin for Design and Certification
PAV's and eVTOL aircraft are more than scaled up drone designs and as such need to be designed and analysed just as you would with any manned aircraft. This means the use of high fidelity simulation, modelling and analysis tools to assure stability and control, handling qualities, and safety in failure modes are all closely studied. This can be achieved through the j2 Universal Tool-Kit. Whether it's a multi-rotor, vectored thrust, or lift-cruise configuration, every design has to go through the process of transition from vertical flight to forward flight and back again smoothly and safely. This can be tested, evaluated and modified throughout the design process with the j2 Universal Tool-Kit. From transition to performance analysis across the entire flight regime or the development and evaluation of flight control systems, everything is possible. The complete certification process can be evaluated offline and cross referenced with real flight test data off full scale aircraft to ensure a safe air vehicle and a successful aircraft certification.
High Fidelity Modelling
Each type of eVTOL has many unique characteristics. To be able to build a model suitable for performing true stability and control analysis and handling qualities assessments across the complete flight envelope takes more than shaping a few curves and flying the eVTOL in a game. With j2 Builder, the model is a complete breakdown of all the aircrafts contributing parts located in the correct position and assembled in a logical hierarchy mimicing the aircrafts configuration. Aerodynamic data can be built from any source using the integrated aerodynamic strip theory in j2 Elements to provide dynamic characteristics from limited airfoil section data through to wind tunnel or CFD data. Multi rotor downwash contributions can be added and focussed on the individual strips without needing to update the aircraft coefficient information. Fuel cells and batteries can be located about the airframe to provide Centre of Gravity and inertia investigations as well as varying payload configurations with the modelling environment easily accommodating the analytical challenges of scaling up in size.
Any number of engines can be adjusted in tilt and toe, throughout the flight, to cover tilt rotor, thrust vectoring or lift cruise configurations or multiple force items can be added to provide multi-rotor configurations. Rotor and system dynamics can be introduced from simple look-up tables through to interlinked tables and equations to provide detailed simulations of the loading and interactions. The model is constructed through the graphical user interface of j2 Builder so items are easy to find and cross reference.
Understanding the aircraft’s behaviour requires many tests and test points throughout the aircraft’s envelope. As the aircraft moves towards certification, so these tests can be in the order of 1000’s. The tests need to be consistent and repeatable so that they can be re-evaluated as the design changes. This approach is not feasible in a real-time simulation. This process of offline simulation (Response Analysis) involves the production of various fixed manoeuvres that will demonstrate the behaviour of the aircraft to different pilot inputs and calculate the response over ranges of flight conditions, configurations and payloads is all possible through the use of j2 Freedom. Cross plots and comparison charts used when assessing airframe safety limits can be produced automatically from user defined templates with j2 Visualize.
Handling Qualities Assessment
As the standards develop so the aircraft will need to be assessed against the requirements for certification. Whether this is identifying natural and augmented stability and FCS design through root locus charts or cross checking the modes of motion against the existing or new standards, this can be achieved with the linear analysis capability available in j2 Classical. Modes of motion are identified along with their respective frequency and damping across the whole flight envelope. State space matrices can be found to support flight control system design.
Flight Control System Design
One of the key aspects of eVTOL’s to remember is that the rotor dynamics do not scale up from a drone to a full scale aircraft. So, when considering FCS design, control systems that work with a drone may not be suitable to control a manned air vehicle. High fidelity models built within the j2 Universal Tool-Kit can have Flight Control components added to them directly through j2 Builder. Alternatively, the complete aircraft model can be added to a Simulink diagram for the AFCS to be wrapped around it. The complete Closed Loop system can then be evaluated back into the j2 Universal Tool-Kit using the j2 Matlab Toolbox.
Offline and Realtime Simulation is available with the j2 Universal Tool-Kit through either j2 Freedom or j2 Pilot respectively. This provides a consistent version controlled high fidelity model that can be used across both systems simultaneously. As the model is developed into a Digital Twin to include electrical, thermal and hydraulic systems, along with onboard avionics, so the offline tests can be re-run and the pilot can evaluate the aircraft without any code needing to be written.
Failure Mode Analysis
With non-conventional designs, the response of the aircraft to failures is less well known and a lot more difficult to evaluate. With a Digital Twin built in the j2 Universal Tool-Kit it is possible to evaluate any range of failure scenarios from single/multiple engine failures, power system failures, thermal failures to system interrupts and interference. Each of these can be evaluated against desired responses and safety criteria to ensure that the safety standards can still be met.