The windmill-brake state
At rates of descent in excess of vih the vortex ring rises above the disk and the rotor is said to be operating in the turbulent windmill-brake state. Although less problematic it is still a working state for which momentum theory is inappropriate. For still higher rates of descent (the windmill-brake state) momentum flow is restored with the rotor extracting sufficient energy from the rising air to maintain the rotor speed and generate thrust. In theory, the rotor can operate satisfactorily up to very high rates of descent in the windmill-brake state. A rotor is said to be operating in an ideal autorotative state if there is no mean flow through the rotor. Consequently the induced power is zero and the helicopter is able to make a controlled descent with potential energy being used to meet the power requirements of the rotorcraft. Theoretically, autorotation is achieved in vertical flight at a rate of descent equal to 1.8 vih. In practice however the requirement to overcome the profile drag of the rotor blades changes the rate of descent to 1.7 vih.
1.7 HORIZONTAL FLIGHT: MOMENTUM THEORY
It has already been stated that the momentum theory is inaccurate for low rates of descent. It is also true that these inaccuracies will persist if the theory is applied at low forward speed. Glauert [2.11] proposed that if the forward speed is large compared with the induced velocity then the momentum theory could be applied successfully. He also suggested that it was possible to make a direct analogy between a rotor and an elliptically loaded wing.