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Rotorcraft Certification by Simulation

Periodic Reporting for period 2 - RoCS (Rotorcraft Certification by Simulation)

Reporting period: 2020-11-01 to 2022-06-30

Before entering into operation, new aircraft must obtain a type certificate from the governing aviation regulatory authority. This certificate testifies that the type of aircraft meets the safety requirements. After the definition of the certification basis, the aircraft manufacturer and the authority must agree on the certification program structure and on how to demonstrate regulation compliance. This compliance demonstration is the most expensive and time-consuming aspect of the certification process, involving high safety risk concerning control system and engine failures. Advanced analysis methods as flight simulation potentially offer an immediate benefit in the certification process for cost/time saving and risk reduction.
RoCS aims to explore the possibilities, limitations, and develop guidelines for best practices for the application of flight simulation to demonstrate compliance to the airworthiness regulations related to helicopters and tiltrotors. While flight simulation cannot fully replace all flight-testing activities, it allows reducing the number of flight tests with respect to current certification standards, within the OFE. Further tests outside the OFE that may lead to catastrophic failure could be conducted by flight simulation.
A three-way approach defines RoCS objectives:
• Developing guidelines for certification of rotorcraft using simulation by consolidating past experiences and dedicated research into a set of guidelines, supported by both industry and the certification authority, defining model and cueing system fidelity metrics. RoCS will develop a set of standard tools to evaluate these metrics in a fast, efficient, and reliable way.
• Developing a low-cost, effective, flight simulation environment for certification compliance demonstration for helicopters and tiltrotors, replacing the concept of flight simulation just for training. This will lead to the development of a new class of simulators that will be competitive to flight tests in terms of acquisition and maintenance costs, affordable also by small rotorcraft companies, and a driver in the development process for certifying aircraft.
• Verifying if certification by simulation could reduce the scope of testing required for the introduction of the next generation of tiltrotors. This final ambitious goal, reached through the issue of final guidelines for CSRFA verified on tiltrotors, will represent a formidable tool to compensate for the lack of experience of both the manufacturers and the certification authorities in using simulation to support certification. It will limit significantly the amount of potentially hazardous flight-testing currently performed in the certification process.
After 38 months, 18 RoCS deliverables were completed and 11 milestones were achieved. In addition to WP2 completed in the first year, WP1, and WP7, we achieved important scientific results:
• WP3: past research, industry practice, and proposed certification application of flight simulation model predictive fidelity metrics and tools for the assessment of simulation credibility in the absence of validation test data were detailed. A secure version-control server for collaborative simulation model development was set up. Extensive time and frequency domain Verification and Validation activities were performed on the AW109 flight simulation model and available flight test data. Various model fidelity enhancements are being developed to further improve the correlation with test data. This includes state-space rotor inflow model identification from a high-fidelity Vortex Particle Method solution, multiple-Aerodynamic Computation Point rotor-on-fuselage interference sampling, maneuver wake distortion and model renovation. Within this WP it has been drafted the first Guideline for Certification by Simulation, a document that contains a structured process to develop credible models to be used for certification.
• WP4: the literature review on simulator cueing fidelity metrics and a proposal for metrics to be used within the simulation campaigns was produced. The definition of possible scenarios for simulation cueing tests has started, alongside the integration of new Virtual/Augmented Reality technology into the NLR’s simulator. A set of test at partners’ simulation facilities was planned. The reduction of COVID-19 restrictions is allowing a restart of simulation testing activities.
• WP6: the modification of the AWARE simulator to integrate it with the new PoliMI projection screen were performed. A new carpentry frame was designed to adapt the AWARE simulation height to the newly developed spherical screen. This system has been mounted and tested at LH facilities.
RoCS proposes a potential breakthrough innovation with significant impacts for the processes used by the target industry. In the domain of certification, the new methodologies and fidelity metrics developed will allow a more frequent, standardised, and cost and time-effective application of flight simulation in rotorcraft certification, going beyond the current practice for fixed-wing aircraft. Significant progress is foreseen in the following areas:
• Modelling of rotor wakes, interference effects and the aerodynamic environment, (including two-way rotor body interference effects and the interference of rotor wakes with obstacles),
• Structural aeroelastic modelling (by adding aeroelastic models of rotor/wing/pylon systems to improve loads and vibrations generated in flight and transmitted to the airframe),
• Control system modelling,
• Model fidelity evaluation metrics producing a methodology for defining task-specific predictive and perceptual fidelity metrics),
• Credibility of simulation models (providing methodologies to verify and validate the credibility of models outside of the tested envelope),
• Physics-based model update methods (providing provide a systematic physical tuning process using flight test data for fidelity enhancement and simulator acceptance and certification
• Flight simulators for certification developing an effective but low-cost COTS simulation system,
• Flight simulator visual system (including a system of mirrors designed to limit the number of projectors to obtain the required wide field of view, possibly complemented by AR devices),
• Motion and vibration cueing systems in flight simulators (proposing a low-cost simple and effective motion platform for motion and vibration cues).
RoCS will have an important impact on certification test safety, economy, duration, and effectiveness. The risks associated with rotorcraft certification compliance demonstration will decrease significantly. Safety can be improved by demonstrating compliance by flight simulation, allowing the most hazardous test to be performed in a safe environment, reducing risks in case of failure and preventing potential accidents. The number of virtual tests can be increased, so the data collection can be improved to gain greater insights into rotorcraft characteristics. Costs of certification will be reduced by removing the possibility of test repetitions. Cost reduction could be up to 88%. RoCS will allow reducing the time required to complete rotorcraft certification and the time-to-market of new products. RoCS expects to form the first steps in the classification of simulation devices for CSFRA topics, to define a new family of flight simulators.
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