The backward transition from cruise to hover flight is the crucial flight maneuver for tilt-wing aircraft, as it allows for a safe vertical landing. At the same time, this flight phase is especially challenging due to low thrust settings and thereby reduced slipstream effects, which push the wing states toward or beyond flow separation. This raises the question of how failure conditions further impact these already severe conditions and if the remaining flight performance suffices to perform the backward transition. This study therefore assesses the tilt-wing's fault tolerance against single and combined component failures. The reference scenario of a longitudinal level backward transition is investigated with optimal-control-based trajectory optimization on a six-degree-of-freedom aircraft model. The results indicate fault tolerance of the tilt-wing aircraft for most failure cases. The majority of failure cases leading to unfeasible transition trajectories suffer from unbalanced hover or cruise conditions. Only for the failure case of control surface runaways on the same side of the canard and main wing, the transition maneuver itself is the limiting factor due to increased thrust settings and thus longer transition times.

[1] Michaelsen O. E. and Martin J. F., "The Aerodynamic Approach to Improve Flying Qualities of Tilt-Wing Aircraft," 9th Anglo-American Aeronautical Conference, AIAA Paper 1963-0484, 1963. https://doi.org/10.2514/6.1963-484 Google Scholar

[2] May M., Milz D. and Looye G., "Transition Strategies for Tilt-Wing Aircraft," AIAA SciTech 2024 Forum, AIAA Paper 2024-1289, 2024. https://doi.org/10.2514/6.2024-1289 LinkGoogle Scholar

[3] Pradeep P. and Wei P., "Energy Optimal Speed Profile for Arrival of Tandem Tilt-Wing eVTOL Aircraft with RTA Constraint," 2018 IEEE CSAA Guidance, Navigation and Control Conference (CGNCC), Inst. of Electrical and Electronics Engineers, New York, 2018, pp. 1-6. https://doi.org/10.1109/GNCC42960.2018.9018748 Google Scholar

[4] Doff-Sotta M., Cannon M. and Bacic M., "Fast Optimal Trajectory Generation for a Tiltwing VTOL Aircraft with Application to Urban Air Mobility," 2022 American Control Conference (ACC), Inst. of Electrical and Electronics Engineers, New York, 2022, pp. 4036-4041. https://doi.org/10.23919/ACC53348.2022.9867852 Google Scholar

[5] Chauhan S. S. and Martins J. R. R. A., "Tilt-Wing eVTOL Takeoff Trajectory Optimization," Journal of Aircraft, Vol. 57, No. 1, 2020, pp. 93-112. https://doi.org/10.2514/1.C035476 LinkGoogle Scholar

[6] Panish L. and Bacic M., "Tiltwing eVTOL Transition Trajectory Optimization," Journal of Aircraft, Vol. 62, No. 1, 2024, pp. 1-13. https://doi.org/10.2514/1.c037862 Google Scholar

[7] Lu Z., Hong H. and Holzapfel F., "Multi-Phase Vertical Take-Off and Landing Trajectory Optimization with Feasible Initial Guesses," Aerospace, Vol. 11, No. 1, 2023, p. 39. https://doi.org/10.3390/aerospace11010039 CrossrefGoogle Scholar

[8] May M. S., Milz D. and Looye G., "Dynamic Modeling and Analysis of Tilt-Wing Electric Vertical Take-Off and Landing Vehicles," AIAA SciTech 2022 Forum, AIAA Paper 2022-0263, 2022. https://doi.org/10.2514/6.2022-0263 LinkGoogle Scholar

[9] Li B., Sun J., Zhou W., Wen C.-Y., Low K. H. and Chen C.-K., "Transition Optimization for a VTOL Tail-Sitter UAV," IEEE/ASME Transactions on Mechatronics, Vol. 25, No. 5, 2020, pp. 2534-2545. https://doi.org/10.1109/TMECH.2020.2983255 CrossrefGoogle Scholar

[10] Oosedo A., Abiko S., Konno A. and Uchiyama M., "Optimal Transition from Hovering to Level-Flight of a Quadrotor Tail-Sitter UAV," Autonomous Robots, Vol. 41, No. 5, 2016, pp. 1143-1159. https://doi.org/10.1007/s10514-016-9599-4 CrossrefGoogle Scholar

[11] Kubo D. and Suzuki S., "Tail-Sitter Vertical Takeoff and Landing Unmanned Aerial Vehicle: Transitional Flight Analysis," Journal of Aircraft, Vol. 45, No. 1, 2008, pp. 292-297. https://doi.org/10.2514/1.30122 LinkGoogle Scholar

[12] Fredericks W. J., McSwain R. G., Beaton B. F. and Klassman D. F., "Greased Lightning (GL-10) Flight Testing Campaign," NASA TM-2017-219643, July 2017. Google Scholar

[13] Wang M., Diepolder J., Zhang S., Söpper M. and Holzapfel F., "Trajectory Optimization-Based Maneuverability Assessment of eVTOL Aircraft," Aerospace Science and Technology, Vol. 117, Oct. 2021, Pper 106903. https://doi.org/10.1016/j.ast.2021.106903 CrossrefGoogle Scholar

[14] Lu Z., Hong H., Diepolder J. and Holzapfel F., "Maneuverability Set Estimation and Trajectory Feasibility Evaluation for eVTOL Aircraft," Journal of Guidance, Control, and Dynamics, Vol. 46, No. 6, 2023, pp. 1184-1196. https://doi.org/10.2514/1.G007109 LinkGoogle Scholar

[15] Vascik P. D., "Systems Analysis of Urban Air Mobility Operational Scaling," Ph.D. Thesis, Massachusetts Inst. of Technology, Massachusetts, CA, 2020. Google Scholar

[16] Al Haddad C., Chaniotakis E., Straubinger A., Plötner K. and Antoniou C., "Factors Affecting the Adoption and Use of Urban Air Mobility," Transportation Research Part A: Policy and Practice, Vol. 132, Deutscher Luft- und Raumfahrtkongress 2023 , Document ID 610319, Feb. 2020, pp. 696-712. https://doi.org/10.1016/j.tra.2019.12.020 CrossrefGoogle Scholar

[17] Babetto L., König R., Fassnacht J. and Stumpf E., "Study on Public Acceptance of eVTOL: Safety & Noise," 2024. https://doi.org/10.25967/610319 Google Scholar

[18] Bauranov A. and Rakas J., "Urban Air Mobility and Manned eVTOLs: Safety Implications," 2019 IEEE/AIAA 38th Digital Avionics Systems Conference (DASC), Inst. of Electrical and Electronics Engineers, New York, 2019, pp. 1-8. https://doi.org/10.1109/DASC43569.2019.9081685 Google Scholar

[19] Thompson E. L., Taye A. G., Guo W., Wei P., Quinones M., Ahmed I., Biswas G., Quattrociocchi J., Carr S., Topcu U. and et al., "A Survey of eVTOL Aircraft and AAM Operation Hazards," AIAA AVIATION Forum, AIAA Paper 2022-3539, 2022. https://doi.org/10.2514/6.2022-3539 LinkGoogle Scholar

[20] Basset P.-M., Vu B., Beaumier P., Reboul G. and Ortun B., "Models and Methods at ONERA for the Presizing of eVTOL Hybrid Aircraft Including Analysis of Failure Scenarios," Proceedings of the Vertical Flight Society 74th Annual Forum, The Vertical Flight Soc., Fairfax, VA, 2018, pp. 1-15. https://doi.org/10.4050/F-0074-2018-12673 Google Scholar

[21] EASA, "Special Condition for Small-Category VTOL Aircraft," 2019. Google Scholar

[22] May M. S., Milz D. and Looye G., "Semi-Empirical Aerodynamic Modeling Approach for Tandem Tilt-Wing eVTOL Control Design Applications," AIAA SciTech 2023 Forum, AIAA Paper 2023-1529, 2023. https://doi.org/10.2514/6.2023-1529 LinkGoogle Scholar

[23] Carroll T. B., "A Design Methodology for Rotors of Small Multirotor Vehicles," M. Sc. Disseration, Ryerson Univ., Toronto, May 2021. https://doi.org/10.32920/ryerson.14649630.v1 Google Scholar

[24] Gill R. and D'Andrea R., "Computationally Efficient Force and Moment Models for Propellers in UAV Forward Flight Applications," Drones, Vol. 3, No. 4, 2019, p. 77. https://doi.org/10.3390/drones3040077 CrossrefGoogle Scholar

[25] Khan W., "Dynamics Modeling of Agile Fixed-Wing Unmanned Aerial Vehicles," Dissertation, McGill Univ., Montreal, Canada, 2016. Google Scholar

[26] Phillips W. F., Mechanics of Flight, Wiley, Hoboken, NJ, 2004, pp. 35-42. Google Scholar

[27] Olson E. D., "Semi-Empirical Prediction of Aircraft Low-Speed Aerodynamic Characteristics," 53rd AIAA Aerospace Sciences Meeting, AIAA Paper 2015-1679, 2015. https://doi.org/10.2514/6.2015-1679 LinkGoogle Scholar

[28] Kirkpatrick D. G. and Murphy R. D., "Planning Wind-Tunnel Test Programs for V/STOL Conversion Studies," AIAA 3rd Aerodynamic Testing Conference, AIAA Paper 1968-0400, 1968. https://doi.org/10.2514/6.1968-400 LinkGoogle Scholar

[29] Cook J. W., "Exploration of Dynamic Transitions of Tiltwing Aircraft Using Differential Geometry," Ph.D. Thesis, Univ. of Colorado, Boulder, CO, 2022. Google Scholar

[30] May M. S., Milz D. and Looye G., "Towards the Determination of the Dynamic Transition Corridor for Tandem Tilt-Wing Aircraft," AIAA AVIATION FORUM AND ASCEND, AIAA Paper 2024-4417, 2024. https://doi.org/10.2514/6.2024-4417 LinkGoogle Scholar

[31] Chauhan S. S. and Martins J. R. R. A., "RANS-Based Aerodynamic Shape Optimization of a Wing Considering Propeller-Wing Interaction," Journal of Aircraft, Vol. 58, No. 3, 2021, pp. 497-513. https://doi.org/10.2514/1.C035991 LinkGoogle Scholar

[32] Goldstein S., "On the Vortex Theory of Screw Propellers," Proceedings of the Royal Society of London. Series A, Containing Papers of a Mathematical and Physical Character, Vol. 123, No. 792, 1929, pp. 440-465. https://doi.org/10.1098/rspa.1929.0078 CrossrefGoogle Scholar

[33] Veldhuis L. L., "Propeller Wing Aerodynamic Interference," Dissertation, Delft Univ. of Technology, Delft, Netherlands, 2005. Google Scholar

[34] Vatistas G. H., Kozel V. and Mih W. C., "A Simpler Model for Concentrated Vortices," Experiments in Fluids, Vol. 11, No. 1, 1991, pp. 73-76. https://doi.org/10.1007/BF00198434 CrossrefGoogle Scholar

[35] Torenbeek E., Synthesis of Subsonic Airplane Design, repr. ed., Kluwer Academic Publ., Dordrecht [u.a.], 1999, p. 502. Google Scholar

[36] Hoerner S. F., Fluid Dynamic Drag, Hoerner Fluid Dynamics, 1965, pp. 2-5 (page 5 in chap. 2). Google Scholar