The wide-field shadowgraph method has been used to photograph the tip
vortices of a hovering helicopter rotor in ground effect. The shadowgr
aphs were used to obtain quantitative measurements of the rotor tip vo
rtex geometry both in and out of ground effect. Many important phenome
na are visible in the rotor wake using this method. These include the
variation in descent and contraction rates of the tip vortices in grou
nd effect, and the interaction between tip vortices in the far wake. T
he tip vortex geometry from the shadowgraphs is compared with the tip
vortex geometry predicted using a free wake hover performance analysis
. The free wake analysis accurately predicts the tip vortex geometry b
oth in and out of ground effect. Performance data from the test is com
pared with the performance predicted using several methods, including
the free wake analysis. All methods provided reasonable predictions of
the helicopter performance in ground effect.