Autorotative Indices

The ability of the pilot to make a safe entry into autorotation and a safe flare from autorotation depends on both his skill and the physical characteristics of the helicopter. Some helicopters have been found to be reasonably forgiving of sloppy piloting; others are dangerous even for skilled test pilots. This situation leads to the desire to quantify the autorotative characteristics with some sort of simple index number that will indicate how a given helicopter compares to other helicopters in this regard. Since both the entryinto autorotation and the flare from autorotation have to do with the kinetic energy stored in the rotor and the rate it is dissipated, a logical index is the equivalent hover time, or the time that the stored kinetic energy could supply the power required to hover before stalling. This and several other indices are discussed in reference 5.15 but the equivalent hover time appears to give the best correlation with the qualitative opinions of test pilots. In Reference 5.15 the equivalent hover time is called t/k. In this book it will be called /cauiv and defined as:

Although reference 5.15 does not specify what value to use for CT/omax, a reasonable approach would be to use the maximum calculated value from isolated rotor hover charts such as those in Chapter 1. Figure 5.13 is taken from reference

5.15 and represents the correlation of pilot opinion and equivalent hover time for several Bell helicopters. The figure indicates that the design goal for single-engine helicopters should be at least 1.5 seconds in order to be considered satisfactory. It is not yet clear what the corresponding goal should be for multiengined helicopters. For what it is worth, the twin-engine example helicopter has an equivalent hover time of 0.8 seconds. A later study* reported in reference 5.16 results in a simple autorotative index for the landing flare in the form:

where the first term corresponds to the altitude to which all the rotor kinetic energy could lift the aircraft, and the second term is the penalty associated with altitude and disc loading. The study as applied to Sikorsky helicopters indicates that an index as low as 60 is satisfactory for single-engine helicopters and 25 for twin-engined. The calculated autorotative index for the example helicopter is 39.

Equivalent Hover Time, tequiv. sec

FIGURE 5.13 Autorotative Ratings of Several Bell Helicopters

Source: Wood, “High Energy Rotor System,” 32nd AHS Forum, 1976.