In an experimental study, the airflow conditions around a ski-jumper during a take-off motion was analyzed. The airflow conditions on the back of a jumper during the take-off were observed by the surface tuft method in a wind tunnel and in actual ski-jumping. The influence of the angle of attack (AOA: the trunk angle measured from a moving direction) during postural transition was investigated.

In wind tunnel test with the wind speed of 16 m/s, the jumper performed the simulated take-off with the low- and the high-AOA. The airflow condition around the vertebra prominence was always turbulent throughout the motion. With the high-AOA (approximately 30 degree), the airflow along the body-sides turned outwards to the body-sides, and the separation of airflow toward both sides was observed at the hip. On the other hand, these changes of airflow condition were not observed in the case with the low-AOA.

The aerodynamic lift and drag forces were calculated from the vertical and the horizontal ground reaction forces in the wind tunnel. In motions with the high-AOA, the aerodynamic forces, especially the drag force was larger (p<0.05) than the cases with the low-AOA.

The airflow conditions were measured in actual ski-jumping. 5 jumpers performed 18 trials at the Miyanomori ski-jumping hill (K=90 m). While a jumper passed a hill-edge, the turbulence of the airflow on the posterior side was generally small. After passing the hill-edge, the airflow condition changed apparently. As the jumper changed his posture, the similar airflow condition to that in the wind tunnel test was observed. The turbulence observed in the actual ski-jumping trials was larger than in the wind tunnel test.

Through this analysis, it was verified that the effect of the AOA on the airflow condition around a jumper is significantly large. The increase of the AOA causes turbulence and the airflow separation on the posterior side of a jumper, and as a consequence, the aerodynamic drag force increases. This suggests that the low AOA during a take-off is a prerequisite to attain a good ski-jump flight.