Computational investigations of Ka-226T helicopter coaxial rotor aerodynamics in the vortex ring state

The vortex ring states are observed when the rotor is flowed at positive angles of attack. For the main rotor, these conditions are realized during a steep descent of the helicopter at low speeds. The rotor vortex ring states are affected by significant phenomena related to the behavior of its aerodynamics, including negative phenomena. The latter is, firstly, referred to decrease in rotor thrust, increase in the required power, pulsations of thrust and torque, unsteady flapping blade motion, etc. In terms of helicopter piloting, it means a sharp loss of altitude, increase in the control force, a high level of vibration, defocusing attenuation of rotor spinning cone, as well as controllability deterioration. All these factors determine the relevance of research on these modes and the importance of solving a problem of defining their boundaries. Recently, due to the rapid development of computer technologies and computational models, it has become practical to perform numerical research of the rotor aerodynamics in vortex ring states. The paper presents the study results of Ka-226T helicopter coaxial rotor aerodynamics in steep descent modes in the field of vortex ring states. The angles of rotor attack α_В = 90...30° and the range of vertical descent velocities V_у = 0...26 m/s are considered. The original nonlinear bladed vortex model of the rotor developed at the Moscow Aviation Institute (MAI) was used. The total and distributed aerodynamic rotor characteristics were calculated. The shapes of the vortex wake and the rotor flow patterns were analyzed. The boundaries of the vortex ring states in velocity coordinates “V_x − V_у” were constructed according to various criteria reflecting the known features of these states. The results obtained significantly complement the existing experience of experimental and numerical research in this field.

1. Akimov, A.I. (1988). Aerodynamics and flight characteristics of helicopters. Moscow: Mashinostroyeniye, 144 p. (in Russian)

2. Petrosian, E.A. (2004). Coaxial helicopter aerodynamics. Moscow: Polygon-Press, 820 p. (in Russian)

3. Leishman, J.G. (2006). Principles of helicopter aerodynamics. Cambridge University Press, 826 p.

4. Johnson, W. (2013). Rotorcraft aeromechanics. Cambridge University Press, 927 p.

5. Johnson, W. (2005). Model for vortex ring state influence on rotorcraft flight dynamics. NASA/TP-2005-213477. NASA, 76 p. Available at: https://rotorcraft.arc.nasa.gov/Publications/files/Johnson_TP-2005-213477.pdf (accessed: 12.12.2023).

6. Anikin, V.A. (2002). Helicopter main rotor aerodynamic performance in descent conditions. In: proceedings of the 58th Annual Forum of the American Helicopter Society. Montreal, Canada, 11–13 June, 15 p.

7. Leishman, J.G., Bhagwat, M.J., Ananthan, S. (2002). Free-Vortex wake predictions of the vortex ring state for single rotor and multirotor configurations. In: Proceedings of the 58th annual forum of the American Helicopter Society. Montreal, Canada, 11–13 June, 30 p.

8. Bailly, J. (2010). A qualitative analysis of vortex ring state entry using a fully time marching unsteady wake model. In: Proceedings of the 36th European Rotorcraft Forum, Paris, France, 7–9 September, 18 p.

9. Brown, R., Leishman, J., Newman, S., Perry, F. (2002). Blade twist effects on rotor behaviour in the vortex ring state. In: Proceedings of the 28th European Rotorcraft Forum, Bristol, UK, 17–20 September, 14 p.

10. Krimskiy, V.S., Shcheglova, V.M. (2014). The investigation of rotor's inflow and main rotor's induced velocity at hover and steep descents. Nauchnyy Vestnik MGTU GA, no. 200, pp. 86–90. (in Russian)

11. Ahlin, G.A., Brown, R.E. (2009). Wake structure and kinematics in the vortex ring state. Journal of the American Helicopter Society, vol. 54, no. 3, pp. 1–18. DOI: 10.4050/JAHS.54.032003

12. Mohd, N.A.A.R., Barakos, G. (2017). Performance and wake analysis of rotors in axial flight using computational fluid dynamics. Journal of Aerospace Technology and Management, vol. 9, no. 2, pp. 193–202. DOI: 10.5028/jatm.v9i2.623

13. Stalewski, W., Surmacz, K. (2020). Investigations of the vortex ring state on a helicopter main rotor using the URANS solver. Aircraft Engineering and Aerospace Technology, vol. 92, no. 9, pp. 1327–1337. DOI: 10.1108/AEAT-12-2019-0264

14. Kinzel, M.P., Cornelius, J.K., Schmitz, S. et al. (2019). An investigation of the behavior of a coaxial rotor in descent and ground effect. In: Proceedings of the AIAA SciTech 2019 Forum, San Diego, USA, 7–11 January, 13 p. DOI: 10.2514/6.2019-1098

15. Ignatkin, Y.M., Makeev, P.V., Shomov, A.I., Shaidakov, V.I. (2019). Computational research of the main rotor steep descent modes based on the nonlinear blade vortex model. Russian Aeronautics, vol. 62, no. 2, pp. 244–253. DOI: 10.3103/S1068799819020107

16. Makeev, P.V., Ignatkin, Yu.M., Shomov, A.I. (2021). Numerical investigation of full scale coaxial main rotor aerodynamics in hover and vertical descent. Chinese Journal of Aeronautics, vol. 34, issue 5, pp. 666–683. DOI: 10.1016/j.cja.2020.12.011

17. Vassiliyev, B.A., Kvokov, V.N., Pavlidi, F.N., Petrosian, E.A., Feofilov, E.B. (2007). The Кa-226 helicopter flight performance and its compliance with the modern requirements. In: Proceedings of the 33th European Rotorcraft Forum. Russia, Kazan, 11–13 September, 12 p.

18. Burtsev, B.N., Ryabov, V.I., Selemenev, S.V. (2007). Mathematical modeling of Кa-226 / Кa-26 Helicopter main rotor blade flapping motion at rotor acceleration / Deceleration in wind conditions. In: Proceedings of the 33rd European Rotorcraft Forum, Russia, Kazan, 11–13 September, 14 p.

19. Green, R., Gillies, E., Brown, R. (2005). The flow field around a rotor in axial descent. Journal of Fluid Mechanics, vol. 543, pp. 237–261. DOI: 10.1017/S0022112005004155

20. Savas, O., Green, R., Caradonna, F. (2009). Coupled thrust and vorticity dynamics during vortex ring state. Journal of the American Helicopter Society, vol. 54, no. 2, pp. 1–10. DOI: 10.4050/JAHS.54.022001