The Atmosphere

Introduction

The atmosphere is the medium in which an aircraft operates. It is the properties of the atmosphere, changed by the shape of the wing, that generate the required lift force.

The most important property is air density (the “thickness” of air).

KEY FACT: If air density decreases, the mass of air flowing over the aircraft in a given time will decrease. Not usually considered during the study of Principles of Flight, keeping the idea of mass flow (kg/s) in the ‘back of your mind’ can aid general understanding.

A given mass flow will generate the required lift force, but a decrease in air density will reduce the mass flow.

To maintain the required lift force if density decreases, the speed of the aircraft through the air must be increased. The increased speed of airflow over the wing will maintain the mass flow and lift force at its required value.

The Physical Properties of Air

Air has substance! Air has mass; not very much if compared to other matter, but nevertheless a significant amount. A mass of moving air has considerable kinetic energy; for example, when moving at 100 knots the kinetic energy of air can inflict severe damage to man-made structures.

Air is a compressible fluid and is able to flow or change its shape when subjected to even minute pressure differences. (Air will flow in the direction of the lower pressure). The viscosity of air is so low that very small forces are able to move the molecules in relation to each other.

When considering the portion of atmosphere in which most aircraft operate (up to 40 000 ft), with increasing altitude the characteristics of air undergo a gradual transition from those at sea level. Since air is compressible, the lower layers contain much the greater part of the whole mass of the atmosphere. Pressure falls steadily with increasing altitude, but temperature falls steadily only to about 36 000 ft, above which it then remains constant through the stratosphere.

Static Pressure

The unit for static pressure is N/m2, the symbol is lower case ‘p’.

•Static pressure is the result of the weight of the atmosphere pressing down on the air beneath.

•Static pressure will exert the same force per square metre on all surfaces of an aeroplane. The lower the altitude, the greater the force per square metre.

•It is called static pressure because of the air’s stationary or static presence.

•An aircraft always has static pressure acting upon it.

Newtons per square metre is the SI unit for pressure. 1 N/m2 is called a pascal and is quite a small unit. In aviation the hectopascal (hPa) is used. (‘hecto’ means 100 and 1 hectopascal is the same as 1 millibar).

The Atmosphere 2

Atmosphere The 2

Static pressure at a particular altitude will vary from day to day, and is about 1000 hPa at sea level. In those countries that measure static pressure in inches of mercury (inHg), sea level static pressure is about 30 inHg.

Temperature

The unit for temperature is °C, or K. It is degrees Celsius (or centigrade) when measured relative to the freezing point of water, or Kelvin when measured relative to absolute zero. (0°C is equivalent to 273 K).

Temperature decreases with increasing altitude up to about 36 000 ft and then remains constant.

Air Density

The unit for density is kg/m3 and the symbol is the Greek letter ρ [rho].

•Density is ‘mass per unit volume’ (The ‘number’ of air particles in a given space).

•Density varies with static pressure, temperature and humidity.

•Density decreases if static pressure decreases.

•Density decreases if temperature increases.

•Density decreases if humidity increases.

Air Density is proportional to pressure and inversely proportional to temperature. This is shown in the ideal gas law formula below.

Density decreases with increasing altitude because of decreasing static pressure. However, with increasing altitude temperature also decreases, which would tend to increase density, but the effect of decreasing static pressure is dominant.

International Standard Atmosphere (ISA)

The values of temperature, pressure and density are never constant in any given layer of the atmosphere. To enable accurate comparison of aircraft performance and the calibration of pressure instruments, a ‘standard’ atmosphere has been adopted. The standard atmosphere represents the mean or average properties of the atmosphere.

Europe uses the standard atmosphere defined by the International Civil Aviation Organization (ICAO).

The ICAO standard atmosphere assumes the following mean sea level values: