The testing of model rotors in closed throat wind tunnels is known to present major problems in the form of tunnel wall interference when operating at low speed. However, it is precisely this region of the aircraft's flight envelope which is of prime importance when investigating areas such as transitional flight, hovering in winds and tail rotor performance and handling qualities. Although much work has been carried out in the past to determine wall interference factors for various model and tunnel configurations, there still exists a problem of flow breakdown due to the presence of tunnel boundaries, for which no corrections are possible. The aim of this experiment was to conduct a short and simple investigation, first to identify the flow breakdown condition for a finite sized rotor in a closed tunnel, then to assess the feasibility of at least delaying the onset of this limiting condition by selectively removing panels from the walls of the working section. Since the experiment was designed only to study gross flow effects, no attempt was made to include wall interference factors.