Two-stage transmission scheme design for perspective helicopter
Conventional commercial and military helicopters have flight speed limit of about 300-350 km/h. High-speed rotary wing aircrafts of the next generation require the increase of horizontal speed up to 450-500 km/h. Aerodynamic limitations, hindering flight speed increase of perspective high-speed helicopters, can be eliminated by reduction of the main rotor rotation frequency, while a helicopter builds up its high speed. This can be provided by implementation of regulated transmission with variable transmission ratio. The existing helicopter transmissions have constant transmission ratio. Thus, the development of regulated transmissions of anew type requires carrying out of exploratory study. The paper presents the results of the design of a new type of helicopter transmission with variable transmission ratio for perspective high-speed helicopters. It allows vary rotation frequency of the main rotor irrespective of engine and anti-torque, or pusher rotors rpm. The paper considers also the possibility of implementation and basic advantages and disadvantages of alternative types of regulated transmissions, such as, mechanical transmissions with stepped and stepless transmission ratio variation, as well as electromechanical versions of high-speed helicopters transmissions. It is shown that from the determining criterion point of view, i. e. minimum weigh and maximum efficiency of the transmission, the most effective and rational drive for a high-speed helicopter is a mechanical dual-mode transmission with stepped transmission ratio variation. The paper presents the description of a developed perspective stepless transmission scheme for a helicopter. The evaluation of dynamic forces acting during transmission transient modes with transmission ratio variation was carried out. Optimal scheme of the dual-mode mechanical transmission for perspective helicopters was developed. The important advantage of the developed scheme is the condition of safe operation, which consists in the fact that the coupling failure will not cause the braking of the force loop and loss of power at the main rotors. These design features provide high reliability of transmission operation and high functional characteristics with minimum mass increase in original structure of the helicopter main-rotor gearbox. This transmission mechanism is optimal solution for main rotors rotation frequency variation of perspective high-speed helicopters.