The Aircraft Research Association's tunnel was designed from the start as a transonic tunnel. Two important decisions were taken at the beginning of the design of this tunnel. First, that a separate auxiliary suction plant would be used to suck air out through ventilated walls of the tunnel instead of using diffuser suction for this purpose. This decision was taken mainly because the total power required with auxiliary suction is appreciably less than for diffuser suction. The plant is also more flexible and provides, for example, the possibility of running a smaller supersonic tunnel from the auxiliary suction plant.