Studying the Possibility of the Tail Rotor Entering the “Vortex Ring” Mode under the Main Rotor Effect

Aeronautical and Space-Rocket Engineering


Аuthors

Makeev P. V.*, Ignatkin Y. M.1**, Shomov A. I.2***, Ivchin V. A.3****

1. Moscow Aviation Institute (National Research University), 4, Volokolamskoe shosse, Moscow, А-80, GSP-3, 125993, Russia
2. National Helicopter Center Mil & Kamov, 26/1, Garshina str., Tomilino, Moscow region, 140070, Russia
3. Design Bureau “Heliburo”, Moscow, Russia

*e-mail: makeevpv@mai.ru
**e-mail: k102@mai.ru; ignatkinym@mai.ru
***e-mail: shomov_aleksandr@mail.ru; a.shomov@nhc.aero
****e-mail: valivchin@mail.ru

Abstract

In horizontal flight modes, the free vortex wake behind the main rotor (MR) blades transforms into a system of right and left longitudinal secondary vortex bundles located along the edges of the rotor disk. The said vortex structures largely determine the velocity field around the rotor. Their inductive effect is most the significant at low horizontal flight speeds, about 7–12 m/s, when they have maximum intensity. While a helicopter hovering in crosswind conditions and during horizontal flight with a sideslip, cases of a tail rotor (TR) hitting one of the vortex bundles of the MR are possible. The TR herewith passes through a significant external induced impact, which may lead to its aerodynamic characteristics deterioration. Rapid development of computer technology and computational models allowed conducting fairly large-scale parametric (not limited to individual cases) studies of problems related to studying aerodynamics of the helicopters MR and TR combination with regard to the aerodynamic interference without limiting to separate cases. The possibility of the TR entering the “vortex ring” state modes during the low-speed flight with sliding was studied on the example of the Mi-8/17 helicomper MR and TR combination employing a nonlinear free wake model developed at the MAI “Helicopter Design” department. The aerodynamic characteristics of the TR of a helicopter in an isolated setting and under the impact of the MR vortex wake (in MR + TR combination) at different flight speeds in the range of V = 0–20 m/s and sliding angles in the range β = –180–180 was considered. A special area of flight modes has been discovered, which are a combination of flight speeds of V = 6.25–7.5 m/s and sliding angles of β = 20–40. The extra induced impact from the right secondary vortex core in this area leads to the TR entering the “vortex ring” state modes. The said TR “vortex ring” state modes are being accompanied by the thrust and TR torque pulsations, as well as increase in the required TR blade pitch angles. As computations revealed, it might lead in separate cases to the increase of the required power for the TR rotation up to 30% compared to the isolated TR without the MR impact. The data obtained in the course of the study allow speaking about the existence of the flight speeds (wind speeds), at which the conditions of the TR flow-around under the impact of the MR vortex wake turn out to be unfavorable at any sliding angles (the angles of the helicopter rotation relative to the external flow). Increasing of the required TR blade pitch angles and required power under such conditions may be one of the prerequisites to the single-rotor helicopter uncontrolled rotations emergence.

Keywords:

free wake model, main rotor, tail rotor, sideslip flight, aerodynamic interference, “vortex ring” mode

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