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CHAPTER 2

THE ATMOSPHERE

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CHAPTER 2: THE ATMOSPHERE

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THE ATMOSPHERE.

Aircraft.

All aircraft, by the very defnition of the word, can fy only when immersed in the air.

Lighter-than-air craft such as hot-air balloons are called aerostats, while heavier- than-air craft which require relative movement between the air and their lifting surfaces are called aerodynes.

Of the aerodynes, fxed-wing craft are called generically aeroplanes. In the word aeroplane, the word-element plane refers to the mainplanes, more commonly known as wings, and the tail-plane, which the Americans often refer to as the horizontal stabiliser. The fn of an aeroplane is a plane, too; the Americans call it a vertical stabiliser. So, you see, plane has a particular technical meaning when referring to aircraft, but the word plane certainly does not refer to the complete aircraft. The complete aircraft may, though, be called an aeroplane. In everyday speech you will often hear people talking about “passenger planes” and “military planes”. But because you are a pilot, you might choose to use the more correct words; it is a personal choice.

Rotary-wing craft are known collectively as helicopters, from the Greek pteron meaning wing and the Greek heliko - from helix, meaning spiral. (Leonardo da Vinci’s unsuccessful late-15th century design for a vertical take-off fying machine featured a rotating spiral wing).

In this book on Principles of Flight we shall be considering the fight of aeroplanes only.

Throughout the book, the words aeroplane and aircraft will be used synonymously.

The Composition of the Atmosphere.

The Principal Gases.

As it is the relative movement of aeroplanes and air which generates the aerodynamic forces which enable an aircraft to fy, we may logically begin our study of the Principles of Flight by examining the nature of the Earth’s atmosphere.

The gaseous atmosphere which surrounds our Earth is similar to a giant ocean of air. The light aircraft fown by most private-pilot licence-holders operate in the lower 10 000 feet of the atmosphere, whereas jet airliners regularly fy at altitudes up to about 40 000 feet. The total depth of the atmosphere has been calculated to be about 500 miles (800 km), but about 90% of the mass of air lies in the lower 50 000 feet (9 miles or 15 km) of the atmosphere.

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Figure 2.1 The four main gases which make up the Atmosphere.

CHAPTER 2: THE

The air in the atmosphere

is made up primarily of Nitrogen (78%) and Oxygen

(21%). The remaining 1% is mainly Carbon Dioxide and Argon.

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ATMOSPHERE

The air in our atmosphere is made up primarily of Nitrogen (78%) and Oxygen (21%). (See Figure 2.1) The remaining 1% consists mainly of Argon and Carbon Dioxide, with traces of Carbon Monoxide, Helium, Methane, Hydrogen and Ozone. It is this mixture of gases which not only enables an aeroplane to fy but which also makes up the air which sustains human life and enables the combustion of fuel to take place to drive piston engines and gas turbine engines.

When air cools to its

saturation point, or dew

point, the invisible gas water vapour condenses out to

its liquid state, and cloud is formed.

Water Vapour and Humidity.

Atmospheric air also contains a small amount of water vapour of varying quantity. The measure of the amount of water vapour contained in an air mass

is termed humidity. Meteorologists measure humidity in several ways: for example, mass of water vapour per unit volume of air (say, 5 gm/m³), or mass of water vapour per unit mass of air (say, 3 gm/kg). As we have said, atmospheric air contains very little water vapour (it is never more than 4% by volume), but the infuence of this water vapour is signifcant, especially on our weather.

Despite the presence of water vapour in the air, the air is normally invisible because water vapour can exist in the air as an invisible gas. The higher the temperature of the air, the more water the air can hold in its gaseous form. As temperature decreases, the air can hold progressively less water vapour, and eventually water condenses out onto microscopic impurities (hygroscopic nuclei) in the air, or onto surfaces in contact with the air. This is why you can see your breath on a cold day, why breathing onto a cold glass surface will cause the glass to mist up, and, of course, it is the reason why clouds form.

When air can no longer hold any more water vapour as gas, the air is said to have reached its saturation point. The temperature of air at its saturation point, that is, the air temperature at which water vapour condenses out to water, is called the dew point. The more water vapour there is in the air, the higher the dew point will be. And,

Figure 2.2 Air is normally invisible but condensed water vapour in the air is visible as cloud.

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of course, on the Earth’s surface, at an airfeld for instance, for any given content of water vapour, the nearer the dew point is to the actual temperature of the air, the greater the danger that the saturation point will be reached, causing condensation to occur and mist or fog to ensue.

Even unsaturated air, as it rises in the atmosphere, under whatever infuence, will cool, and its temperature decreases suffciently for the rising air eventually to reach its saturation point. At the saturation point, cloud is formed. Mist and fog, of course, are just cases of low level cloud. If the visibility is less than 1000 m, the condition is termed fog, and if 1000 m or more, mist.

You will often hear the term relative humidity used in aviation circles. Relative humidity is an expression of the ratio of the amount of water vapour actually present in the air to the amount of water vapour the air can “hold” at any given temperature. When the temperature of the air falls to the dew point, relative humidity will become 100%. The air will then be saturated and the water vapour will condense out, changing from the gaseous to the liquid state.

Water vapour is lighter than the same volume of dry air at equal pressure. Therefore, for a given temperature and pressure, a mixture of air and water vapour will be less dense if the water vapour content is high than if the water vapour content is low (see Figure 2.3).

Relative

humidity is the ratio of

the amount of

water vapour present in the air to the amount of water vapour the air can hold at the same temperature.

When

temperature falls to the

dewpoint,

relative humidity is 100%, and water vapour condenses out into its liquid state.

If the water

vapour content of air increases,

air density

will decrease at constant temperature and pressure.

Figure 2.3 Moist air is less dense than dry air.

Air Pressure and Air Density.

The atmosphere was frst formed when its gases were released from the Earth during the Earth’s formation, over 4 billion years ago. The gases which now make up the air of our atmosphere were prevented from escaping into space by the Earth’s force of gravity, so, over an unknown period of time, molecules of air spread out to cover the entire surface of our planet. The gravitational force acting between objects is proportional to the mass of those objects, but gets weaker as the distance between the objects increases, so many more air molecules are held in contact with the Earth’s surface than are present in the higher reaches of the atmosphere. This fact and the fact that the air near the surface is compressed by the weight of the mass of air above it mean that air pressure and air density are greatest near the surface of the Earth, and decrease with increasing altitude.

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Figure 2.4 Air Density and Pressure decrease with increasing altitude.

CHAPTER 2: THE

Air density and air

pressure decrease with

increasing altitude.

At constant temperature,

an increase in air pressure

causes an increase in air density.

At constant pressure, an

increase in air temperature

will cause a decrease in air density.

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ATMOSPHERE

A useful analogy of the variation of pressure and density with altitude is to consider foam rubber blocks piled on top of one another. If we consider any one of the blocks, we can see, in our mind’s eye, that it is compressed by an amount which is proportional to the number and weight of blocks above it, and that the maximum compression is experienced by the block at the very bottom of the pile. Similarly, air pressure and air density are highest at the Earth’s surface. (See Figure 2.4).

Air density refers to the number of air molecules contained within a given volume of air and is measured in terms of mass per unit volume. The standard units of air density are kilograms per

cubic metre. The greater the pressure acting on a given volume of air, the greater the number of air molecules that are contained within that volume. Consequently, air density is directly proportional to pressure. When a given mass of air is heated at constant pressure it expands and its volume increases. Because of this increase in volume, the molecules of air are contained within a larger space and, thus, the mass per unit volume of the air – that is the density of the air – decreases. Air density, then, is inversely proportional to temperature; that is, it decreases with increasing temperature.

In general, both engine and fight performance decrease with decreasing air density which is why pilots need to be especially careful in their performance calculations when operating from airfelds which are “hot and high”: on the continent of Africa, for example.

Both pressure and temperature decrease with increasing altitude. But although a decrease in pressure will cause density to decrease while a decrease in temperature causes density to increase, the effect of the decreasing pressure on air density is the greater.

Air density is of considerable importance for the measurement of aircraft performance. Lift, service ceiling, and the relationship between true and indicated airspeed all depend on air density. If air density is low, not only will the lift generated by the wings be less for any given true airspeed, but the power output of the engine will be lower too. Consequently, in low density conditions at an airfeld (e.g. high temperature, and high airfeld elevation), longer take-off runs will be required for an aircraft of any given take-off mass.

Pressure is a description of the way in which a force is spread over a contact area.

Pressure is defned as “force per unit area”. In Principles of Flight, the pressure exerted by the atmosphere on objects immersed in it, when neither the air nor the object is in motion, is known as atmospheric pressure or static pressure. The standard unit of pressure is the Newton per square metre, but, in Principles of Flight you will rarely, if ever, hear pressure expressed in those units. In Britain and, especially, the United States, you might still hear pressure expressed, generally, in pounds per square

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inch. In engineering, the bar or millibar is often used, as is the Pascal or hectopascal. The millibar and the hectopascal are also used in Meteorology and Altimetry. In the United States, inches of mercury are the units of pressure in Altimetry. Like air density, atmospheric pressure (static pressure) decreases with increasing altitude.

Though the pressure exerted by the atmosphere at the Earth’s surface varies from day to day for reasons you will learn about in Meteorology, at sea level, atmospheric pressure is in the order of 100 000 Newtons per square metre, 1 bar, 1 000 millibars, 1 000 hectopascal, 14.7 pounds per square inch or 30 inches of Mercury.

The pressure of the atmosphere acts in all directions, acting on every square inch of every object immersed in it. (See Figure 2.5) For instance, a 6 foot (1.83 metres) human being on the surface of the Earth carries a total load of over ten tons. This is, of course, the equivalent of a force of 14.7 lbs (6.7 kg force) acting on every square inch of his body surface. But despite the fact that atmospheric pressure on the surface of the Earth is very high, we do not notice the pressure, because the pressure inside our own bodies balances this atmospheric pressure. But when the pressure inside any hollow object is less than atmospheric pressure, the difference in pressure can be withstood only by the strength of the object’s structure. You have probably all witnessed during school Physics lessons that an empty tin can will collapse if the air inside it is removed.

Atmospheric

pressure, also known as static

pressure, acts

in all directions on a body immersed in the atmosphere.

Figure 2.5 Atmospheric pressure acts in all directions.

We see then that air possesses mass, and that the force of gravity acting on that mass gives air weight which is the ultimate reason why our atmosphere exerts a pressure on objects immersed in it, and why the pressure and the density of the air decrease with altitude. As you will learn in subsequent chapters, these are the properties of air which enable aeroplanes to fy.

Variations in atmospheric pressure and density, along with variations in humidity, have a signifcant infuence on aircraft performance and on the functioning of fight instruments, as you will learn in the Aeroplanes (General) volume of this series.

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CHAPTER 2: THE ATMOSPHERE

The Temperature of the Atmosphere.

The temperature of atmospheric air, like air density and atmospheric pressure, also decreases with increasing altitude. The air is not heated directly by the sun. The sun’s short wave radiation passes through the atmosphere without heat being absorbed by the air. The Earth’s surface, however, is heated up by solar radiation, and it is the Earth which heats up the air in contact with, and near, its surface by conduction, convection and long-wave radiation. Not surprisingly, then, it is the lowest layer of atmospheric air which is heated through its proximity to the Earth’s surface and it is in that lowest layer where a clear and steady decrease in temperature with increasing altitude occurs. (See Figure 2.7.) The lowest layer of the atmosphere is known as the Troposphere, from the Greek word tropos meaning mixing or turning, which undoubtedly refers to the fact that it is in the Troposphere that temperature and pressure changes cause the meeting and mixing of air which gives rise to our weather. Almost all of the Earth’s weather occurs in the Troposphere, so if you fnd yourself fying in an airliner on a European route, at 38 000 feet, you are indeed, in all probability, fying above the weather.

The Troposphere rises from the Earth’s surface to about 50 000 feet over the Equator, 25 000 feet over the Poles, and about 36 000 feet at mid-latitudes. The Troposphere contains approximately 75% of the total mass of the atmosphere, and all of the water vapour.

Figure 2.6 The various layers of the atmosphere with approximate heights in kilometres, one kilometre being 3 281 feet.

The boundary between the Troposphere and the layer immediately above it, the Stratosphere, is called the Tropopause. At the Tropopause, the temperature is around - 56.5º Celsius ( - 69º Fahrenheit), and this temperature remains constant to an altitude of about 18 miles, or 35 kilometres. At altitudes greater than that, the temperature begins to rise again. But 18 miles high is 95 000 feet, so we will end our account of temperature variation with altitude there, and leave these higher regions to astronauts.

The Atmosphere and Flight.

The important facts to retain about the physical properties of the atmosphere in terms of your study of the Principles of Flight, is that air has mass, and that the pressure, density, temperature and relative humidity of the air change in certain circumstances.

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Another property of the air which is important in Principles of Flight is its viscosity.

The viscosity of air is a measure of its resistance to fow because of a kind of internal friction acting between the air molecules as they move relative to one another. The viscosity of a fuid is often being described when we talk of a fuid being thick or thin.

Air and water have a low viscosity and might be described as thin, whereas treacle and tar have a high viscosity and are seen as thick fuids. The viscosity of air, then, is low, but air does possess a measurable degree of viscosity and this viscosity has consequences for an aircraft in fight.

The ICAO Standard Atmosphere (ISA).

Changes of air pressure, air density, air temperature and humidity within the atmosphere greatly affect the performance of an aircraft in fight as well as the readings of certain fight instruments. In the real atmosphere, of course, these properties are changing continuously with altitude, with passing time and from place to place. In order, therefore, that aerodynamicists, aircraft manufacturers and engineers might have a set of standard values for pressure, density temperature etc, against which to measure aircraft performance and to calibrate instruments, a so-called standard atmosphere was defned by the International Civil Aviation Organisation (ICAO) in

1964. The ICAO Standard Atmosphere, generally known by its initials ISA, shows a standard variation of pressure, temperature, density, and viscosity, with altitude. ISA, then, serves as an international standard reference so that, when dealing with the measurement of aircraft performance and the calibration of instruments, everyone can be sure that they are working to the same set of atmospheric conditions.

Changes of

pressure, density, and

humidity of

the air, all affect aircraft performance.

The ICAO Standard Atmosphere, with its signifcant values for the variation of temperature, pressure and density with altitude, is illustrated at Figure 2.7. Mean Sea Level air pressure in the ICAO Standard Atmosphere (which we will, henceforth in this volume, refer to as ISA) is 1013.2 millibars (1013.2 hectopascals) or 29.92 inches of Mercury. The ISA temperature at Mean Sea Level is 15º Celsius. In the ISA, temperature decreases with altitude at approximately 2º Celsius for every 1 000 ft.

Density, and

pressure decrease with

altitude, and

temperature decreases with altitude up to the tropopause.

Figure 2.7 The ICAO Standard Atmosphere.

Any values for atmospheric pressure, density and temperature given in this volume will be ISA values. It is important that you remember, though, that the actual values for atmospheric pressure, density and temperature which prevail on any given day

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CHAPTER 2: THE ATMOSPHERE

are inevitably different from the ISA values. (It would be in the order of a million to one chance that all actual values were the same as ISA values.) Consequently, as the calibrations of fight instruments, such as the altimeter and the air speed indicator, as well as manufacturers’ fgures for aircraft performance, assume that an aircraft is fying in ISA conditions, it is very important, when reading instruments and measuring aircraft performance, on an actual fight, that pilots and engineers understand the effect that the atmosphere’s deviation from ISA conditions has on the information they are reading. The topic of ISA Deviations is dealt with in detail in the Meteorology and Aircraft (General) volumes in this series.

The ISA sea level pressure of 1013.2 millibars is also the altimeter subscale setting, which a pilot selects when reading his altitude in terms of Flight Level. Flight Levels are also known as Pressure Altitudes.

The Measurement of Temperature.

Before we leave our brief look at the atmosphere, there is one more observation that needs to be made on the measurement of temperature.

The standard unit of measurement of temperature in the aviation world, outside the United States, is degrees Celsius (formerly Centigrade). However, the Fahrenheit scale was the primary scale of temperature measurement for non-scientific purposes in most English-speaking countries until the 1960s. Consequently, you will meet degrees Fahrenheit frequently in the United States, and still occasionally in Britain. So, although aviation meteorological reports and forecasts mention temperature in degress Celsius, it is still useful to be able to convert between the two scales.

In degrees Fahrenheit, water freezes at 32ºF and boils at 212ºF; in degrees Celsius, 0ºC degrees is the freezing point of water, and 100ºC its boiling point. So in the Fahrenheit scale there are 180º between the boiling points of water, whereas, in the Celsius scale, there are, of course, 100º. Therefore, one Fahrenheit degree is only 5/9 the value of a Celsius degree, (100/180 = 5/9).

The formulae for converting from one scale to the other are:

Conversion from	To	Formula
Fahrenheit	Celsius	ºC = (ºF - 32) × 5/9
Celsius	Fahrenheit	ºF = (ºC × 9/5) + 32

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