Experimental and numerical study of tail boom strake on helicopter

Some helicopters may suffer from loss of directional control due to insufficient tail rotor thrust when in starboard sideward flight or cross-wind (with counter-clockwise main rotors). In such flight conditions, according to literature, an adverse suction force that contrasts the tail rotor thrust could be produced due to establishment of circulation or lift on the tail boom and the use of a strake is proposed in the literature as a means to destroy circulation and prevent the suction force from being produced. The present work combines numerical (CFD) and experimental (PIV) approaches, and through modelling of the tail boom cross-section as a low aspect-ratio ellipse (AR equal to 1.25), demonstrates that circulation is established when the airflow impinges on the tail boom at a non-zero angle of attack, provided that the flow regime is critical. In fact, a phenomenon scarcely reported in the literature called lift crisis is observed along with the well-known drag crisis. Then, the work demonstrates that the strake is effective in the critical regime, where the majority of helicopter tail booms operate, to such an extent that not only is the adverse sideforce suppressed, but also a beneficial sideforce is produced. Conversely, the strake is ineffective in the subcritical regime. Finally, the strake configuration is numerically optimized and the one yielding the lower tail rotor thrust requirement is thus determined. The application of the optimum strake on the tail boom of the A109 helicopter shows that a tail rotor thrust reduction as high as 17% can be obtained at the expense of a main rotor power increment due to download less than 3%, that represents a promising result for improving the handling qualities and the directional control of the real helicopter.