Читать книгу Flight Theory and Aerodynamics - Joseph R. Badick - Страница 47

The static pressure of the air, P, is simply the weight per unit area of the air above the level under consideration. Air has mass and as we have discussed thus has weight, which means it exerts a force. For instance, the weight of a column of air with a cross‐sectional area of 1 ft² and extending upward from sea level through the atmosphere is 2116 lb. The sea level static pressure is, therefore, 2116 pounds per square foot (psf), or 14.7 pounds per square inch (psi). Another commonly used measure of static pressure is inches of mercury. On a standard sea level day, the air’s static pressure will support a column of mercury (Hg) that is 29.92″ high (Figure 2.1). Weather reports express pressure in millibars; standard atmospheric pressure is 1013.2 mb. In addition to these rather confusing systems, there are the metric measurements in use throughout most of the world. For the discussion of performance problems in this textbook, we will primarily use the measurement of static pressure in inches of mercury is the standard used unless stated otherwise.

Static pressure is reduced as altitude is increased because there is less air weight above. At 18 000 ft altitude, the static pressure is about half that at sea level, the higher you go the less air there is above. The accepted standard pressure lapse rate is approximately 1″ Hg decrease in pressure for every 1000 ft gain in altitude from sea level (Figure 2.2). This change in atmospheric pressure with altitude is an important concept during evaluation of aircraft performance as well as the operation of aircraft flight instruments.

Figure 2.1 Standard pressure.

Source: U.S. Department of Transportation Federal Aviation Administration (2008a).

Figure 2.2 Properties of a standard atmosphere.

Source: U.S. Department of Transportation Federal Aviation Administration (2016b).

In aerodynamics, it is convenient to use pressure ratios, rather than actual pressures; thus the units of measurement are canceled out. When at sea level on a standard day, the pressure ratio can be determined using equation:

(2.1)

where P₀ is the sea level standard static pressure (2116 psf or 29.92″ Hg). Thus, a pressure ratio of 0.5 means that the ambient pressure is one‐half of the standard sea level value. At 18 000 ft, on a standard day, the pressure ratio is 0.4992.