In this paper results arc discussed of wind‐tunnel smoke tests with a small helicopter rotor. The test apparatus includes a specially developed hot‐wire smoke‐generator. An attempt is made to describe the flow pattern in the neighbourhood of a helicopter rotor in the vortex ring state by introducing the spread of the slipstream, causing an airbody around the rotor. By considering the proportions of this airbody the unpleasant behaviour of helicopters in the partial power‐off descent at low forward speeds can be more clearly understood.