TRAILING VORTEX INSTABILITY AND ITS IMPLICATIONS FOR AERIAL SPRAYING

Data on the stability of the trailing vortex system shed by a small, c lean aircraft cruising in ground effect are presented for a flight spe ed of 57 m/s at altitudes varying between 2.8 and 19.2 m over flat ter rain. The aircraft used during the experiments, a Harvard Mark IV, clo sely simulates the vortex strength and wing span of a Cessna Agtruck. In all of the 46 flights analyzed, the vortices were observed to decay by core bursting following asymmetric, sinuous motion without exhibit ing any tendency to link. In an early series of tests, the port vortex was always observed to burst first, most probably because of a small antenna mounted near the wing tip. Upon removal of the antenna, neithe r vortex consistently burst first. Also, a ground marker used for some tests was found to induce early bursting when it interacted with the downwind member of the vortex pair. The time-to-burst was correlated w ith the atmospheric turbulent dissipation rate and the results agreed well with other experimental data and with the predictions of a model. It was observed that each vortex became extinct after core bursting w ithin an 18 s time interval. The implications of these results for aer ial spraying are discussed.