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Chapter 2.

The Flight Environment

Introduction

In order to study the principles of flight it is first necessary to understand the medium in whichflight takes place.

The Atmosphere

The atmosphere is a region of air surrounding the earth up to a height of approximately 500miles (900 Km). Air is a mixture of gases, the principal ones being oxygen 21% and nitrogen78% by volume. Up to a height of 6 miles (11 Km) water vapour is also found in varying

quantities. The actual amount of water vapour in a given mass of air depends on thetemperature and whether the air has recently passed over a large area of water. Generally thehigher the temperature, the greater the amount of water vapour a given mass of air can hold.Air has weight and is also compressible. Its pressure, density and temperature all decreasewith increasing altitude. An aircraft does work on the air to sustain flight, and any change inpressure, density and temperature will effect the amount of energy that the aircraft can extractfrom the air.

The Universal Gas Law

The relationship between pressure, density and temperature is:

constantT

P=

where P = Pressure (N/m),T = Absolute Temperature (Kelvin),= Density or Mass per unit volume (kg/m

3)

This equation only applies to a perfect gas when a given mass occupies a given volume, butfrom this equation it is possible to establish effect of pressure and temperature on density.

The Effect of Pressure on Density

Air is a gas, and can be easily compressed or expanded. When air is compressed the numberor mass of molecules in a given volume increases and the density rises. Conversely as air isexpanded the original volume will contain fewer molecules and the density will reduce. Densityis therefore directly proportional to pressure, i.e. if the pressure is doubled then the density is

also doubled. This statement is only true if the temperature remains constant.

Density Pressure

Effect of Temperature on Density

As air is heated the molecules tend to speed up and increase the spacing between them. Agiven volume will contain fewer molecules and air density will decrease. Conversely as air iscooled the degree of molecular movement will decrease and the given volume will now containa greater number of molecules, so the density will increase. Air density is inversely proportionalto temperature, i.e., if the temperature is doubled the density will half. This statement is onlytrue if the pressure remains constant.

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Density 1/Temperature

The Effect of Altitude on Density

With increasing altitude pressure and temperature both decrease. As stated previously the dropin temperature will cause an increase in density whilst the drop in pressure will cause adecrease in density. These factors act in opposition to each other, but pressure has adominating influence over density. Consequently, pressure, temperature and density alldecrease with increasing altitude.

The Effect of Humidity on Density

It has been assumed that air is perfectly dry. In fact there is always a certain amount of watervapour in the atmosphere, which varies from place to place, and day to day. When watervapour is present in the air it affects its density because it is less dense than dry air, and

therefore lighter. The density of water vapour under standard sea level conditions is 0.760kg/m

3, whereas the density of dry air is 1.225 kg/m

3. Water vapour therefore weighs 5/8 as

much as dry air. This means that, for a given volume, air is least dense when it contains amaximum amount of water vapour and most dense when it is perfectly dry. Humidity willtherefore affect aircraft performance.

The International Standard Atmosphere

In order to compare aircraft performance and calibrate aircraft instruments, it is necessary tohave an internationally agreed Standard Atmosphere covering temperature, pressure anddensity for varying altitudes. The International Standard Atmosphere (ISA), part of which isshown below, is laid down by the International Civil Aviation Organisation (ICAO).

HEIGHT ABOVE SEA LEVEL

FEET METRES

52,496

45,934

39,372

32,810

26,248

19,686

13,124

6,562

0

16,000

14,000

12,000

10,000

8,000

6,000

4,000

2,000

0

DENSITY

KG/M3

PRESSURE

MILLIBARS

TEMPERATURE

C

0.166

0.288

0.312

0.414

0.526

0.660

0.819

1.007

1.225

104

142

194

265

357

472

612

795

1013.25

-56.6

-56.5

-56.5

-50

-37

-24

-11

2

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