Computational modeling of aerodynamic interference of helicopter main and tail rotor during vertical descent, including vortex ring state of the main rotor
Аuthors1*, 1**, 2***
2. National Helicopter Center Mil & Kamov, 26/1, Garshina str., Tomilino, Moscow region, 140070, Russia
AbstractThe paper is focused on the problems of providing flight safety for a single-rotor helicopter at special flight modes. At these modes the tail rotor can be exposed to some conditions, under which its aerodynamic characteristics change drastically. In case of tail rotor thrust decay its effectiveness as a control unit decreases. This can lead to loss of directional trim of the helicopter, its uncontrolled rotation and crash.
The paper aims to carry out the computational study of main and tail rotor interference during vertical descent of the helicopter, including the range of vortex ring states of the main rotor.
The research was carried out by using the specially created software, which is based on the non-linear rotor vortical blade model developed at the “Helicopter Design” Department of MAI.
The research was carried out for a calculation model of main and tail rotor combination of Mi-2 helicopter. The case of horizontal flight with angle of attack of main rotor αн = 0º and flight speed V = 10 mps was modeled during the research.
The descent with the range of speeds Vу = 0…15 mps for angle of attack of main rotor αн =0º was considered. The shape of the free vortical wake of main and tail rotors was built for the mentioned above modes with and without taking into account interference. Diagrams of induced velocities of main rotor vortical wake in the area of tail rotor location were constructed. Reasons of interference effect emergence were analyzed and its influence on tail rotor aerodynamic characteristics was estimated.
As a result of the research it was established that main and tail rotor interference develops to a large degree at vortex ring state mode of the main helicopter rotor. At that the interference is determined by vortex wake structure of the main rotor and its location relative to the tail rotor. In addition the severity of exposure of the tail rotor to main rotor influence depends on the operation mode and rotation direction of the tail rotor. The research results show that the decrease of tail rotor thrust due to interference reaches its maximum of 26 % compared to the trust of the isolated tail rotor at Vy = 6 mps. At that the main rotor works in a mode of intensive vortex ring state and the tail rotor ends up in the core of the main rotor vortex ring and gets exposed to heavy influence of its inductive velocities field.
The research results can be used for estimating and choosing optimal direction of tail rotor rotation and determining directional control margin during design and modernization of helicopters.
Keywords:main rotor, tail rotor, computational methods of rotor's aerodynamics, non-linear vortical model, vertical descent, rotor interference, aerodynamic characteristics
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