Density Altitude as a Performance Standard for Helicopter Research in Forestry

Abstract

ABSTRACT DENSITY altitude at a site is the altitude in the standard atmosphere corresponding to ambient air density at the site. As helicopter performance is directly related to air density, the results of research evaluations that are dependent upon helicopter performance can be standardized by means of density altitude. This paper uses the definition of the standard atmosphere to derive, for dry air, a differential equation relating changes in air pressure and temperature to shifts in altitude. In-tegrating this equation provides an expression by which the density altitude at a site can be computed directly from ambient pressure altitude and temperature at the site. Further analysis shows also that the effect of humidity is to increase density altitude, thus degrading aircraft performance. An expression is derived for ad-justing the measured pressure altitude for the effect of humidity. This adjusted pressure altitude is then used to compute the humidity-corrested density altitude. Analysis of the errors that would result if humidity were neglected suggests that onsite humidity measurements will be required for accurate standardization of most helicopter research. Forest operations increasingly utilize helicopters where terrain or other obstacles must be surmounted or where delivery speed is important. Examples of recent research applications include logging (Dykstra, 1976), broad-casting of fertilizers (Page and Gustafson, 1969), delivery of herbicides or insecticides (Orchard et al., 1974), and collection of seed cones (Dobbs et al., 1977). Helicopters also are widely used in direct seeding operations, in reconnaissance, in fire detection and suppression, in search and rescue operations, and for low-level photography. Many forested regions lie at relatively high elevations above sea level. Air density, which provides aerodynamic force and also influences engine performance, varies in-versely with altitude (Dommasch et al., 1959). Although all aircraft are influenced by air density, helicopter per-formance is seriously degraded under low density condi-tions (Saunders, 1975). As an example, the net lifting capacity of a helicopter commonly used in logging opera-tions, the Sikorsky S61L, can be reduced as much as 50 percent as the result of variation in air density over the course of a single day (McGonnagill, 1973). Although empirical curves relating helicopter perfor-mance to ambient air density are published by helicopter. manufacturers (Saunders, 1975), none of the researchers cited earlier used this information to standardize their results. Therefore, results are difficult to assess, and predictions based on the research are likely to be in er-ror. Most researchers would agree that research results should be standardized. The lack of standardization in forestry helicopter research can be attributed to the lack of a convenient and precise method for determining den-sity altitude, which is the standard upon which helicopter performance curves are based. For normal helicopter operations, density altitude charts (Fig. 1) are often us-ed. For research purposes, however, these charts may not provide sufficient precision. Also, they are inconvenient if data are to be processed on a computer. This paper presents a method for conveniently and precisely deter-mining density altitude.