Flight 101 - Properties of the Atmosphere - free Suite101 course

Lesson 1: Aerodynamics as a Science

Lesson 2: Aerodynamic Forces

Lesson 3: Properties of Fluids

Lesson 4: Fluid Flow

Lesson 5: Properties of the Atmosphere
- Section 1 – A History and Summary
- Section 2 - The Atmosphere
- Section 3 - Parts of the Airplane

Lesson 6: Flow Effects and Flight

Lesson 7: A History of Aerodynamics - Part I

Lesson 8: A History of Aerodynamics – Part II

Lesson 5: Properties of the Atmosphere

Section 2 - The Atmosphere

In studying the nature of the atmosphere, the aerodynamicist is concerned about one fluid, namely air. Air makes up the Earth's atmosphere-the gaseous envelope surrounding the Earth-and represents a mixture of several gases. Up to altitudes of approximately 90 km, fluctuating winds and general atmospheric turbulence in all directions keep the air mixed in nearly the same proportions.

Interestingly, nitrogen and oxygen taken together represent 99 percent of the total volume of all the gases. That the local composition can be made to vary has been brought dramatically to light in recent times by the air pollution problem where in industrialized areas the percentages of carbon monoxide, sulfur dioxide, and numerous other harmful pollutants are markedly higher than in non-industrialized areas.

Above about 90 km, the different gases begin to settle or separate out according to their respective densities. In ascending order one would find high concentrations of oxygen, helium, and then hydrogen which is the lightest of all the gases.

There are then, based on composition two atmospheric strata, layers, or "shells." Below 90 km where the composition is essentially constant the shell is the homosphere. Above 90 km where composition varies with altitude, the shell is called the heterosphere.

Although composition is one way of distinguishing shells or layers, the most common criterion used is the temperature distribution. In ascending order are the troposphere, stratosphere, mesosphere, thermosphere, and exosphere.

It is the troposphere which is the most important atmospheric layer to aeronautics since most aircraft fly in this region. Most weather occurs here and, of course, man lives here also. Without the beneficial ozone layer in the stratosphere absorbing harmful solar ultraviolet radiation, life as we know it would not have developed.

The ionosphere, a popularly known layer, begins in the mesosphere and extends indefinitely outwards. It represents the region in which ionization of one or more of the atmospheric constituents is significant. The exosphere represents the outer region of the atmosphere where the atmospheric particles can move in free orbits subject only to the Earth's gravitation.

It is interesting to note that at altitudes greater than 500 km, the solar wind which is streams of high-energy particles of plasma from the Sun, becomes a dominant influence so that one has an "atmosphere" which extends all the way to the Sun. The density of the solar wind, however, is negligibly small. In regards to the standard atmosphere, for purposes of pressure altimeter calibrations, aircraft and rocket performance and their design, etc., knowledge of the vertical distribution of such quantities as pressure, temperature, density, and speed of sound is required.

Since the real atmosphere never remains constant at any particular time or place, a hypothetical model must be employed as an approximation to what may be expected. This model is known as the standard atmosphere. The air in the model is assumed to be devoid of dust, moisture, and water vapor and to be at rest with respect to the Earth. In other words, where there is no wind or turbulence.

The first standard atmospheric models were developed in the 1920's in both Europe and the United States. The slight differences between the models were reconciled and an internationally accepted model was introduced in 1952 by the International Civil Aviation Organization (ICAO).

This new ICAO Standard Atmosphere was officially accepted by NACA in 1952 and forms the basis of tables in NACA report 1235. The tables extended from 5 km below to 20 km above mean sea level.

Since 1952, and with increased knowledge because of the large scale use of high-altitude sounding rockets and satellites, extended tables above 20 km were published. Finally in 1962, the U.S. Standard Atmosphere (1962) was published to take into account this new data.

As the requirements of the nation’s space program expanded, a need was generated for information on the variability of atmospheric structure that would be used in the design of second-generation scientific and military aerospace vehicles.

Systematic variations in the troposphere due to season and latitude had been known to exist and thus a new effort was begun to take those variations into account. The result was the publication of the most up-to-date standard atmospheres-the U.S. Standard Atmosphere Supplements (1966).

In this standard, there are two sets of tables-one set for altitudes below 120 km and one for altitudes, 120 km to 1000 km. The model atmospheres below 120 km are given for every 15° of latitude for 15° N to 75° N and in most cases for January and July (or winter and summer).

Above 120 km, models are presented to take into account varying solar activity. The older 1962 model is classified in the 1966 supplements as an average mid-latitude (30° N to 60° N) spring/fall model.

The 1962 U.S. Standard Atmosphere is the more general model. Here it is helpful to list the standard sea level conditions:

In the troposphere (from sea level to 10 to 20 km in the standard atmosphere), the temperature decreases linearly with altitude. In the stratosphere it first remains constant at about 217 K before increasing again. The speed of sound varies accordingly. Both the density and pressure decrease rapidly with altitude. The lift on an airfoil is directly dependent on the density.

The real atmosphere is dependant upon thermal effects of the Sun, the presence of continents and oceans, and the Earth's rotation all combine to stir up the atmosphere into a nonuniform, nonstandard mass of gases in motion.

Although a standard atmosphere provides the criteria necessary for design of an aircraft, it is essential that "nonstandard" performance in the real atmosphere be anticipated also.

Winds and turbulence have a direct affect on aircraft. This concerns the relative motion of the atmosphere. Although in the standard atmosphere the air is motionless with respect to the Earth, it is known that the air mass through which an airplane flies is constantly in a state of motion with respect to the surface of the Earth. Its motion is variable both in time and space and is exceedingly complex.

The motion may be divided into two classes: (1) large-scale motions and (2) small-scale motions. Large-scale motions of the atmosphere (or winds) affect the navigation and the performance of an aircraft.

(a) the pilot is attempting to fly his aircraft from point A to point B. He sets his heading and flies directly for point B but winds (representing large-scale motion of the atmosphere relative to the ground) are blowing crosswise to his intended flight path.

After the required flight time which would have brought the pilot to point B if there were no winds, the pilot finds himself at point C. The winds, which were not taken into account, had forced him off course.

In order to compensate for the winds, the pilot should have pointed the aircraft slightly into the wind as illustrated in (b). This change would have canceled out any drifting of the aircraft off course. Compensation for drift requires knowledge of both the aircraft's velocity and the wind velocity with respect to the ground.

The small-scale motion of the atmosphere is called turbulence (or gustiness). The response of an aircraft to turbulence is an important matter. In passenger aircraft, turbulence may cause minor problems such as spilled coffee and in extreme cases injuries if seat belts are not fastened.

Excessive shaking or vibration may render the pilot unable to read instruments. In cases of precision flying such as air-to-air refueling, bombing, and gunnery, or aerial photography, turbulence-induced motions of the aircraft are a nuisance.

Turbulence-induced stresses and strains over a long period may cause fatigue in the airframe and in extreme cases a particular heavy turbulence may cause the loss of control of an aircraft or even immediate structural failure.

There are several causes of turbulence. The unequal heating of the Earth's surface by the Sun will cause convective currents to rise and make the plane's motion through such unequal currents rough.

On a clear day the turbulence is not visible but will be felt; hence, the name "clear air turbulence (CAT)." Turbulence also occurs because of winds blowing over irregular terrain or, by different magnitude or direction, winds blowing side by side and producing a shearing effect.

In the case of the thunderstorm, this is one of the most violent of all turbulences where strong updrafts and downdrafts exist side by side. The severity of the aircraft motion caused by the turbulence will depend upon the magnitude of the updrafts and downdrafts and their directions.

Many private aircraft have been lost to thunderstorm turbulence because of structural failure or loss of control. Commercial airliners generally fly around such storms for the comfort and safety of their passengers.

Another real atmospheric effect is that of moisture. Water in the air, in either its liquid or vapor form, is not accounted for in the pure dry standard atmosphere and will affect an aircraft in varying degrees.

Everyone is familiar with the forms of precipitation that can adversely affect aircraft performance such as icing on the wings, zero visibility in fog or snow, and physical damage caused by hail. Water vapor is less dense than dry air and consequently humid air (air containing water vapor) will be less dense than dry air. Because of this, an aircraft requires a longer take-off distance in humid air than in the more dense dry air.

Air density is a very important factor in the lift, drag, and engine power output of an aircraft and depends upon the temperature and pressure locally. Since the standard atmosphere does not indicate true conditions at a particular time and place, it is important for a pilot to contact a local airport for the local atmospheric conditions.