Inertia in the main rotor is valuable for two purposes: to prevent the rotor from decelerating too quickly following an unexpected loss of power, and to provide a source of energy for making a landing flare at the end of an autorotational descent. In either case, the level of rotor inertia is more important to a single-engine helicopter than to a multiengine machine, since a sudden power failure of two or more engines is unlikely and there is not the requirement to practice autorotational landings continuously as there is with single-engine helicopters.
Flight test experience with a number of single-engine helicopters has led to the conclusion that the rotor kinetic energy stored in the rotor at normal rotor speed should be sufficient to provide the equivalent of at least one and a half seconds of hover time to insure a satisfactory flare capability. It is believed that an equivalent hover time of 0.75 seconds is sufficient for a twin-engine helicopter. The methods for calculating the equivalent hover time are developed in Chapter 5.