Читать книгу Flight Theory and Aerodynamics - Joseph R. Badick - Страница 48

The commonly used measures of temperature are the Fahrenheit, °F, and Celsius, °C scales. Aviation weather reports for pilots, as well as performance calculation tables, will usually report the temperature in °C. In a standard atmosphere, the sea level surface temperature is 15 °C or 59 °F.

Since neither of these scales has absolute zero as a base, neither can be used in calculations; absolute temperature must be used instead. Absolute zero is −460 °F, or −273 °C. To convert from the Fahrenheit system to the absolute system, called Rankine, R, add 460 to the °F. To convert from the Celsius system to the absolute system, called Kelvin, K, add 273 to the °C. The symbol for absolute temperature is T and the symbol for sea level standard temperature is T₀:

By using temperature ratios, instead of actual temperatures, the units cancel. The temperature ratio is the Greek letter theta, θ:

(2.2)

At sea level, on a standard day, θ_0° = 1.0. Temperature in a standard atmosphere decreases with altitude until the tropopause is reached (36 089 ft on a standard day). The rate of change of temperature with altitude is known as the lapse rate. The standard lapse rate is approximately a 2 °C decrease in temperature for every 1000 ft increase in altitude from sea level (Figure 2.2) up to the tropopause. Temperature then remains constant until an altitude of about 82 023 ft.

As an example, if the temperature at the tropopause is −69.7 °F, calculate θ: