Properties of the Atmosphere

# Properties of the Atmosphere - Properties of the Atmosphere...

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Properties of the Atmosphere The performance of an aircraft depends on the density and viscosity of the atmosphere in which it operates. The density in turn depend on pressure and temperature, while viscosity depends on temperature. Density is the amount of mass in a certain volume and is expressed as mass per unit volume . Typical units in the English system are slugs per cubic feet or slugs per cubic inch . At sea level, there are about 2.38 x 10 -3 slugs per cubic foot, although we shall see shortly that this quantity can vary enough to affect aircraft performance depending on atmospheric conditions. In the atmosphere, density depends on local temperature and pressure. Viscosity is a property that describes the resistance of a fluid to flow. The higher the viscosity, the harder it is for a fluid to flow. Molasses is a more viscous fluid than alcohol, i.e, if poured, the molasses flows more slowly. Viscosity is a result of the internal friction of a fluid caused by molecular attraction. For an object moving through a fluid, the higher the viscosity, the harder it is for the object to move, i.e., drag increases in a fluid of higher viscosity. For a liquid, viscosity decreases as temperature increases. The oil is your car is an example of this. For a gas (including air), however, viscosity increases as the temperature increases. Since temperature varies in the atmosphere, viscosity also varies in the atmosphere. The actual resistant force on a body that results from viscosity depends the distribution of velocity within the fluid (to be discussed later in boundary layer flow) and the coefficient of viscosity. Viscosity can be expressed in different ways. We shall express in terms of the absolute viscosity which has dimensions of mass per length-time . A typical value for absolute viscoscity at sea level is 3.7373 x 10 -7 slugs per foot-second. Viscosity is a property that is important in aircraft design, however, viscosity is not considered by pilots in calculating aircraft performance since its effects are buried in other parameters that are used. Pressure is a force per unit area. It exists in the atmosphere as a result of air molecules (mostly nitrogen and oxygen) striking a surface and transferring some of their momentum to the surface. The force acting on a surface due to pressure is always perpendicular, or "normal", to the surface. The total force on the surface can be calculated by multiplying the average pressure times the surface area. A typical value for

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## This note was uploaded on 09/12/2011 for the course ATF 1100 taught by Professor Abbitt during the Spring '09 term at Santa Fe College.

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Properties of the Atmosphere - Properties of the Atmosphere...

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