Scale-model tests of coaxial rotors in water tunnel via particle image velocimetry technique

Abstract

In this work, the flow features of coaxial rotors measured in water tunnel are investigated via particle image velocimetry technique. The experiments are conducted using a two-bladed model-scale coaxial rotor and a single rotor with the same geometry. Some noticeable flow features are found. For the coaxial rotor in hover, there is an annular region radially inboard of the lower rotor disk, where there are almost no intrusions from the upper rotor wake. At the same time, upwash occurs at the outboard edge of the lower rotor, while no such phenomenon occurs near the upper rotor blade tip. Sometimes, two upwash phenomena are found both at the blade tip and root in the single-rotor case. For both the single and coaxial rotors in forward flight, there is a sudden velocity change at about 0.7 R on the retreating side. From comparisons with the prior performance experiments, it can also be found that mutual interferences are playing a positive role in the hovering coaxial rotor configuration. The previously prescribed wake models can well describe the tip vortex trajectories of the single rotor and coaxial rotor in hover and forward flight, especially at high advance ratios. However, in this experiment, some special features are also found slightly different with that of the experiment conducted in air. Some valuable results obtained from this experiment would be helpful to reasonable designs of coaxial helicopters in the future.

Keywords

Particle image velocimetry flow feature scale-model tests coaxial rotor hover and forward flight tip vortex trajectory water tunnel

Get full access to this article

View all access options for this article.

References

Leishman JG and Ananthan S. Aerodynamic optimization of a coaxial PropRotor. In: 62nd Annual national forum of the American Helicopter Society, Phoenix, AZ, USA, 9–11 May 2006.

Boulet

. The history of the helicopter as told by its pioneers 1907–1956, Paris: Editions France-Empire, 1984.

Liberatore

. Helicopters before helicopters, Malabar, FL: Krieger Publishing, 1998.

Chaney

. The ABC helicopter. J Am Helicopt Soc 1969; 14: 10–19.

Green

Acosta

. Unsteady flow in trailing vortices. J Fluid Mech 1991; 227: 107–107.

Leishman

. Principles of helicopter aerodynamics, Cambridge: Cambridge University Press, 2001.

McCroskey WJ. Vortex wake of rotorcraft. In: 33th AIAA aerospace science meeting and exhibition, Reno, USA, 9–12 January 1995.

Conlisk

. Modern helicopter rotor aerodynamics. Prog Aerosp Sci 2001; 37: 419–419.

Lakshminarayan

Baeder

. Computational investigation of micro-scale coaxial rotor aerodynamics in hover. J Aircraft 2010; 47: 940–955.

10.

Lakshminarayan

Baeder

. High resolution computational investigation of trimmed coaxial rotor aerodynamics in hover. J Am Helicopt Soc 2009; 54: 042008-1–042008-21.

11.

Akimov AI, Butov VP and Selemenev SV. Flight investigation of coaxial rotor tip vortex structure. In: Proceedings of the 50^th annual forum of the American Helicopter Society, Washington, D.C., May 1994.

12.

Zheng

. Recent achievements in the field of helicopter aerodynamics, Beijing: Aviation Industry Press, 1999.

13.

Burley CL, Brooks TF, van der Wall BG, et al. Rotor wake vortex definition and validation from 3-C PIV HART II study. In: 28th European rotorcraft forum, Bristol, UK, 2006.

14.

Deng

. PIV experimental investigation of coaxial rotors’ induced velocity field in hover. J Exp Fluid Mech 2012; 1: 16–20.

15.

Adrian

. Particle imaging techniques for experimental fluid mechanics. Ann Rev Fluid Mech 1991; 23: 261–304.

16.

Raffel

Richard

Ehrenfried

. Recording and evaluation methods of PIV investigation on a helicopter rotor model. Exp Fluids 2004; 36: 146–156.

17.

Leishman

. Measurements of the aperiodic wake of a hovering rotor. Exp Fluids 1998; 25: 352–361.

18.

Leishman

Baker

Coyne

. Measurements of rotor tip vortices using three-component laser Doppler velocimetry. J Am Helicopt Soc 1996; 41: 342–353.

19.

Kindler

Goldhahn

Leopold

. Recent developments in background oriented Schlieren methods for rotor blade tip vortex measurements. Exp Fluids 2007; 43: 233–240.

20.

Elliott JW, Althoff SL and Sailey RH. Inflow measurements made with a laser velocimeter on a helicopter model in forward flight. In three volumes: Volume I: Rectangular planform at an advance ratio of 0.15, NASA TM 100545. Volume II: Rectangular planform at an advance ratio of 0.13, NASA TM 100546. Volume III: Rectangular planform at an advance ratio of 0.30, NASA TM 100547, 1988.

21.

Bousman

. A quantitative examination of dynamic stall from flight test data. J Am Helicopt Soc 1998; 43: 279–295.

22.

Landgrebe

. The wake geometry of a hovering rotor and its influence on rotor performance. J Am Helicopt Soc 1972; 17: 3–15.

23.

Kocurek

Tangler

. A prescribed wake lifting surface hover performance analysis. J Am Helicopt Soc 1977; 22: 24–35.

24.

Egolf TA and Landgrebe AJ. Helicopter rotor wake geometry and its influence in forward flight, Vol. I-Generalized wake geometry and wake effect on rotor airloads and performance. NASA CR-3726, 1983.

25.

Beddoes TS. A wake model for high resolution airloads. In: 1st International conference on basic rotorcraft research, Triangle Park, NC, USA, 1985.