Читать книгу Flight Theory and Aerodynamics - Joseph R. Badick - Страница 49

Density is the most important property of air in the study of aerodynamics, and is directly impacted by pressure, temperature, and humidity changes. Since air can be compressed and expanded, the lower the pressure the less dense the air: density is directly proportional to pressure. Increasing the temperature of the air (giving the particles greater kinetic energy) also decreases the density of the air, so in this case density and temperature have an inverse relationship.

Less dense, thinner air has a lower air density and is said to be a higher density altitude (decreasing aircraft performance); more dense, thicker air is said to be a lower density altitude (improved aircraft performance). Decreasing the density of the air results in a higher density altitude, while increasing the density of the air results in a lower density altitude.

Atmospheric pressure decreases as altitude increases. Temperature also decreases with increasing altitude, with two exceptions: first, when a temperature inversion layer exists, and second, in the troposphere, where the temperature remains constant and may even rise with increasing altitude. The discussion above would indicate that greater the altitude (less dense air) and colder temperature (more dense air) would result in a conflict in regard to density. But usually, the effect of a decrease in pressure with altitude overcomes any improvement in performance the colder, dense air may have, and a lower density altitude is the rule of thumb the higher in altitude an aircraft climbs.

The effect of moisture content on performance will be largely ignored in this textbook because most textbooks treat the effect of humidity as being negligible for practical purposes, but it is important to understand that water vapor is lighter than air, so moist air is lighter than dry air. As the amount of water vapor increases, the density of the air decreases, resulting in a higher density altitude (decrease in aircraft performance).