Investigation of Aerodynamic Interactions Between a Rotor and a T-Tail Empennage

Erwin Moedersheim and Dr. J. Gordon Leishman

This is the HTML version of a paper presented at:
The American Helicopter Society International Specialists' Meeting, Stratford, Connecticut, October 11-13, 1995.

Summary and Conclusions

The following conclusions have been drawn from this investigation:

1. Flowfield measurements showed that the flow below and behind the rotor was highly asymmetric, and varied considerably with advance ratio. The tip vortices generated by the rotor blades were found to roll-up quickly behind the rotor disk to form two vortex bundles. These vortex bundles were the dominant feature in the flow behind the rotor disk, and greatly affected the vertical and horizontal velocity components at the tail.

2. The geometry of the wake boundary showed that at low advance ratios the wake impinged on the rotor tailboom, having little effect on the horizontal stabilizer. As the advance ratio was increased, however, the wake skew angle increased, and the wake was observed to impinge on the horizontal tail. For some conditions, the wake passed over the top of the tail. At high advance ratios, the wake skew angle remained nearly constant, and the upper wake boundary was convected almost parallel to the rotor tip-path-plane.

3. At low advance ratios, the tail encountered high downwash angles, and the flow was generally stalled. The flow progressively reattached as the advance ratio was increased. The steady pressure loading was asymmetric due, primarily, to the difference in strength of the vortex bundles trailed from the sides of the disk. Under some test conditions, a sharp trailing edge pressure peak was observed on the retreating side, perhaps indicating that a scarf vortex originated at the junction of the vertical fin and the horizontal stabilizer.

4. Unsteady pressure measurements showed that flowfield near the horizontal stabilizer was highly unsteady and dependent on the position of the rotor wake boundary, the tip vortex strengths and their convection velocity. Below an advance ratio of 0.15, the unsteady pressure responses were generally small since the rotor wake was far from the stabilizer. As the advance ratio was increased and the wake boundary encroached on the stabilizer, the unsteady pressures increased significantly. When the wake impinged on the tail, a sharp rise in 8P pressures was observed.

5. The unsteady pressures were characterized by two events. The first and more dominant, was due to the unsteady lift produced on the horizontal tail that was induced by the tip vortices convecting along the rotor wake boundary. This unsteady pressure response was in-phase over the chord. The second event, smaller in magnitude, was due to the disturbances produced by the convection of the individual tip vortices past the measurement points. These pressure disturbances were out of phase over the chord.

Acknowledgments

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