Reliability assessment of convertible aircraft with hybrid propulsion system and multirotor lifting system

Projects and experimental models of innovative concepts of VTOL aircraft with a hybrid propulsion system are attracting great interest and investment inflow all over the world. In this regard, when developing new concepts, it is important to understand how much better they will be than the currently operated rotorcraft and convertible aircraft in terms of reliability and flight safety, to be able to use them for passenger transportation in the future. In addition, when designing and choosing the optimal layout, it is necessary to know the contribution of each element and unit to the reliability of the aircraft as a whole in order to meet the requirements. To calculate the reliability indicators, the method of structural diagrams was chosen, and the calculation methodology was developed. The general classification of modern innovative concepts of convertible aircraft is considered, schematic diagram of hybrid propulsion system and its main parameters are determined. The article discusses the influence of the number of lifting rotor groups and their location on the possibility to continue the flight in hover mode in case of failure of one rotor group, the necessary power reserve of lifting electric motors is determined to ensure the given safety condition. In accordance with the adopted structural diagram, the main functional groups of the hybrid propulsion system of convertible aircraft operating in different flight modes are determined. The basic modes of a typical flight profile of a convertible aircraft are considered, time intervals characteristic for each mode are set. For each flight mode, a structural scheme of reliability of functional groups of a hybrid propulsion system is constructed, having a serial or parallel connection of elements, depending on their influence on the consequences of failure, the equation for calculating the probability of fail-free operation is derived. For lifting rotor groups, a combination of critical failures of more than one group is considered, and the equation is composed to calculate the probability of a catastrophic event in hover mode. Based on the obtained equations, the resulting calculation of the probability of fail-free operation, the probability of failure per flight hour for each flight mode was carried out separately and in total for the entire flight. Thus, for all flight modes, the probabilities of a catastrophic and emergency event, as well as a difficult situation in flight, are considered. A comprehensive analysis of the obtained results of reliability indicators calculation for convertible aircraft with six lifting rotors and two turboprop engines made it possible to conclude that it meets the requirements of the 25th part of aviation regulations for transport aircraft. A good potential margin of up to 10-2 was determined for the probability of a catastrophic failure in take-off, landing and transitional modes. The elements and subsystems that are critical for fail-free operation are identified, and ways to improve their reliability and the aircraft as a whole are proposed.

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