Trajectories and Stability of Trailing Vortices Very Near the Ground
NATIONAL RESEARCH COUNCIL OF CANADA OTTAWA (ONTARIO) INST FOR AEROSPACE RESEARCH
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The behaviour of the trailing vortices of a Harvard aircraft used as the spraying vehicle during a set of experiments in aerial spraying over flat terrain for agricultural applications is discussed. The aircraft flew at a nominal altitude of 3 m 10 ft. above ground at a speed of 56.7 ms 110 knots. The stability and trajectory of a chosen element of the trailing vortices were measured by analyzing movie films taken by a ground-based camera and by a camera in a helicopter hovering at about 244 m 800 ft. above the aircraft. The vortices decayed by core bursting in every case and the time to burst was usually in agreement with other published data for a light aircraft out of ground effect. The downwind vortex almost always burst before the upwind vortex and in most cases, both upwind and downwind vortices exhibited about the same amount of rebound even though the downwind vortex generally had a shorter lifetime. The classical inviscid theory for vortex descent was not a good model for the current experiments but it was able to predict with some success the lateral separation between the vortices when the aircraft wing tip height was arbitrarily reduced by a factor of 0.85. It was concluded that vortex core bursting and rebound must be included in any procedure for calculating aerial spray deposit on the ground.