The traditional concept is that flying vehicle is a land and air amphibious vehicle with the functions of flying in the air and driving on the ground. With the rise of the concept of urban air mobility (UAM), the vertical take off and landing (VTOL), which only has the function of air flight, is also called flying vehicle to solve the problem of urban traffic congestion. Flying vehicles will be available in the aircraft and automobile market from 2020 to 2025. Many start-ups such as Terrafugia, aeromobil, pal-v, opener, Kitty Hawk, etc., as well as mature automobile and aircraft manufacturing companies such as Airbus, Audi, Rolls Royce, Aston Martin, etc., are constantly showing their flying car prototypes, and plan to introduce flying cars. With the rapid development of flying vehicles, they are facing many problems, such as flying vehicle performance, airworthiness certification, air traffic management and so on. As far as the performance of flying vehicles is concerned, high power density propulsion and high lift drag ratio lightweight car body are the main bottlenecks and key technologies in the research and development of flying vehicles. While constantly seeking for high-power, lightweight and high thrust to weight ratio flying vehicles, it is bound to bring more severe challenges to the safety and reliability of the transmission system. As an important part of the movement and power transmission of flying vehicles, the reliability of gears will directly determine the driving safety of flying vehicles.
The failure of gear transmission will lead to aircraft engine shutdown, important parts damage and major man-machine safety accidents. Fatigue is one of the main causes of gear failure. Aircraft gear failure mainly includes tooth root bending fatigue failure and tooth surface contact fatigue failure. According to the failure of aeroenginetooth surface, Cheng et al.  found that the contact fatigue failure was related to the material composition, metallographic structure, microhardness, carburizing depth and other factors by using the failure tree analysis method, SEM, EDS, metallographic examination and hardness measurement. Manda et al. Studied the fatigue fracture of helicopter cooling fan drive gear, and found that the microstructure of the non failure gear is finer than that of the failure gear, and the fracture is mainly caused by the root fatigue crack. In addition, many scholars have carried out a lot of research on reliability analysis and optimization design of aircraft gear transmission.
In view of the reliability problem of aircraft landing gear gear, Chang et al. Combined with the dynamic simulation model based on neural network and the first order second moment method, obtained the reliability of the landing gear gear transmission, and found that the load torque and working temperature have a great influence on the reliability. Sato et al. Carried out a lot of research on the planetary gear train of the new generation of open rotor aero-engine, combined with computational fluid dynamics, finite element analysis and other methods, to meet the requirements of transmitting 14 700 kW power, efficiency up to 99.5%, and mean time to failure up to 5 000 H. In addition to the further development of structure type design, the reliability evaluation and optimization method of gear transmission of flying vehicle is also very lacking. Different from the traditional vehicle and most aircraft, the gear transmission of flying vehicle meets the requirements of vertical take-off and cruise flight, and the transmission is in high-speed operation for a long time, once it fails, it will bring serious consequences. The offset compound gear (OCG) transmission of a large tilting wing flying vehicle test is taken as the research object. The reliability analysis and structure optimization model of transmission gear transmission is established to analyze the fatigue reliability of all levels of transmission and the overall structure, and obtain high reliability and lightweight gear transmission structure parameters, which provides support for the development of automobile transmission system.