The atmosphere provides the medium in which aircraft operate. Its properties, including pressure, density, temperature, and humidity, directly affect aircraft performance, engine operation, and aerodynamic forces. Understanding the characteristics of the atmosphere is therefore essential to understanding the principles of flight.
Before examining the fundamental laws of flight, several basic facts must be considered, namely that an aircraft operates in the air. Therefore, those properties of air that affect the control and performance of an aircraft must be understood.
The air in the Earth’s atmosphere is composed mostly of nitrogen and oxygen. Air is considered a fluid because it fits the definition of a substance that has the ability to flow or assume the shape of the container in which it is enclosed. If the container is heated, pressure increases; if cooled, the pressure decreases. The weight of air is heaviest at sea level where it has been compressed by all of the air above. This compression of air is called atmospheric pressure.
Pressure
Atmospheric pressure is usually defined as the force exerted against the Earth’s surface by the weight of the air above that surface. Pressure is defined as force applied over a given area. Force (F) equals area (A) times pressure (P), or F = AP. Therefore, to find the amount of pressure, divide area into force (P = F/A). A column of air (one square inch) extending from sea level to the top of the atmosphere weighs approximately 14.7 pounds; therefore, atmospheric pressure is stated in pounds per square inch (psi). Thus, atmospheric pressure at sea level is 14.7 psi.
Atmospheric pressure is measured with an instrument called a barometer, which uses a column of mercury in a tube to indicate atmospheric pressure in inches of mercury ("Hg). [Figure 1]
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| Figure 1. Barometer used to measure atmospheric pressure |
The standard measurement in aviation altimeters and U.S. weather reports has been "Hg. However, worldwide weather maps and some non-U.S. manufactured aircraft instruments indicate pressure in millibars (mb), a metric unit. At sea level, when the average atmospheric pressure is 14.7 psi, the barometric pressure is 29.92 "Hg, and the metric measurement is 1013.25 mb.
An important consideration is that atmospheric pressure varies with altitude. As an aircraft ascends, atmospheric pressure, air density, and temperature generally decrease. Although the percentage of oxygen in the air remains nearly constant, the amount of oxygen available in a given volume of air decreases with altitude. The changes in altitude affect an aircraft’s performance in such areas as lift and engine horsepower. The effects of temperature, altitude, and density of air on aircraft performance are covered in the following paragraphs.
Density
Density is the mass of a substance per unit volume. Since air is a mixture of gases, it can be compressed. If the air in one container is under half as much pressure as an equal amount of air in an identical container, the air under the greater pressure weighs twice as much as that in the container under lower pressure. The air under greater pressure is twice as dense as that in the other container. For the equal weight of air, that which is under the greater pressure occupies only half the volume of that under half the pressure.
The density of gases is governed by the following rules:
- Density varies directly with pressure.
- Density varies inversely with temperature.
Thus, air at high altitudes is less dense than air at low altitudes, and a mass of hot air is less dense than a mass of cool air.
Changes in air density affect aircraft aerodynamic performance and engine performance. At higher altitudes, reduced air density decreases aerodynamic drag, which can allow an aircraft to achieve higher true airspeeds, provided sufficient engine power is available. This is because air offers less resistance to the aircraft when it contains a smaller number of air particles per unit of volume.
Humidity
Humidity is the amount of water vapor in the air. The maximum amount of water vapor that air can hold varies with the temperature. The higher the temperature of the air, the more water vapor it can contain.
- Absolute humidity is the weight of water vapor in a unit volume of air.
- Relative humidity is the ratio, in percent, of the moisture actually in the air to the moisture it would hold if it were saturated at the same temperature and pressure.
Assuming that the temperature and pressure remain the same, the density of the air varies inversely with the humidity. Under the same temperature and pressure conditions, air density is lower on humid days than on dry days. Because humid air is less dense than dry air, high humidity can slightly reduce aircraft performance and contribute to increased takeoff distance.
Water vapor has a lower molecular weight than the nitrogen and oxygen that make up most of the atmosphere. As a result, moist air is less dense than dry air under the same temperature and pressure conditions. Therefore, moist air is less dense than dry air under the same temperature and pressure conditions.