TAV0013

BASIC AERODYNAMIC

Prepared by Zuliana Ismail for Diploma in Aircraft Maintenance, MSU

Chapter 1: Physics of the atmosphere

Contents

The Atmosphere Gas Composition Regions of the Atmosphere Pressure Temperature Air Density Humidity

The Gas Laws The International Standard Atmosphere

(ISA)

Air is one of the primary things that makes life on Earth possible. Air is a synonym for atmosphere. The Earth’s atmosphere or air is made up of a variety of gases and other particles.

Introduction

Gas Composition• What is the Atmosphere??

– layer of gases surrounding the planet Earth that is retained by Earth's gravity.

• Gases in atmosphere:– 78% - Nitrogen– 21% - Oxygen– 1% - Other gases (Argon,

Carbon Dioxide, Neon, Helium)

Regions of the Atmosphere

Regions of the Atmosphere

Troposphere Stratosphere Mesosphere Ionosphere Thermosphere

Troposphere starts at Earth's surface and goes up to a height of 14 km above sea level (0-36’000 feet).

• Almost all weather occurs within this layer (below 15’000 feet). • Temperature will decrease with altitude.• Almost all clouds in this layer.

Troposphere

Tropopause Boundary between troposphere and stratosphere. Temperature stop to decrease. Height of tropopause is important because it MARKS:

• Max. Height of Cloud.• Presence of Jetstream. (a band of strong winds at high altitude)• Presence of Clear Air Turbulence (CAT). turbulence

encountered in air where no cloud is present

Stratosphere The stratosphere

occurring between about 14 km to 50km or (36’000 feet-160’000 feet)

Very small increase in temperature (almost constant) except at the top where temperature is zero at 50 kms.

Stratosphere

• Commercial jet aircraft fly in the lower stratosphere to avoid the turbulence which occur in the troposphere layer.

• Air is thinner at the top of the stratosphere than it is at sea level. Aircraft reach their maximum operational altitudes

Variation in Atmosphere

The earth’s atmosphere varies vertically and horizontally in

Pressure Temperature Air Density Humidity

Pressure Pressure:

Pressure is the force per unit area:

Pressure can be in the unit of mmHg, atmosphere (atm), Pascal (Pa), Pa, milibar (mb), Inch of mercury etc.

Static pressure (PS) Static pressure is the pressure particles of air exert on adjacent bodies. Ambient

static pressure is equal to the weight of a column of air over a given area. The force of static pressure always acts perpendicular to any surface that the air particles collide with, regardless of whether the air is moving with respect to that surface.

Cont.

Pressure is the cumulative effect of air molecules colliding with a surface.

Air Pressure Depends On:• The mass of the molecules (density)

• Temperature

Pressure Variation with Altitude As altitude increases, there is less air in

the column above, so it weighs less.

Thus atmospheric static pressure decreases with an increase in altitude.

At troposphere (pressure decrease 1 inch for each 1000 ft increase in altitude).

The higher the altitude, the lower the air pressure

Temperature

Temperature (T) is a measure of the average random kinetic energy of air particles.

Scale Fahrenheit scale: from -32 to 212 deg

Celcius (centigrade) scale: from 0 to 100 deg

Kelvin (absolute) scale: from -273 to 373 deg

Temperature Variation with Altitude

Air temperature decreases linearly with an increase in altitude at a rate of 2 °C per 1000 ft until approximately 36,000 feet.

This rate of temperature change is called the average lapse rate.

From 36,000 feet through approximately 66,000 feet, the air remains at a constant −56.5 °C. This layer of constant temperature is called the isothermal layer.

Pressure and Temperature decrease with altitude

Altitude (Feet)

Pressure (mm/ Hg)

Temperature (degree Celsius)

0 760 15.0

18,000 380 -5.3

34,000 190 -62.3

48,000 95 -67.3

63,000 47 -67.04

Air Density

Density : mass per unit volume (m/v)

Air density (ρ) is the total mass of air particles per unit of volume.

Density inversely proportional to temperature. When air heated (temperature increase), the

volume of air will expand. Therefore the mass of air decreases and lead to density decreases.

The distance between individual air particles increases with altitude resulting in fewer particles per unit volume.

Therefore, air density decreases with an increase in altitude.

Density Variation with Altitude

How Change of Density affect the aircraft operation

The higher the aircraft altitude the thinner the air. LOWER AIR DENSITY.

As the air becomes thinner/less dense, it creates less resistance to objects flying through it. REDUCE DRAG.

Therefore, LESS THRUST is needed to move the aircraft forward at a given speed.

As a result, airplanes can fly more efficiently at higher altitudes with LESS FUEL.

However if airplane is too high (>60,000ft), AIR DENSITY TOO LOW or ZERO AIR DENSITY it will cause LIFT force highly reduce or no Lift force. This condition is dangerous where it can cause airplane lose it control (STALL) .

Humidity

Humidity is the amount of water vapor in the air.

Water vapor: water in its gaseous state

What change in air density occurs with an increase in humidity. As humidity increases, water

molecules displace an equal number of air molecules.

Since water molecules have less mass and do not change the number of particles per unit volume of air, density decreases.

Therefore, as humidity increases, air density decreases.

Conclusion

The Earth’s atmosphere or air is made up of a variety of gases, other particles and in form of several layers.

The physical condition of atmosphere different for each layer.

The earth’s atmosphere varies vertically and horizontally in temperature, pressure, density and humidity.

THE GENERAL GAS LAW

The General Gas Law sets the relationship between three properties of air: pressure (P), density (ρ), and temperature (T). It is expressed as an equation where R is a constant for any given gas (such as dry air):

P = ρRT One method to increase pressure is to keep density

constant and increase temperature (as in a pressure cooker).

If pressure remains constant, there is an inverse relationship between density and temperature.

An increase in temperature must result in a decrease in density, and vice versa.

The International Standard Atmosphere (ISA)

ISA = an atmospheric model of how the pressure, temperature, density, and viscosity of the Earth's atmosphere change over a wide range of altitudes.

To provide a common reference for temperature and pressure and consists of tables of values at various altitudes.

The International Civil Aviation Organization (ICAO) published their "ICAO Standard Atmosphere".

It has the same model as the ISA, but extends the altitude coverage to 80 kilometres (262,500 feet.)

The standard is very useful in Meteorology for comparing against actual values.