From the start of the development of helicopters at the Bristol Aeroplane Company the necessity was appreciated of providing means of testing, independently of the aircraft, rotor hubs and blades. The usual method of testing on the helicopter suffers from several disadvantages, the most important being that proximity of the ground beneath the rotor greatly affects the flow through it and makes such performance figures as can be obtained unrepresentative. Others are the probable inconvenience of having to wait for the completion of the aircraft before rotor-testing can start, the comparative difficulty of instrumentation, and the risk of damage to the aircraft by failure or misbehaviour of the rotor.