Читать книгу Introduction to Flight Testing - James W. Gregory - Страница 29

Earth's atmosphere is an envelope of air surrounding the planet Earth, where dry air consists of 78.08% nitrogen, 20.95% oxygen, 0.93% argon, 0.031% carbon dioxide, and small amounts of other gases (NOAA et al. 1976). In addition, air contains a small amount of water vapor (about 1%). The entire atmosphere has an air mass of about 5.15 × 10¹⁸ kg (1.13 × 10¹⁹ lb), and three quarters of the total air mass are contained within a layer of about 11 km (∼36,000 ft) from the Earth's surface.

There is a general stratification of Earth's atmosphere, which leads to the definition of distinct regions of the atmosphere: the troposphere (0–11 km), stratosphere (11–50 km), mesosphere (50–85 km), and thermosphere (85–600 km). The atmosphere becomes thinner as the altitude increases, and there is no clear boundary between the atmosphere and outer space. However, the Kármán line has been defined at 100 km and is often used as the border between the atmosphere and outer space. Atmospheric effects become noticeable during atmospheric reentry of spacecraft at an altitude of around 120 km. Aircraft propelled by internal combustion engines and propellers are generally limited to operating in the troposphere, while jet‐propelled aircraft routinely operate in the stratosphere.

Figure 2.1 illustrates the bottom three layers of Earth's atmosphere, which is where all atmospheric flight vehicles conduct flight. The delineation between the various regions of the atmosphere is based on historical measurements of temperature profiles, which lead to distinct regions with different temperature lapse rates. In the troposphere (the layer of the atmosphere nearest the surface), the air temperature generally decreases linearly with the altitude. This temperature reduction with altitude is due to the increasing distance from Earth and a concomitant reduction in heating from Earth's surface. Weather phenomena are directly dependent on this temperature reduction with altitude, causing most storms and other weather phenomena to develop and reside within the troposphere. The dividing boundary between the troposphere and the next layer (the stratosphere) is called the tropopause, at 11 km. Within the lower portion of the stratosphere (11–20 km), the air temperature remains constant; it then increases with altitude in the upper stratosphere (20–50 km), due to absorption of the sun's ultraviolet radiation by ozone in this region of the atmosphere.

Figure 2.1 The layers of Earth's atmosphere.

In contrast with the temperature–altitude profile, the variation of pressure with altitude is highly repeatable and deterministic. Air pressure continually decreases with altitude from Earth's surface all the way to the edge of the atmosphere. The primary reason for this is the action of Earth's gravitational acceleration on air, causing a given mass of air to exert a force on the air below it. Air at a given altitude must support the weight of all of the air mass above it, and it balances this force by pressure. As altitude increases, there is less air mass above that altitude, so there is less force (weight) acting on the air at that point and the pressure decreases. Thus, pressure decreases as altitude increases. We will discuss this physical mechanism in greater detail in Section 2.2, when we derive an expression for the variation of pressure with altitude.