For helicopters, the H-V avoid area is usually given as a function of IAS. Takeoff and landing procedures must be developed to preclude entering the H-V avoid area, and pilots use IAS during takeoff and landing.
H-V diagrams are validated during full scale flight test by introducing engine failures at discrete points along the perimeter of the H-V avoid area. The test points are set-up by flying at a specified IAS and height. Some H-V diagrams are shown as a series of density altitude lines for a fixed weight, some are shown as a series of weight lines for a fixed density altitude, while others are shown as a fixed line but require that weight be reduced with increasing density altitude in a prescribed way. Regardless of how the H-V diagram is portrayed, it must be validated by flight test over the range of density altitudes for which the helicopter is approved for takeoff and landing.
I’m puzzled by Shawn Coyle’s input that “there is no way to measure the airspeed accurately below 40 Knots in any case.” On the helicopter models I’ve tested the airspeed system position errors were measured at airspeeds in the 15-30 kt range.