ppl pof ch 2

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Questions and Answers

What is the term for a heavier-than-air craft that requires relative movement between the air and its lifting surfaces?

Aeroplane

Aerostat

Helicopter

Aerodyne (correct)

What does the word 'plane' refer to in the context of an aeroplane?

The horizontal stabiliser

The vertical stabiliser

The complete aircraft

The main wings and tail-plane (correct)

What is the term for a lighter-than-air craft?

Aeroplane

Helicopter

Aerodyne

Aerostat (correct)

What is the term for a rotary-wing craft?

Helicopter Signup and view all the answers

What is the focus of this book on Principles of Flight?

The flight of aeroplanes Signup and view all the answers

What is the meaning of the Greek word 'heliko' in the context of helicopters?

Spiral Signup and view all the answers

What percentage of the atmosphere is composed of Oxygen?

21% Signup and view all the answers

What is the approximate total depth of the atmosphere?

500 miles Signup and view all the answers

What is the primary function of the atmosphere in relation to aeroplanes?

To enable an aeroplane to fly Signup and view all the answers

What is the approximate percentage of the atmosphere that lies in the lower 50 000 feet?

90% Signup and view all the answers

What is the primary component of the atmosphere?

Nitrogen Signup and view all the answers

What is the approximate altitude that jet airliners regularly fly at?

40 000 feet Signup and view all the answers

What is the term used to describe the amount of water vapour in an air mass?

Humidity Signup and view all the answers

What is the maximum percentage of water vapour in atmospheric air by volume?

4% Signup and view all the answers

What happens to the air when it reaches its saturation point?

Water condenses out Signup and view all the answers

What is the term for the temperature at which water vapour condenses out to water?

Dew point Signup and view all the answers

Why can you see your breath on a cold day?

Because water condenses out onto microscopic impurities in the air Signup and view all the answers

What happens to the dew point when there is more water vapour in the air?

It increases Signup and view all the answers

What occurs when air reaches its saturation point?

Cloud is formed Signup and view all the answers

What is the result when the air reaches its saturation point?

Mist or fog ensues Signup and view all the answers

What is the difference between fog and mist?

Fog has a visibility of less than 1000 m, mist has a visibility of 1000 m or more Signup and view all the answers

Why does breathing onto a cold glass surface cause the glass to mist up?

Because water condenses out onto the surface Signup and view all the answers

What is relative humidity an expression of?

The ratio of the amount of water vapour present to the amount the air can 'hold' at a given temperature Signup and view all the answers

What happens when the air temperature falls to the dew point?

The air becomes saturated and water vapour condenses Signup and view all the answers

What is the characteristic of a mixture of air and water vapour with high water vapour content?

It becomes less dense than dry air Signup and view all the answers

What is the relationship between water vapour and dry air at equal pressure?

Water vapour is lighter Signup and view all the answers

What is the main reason why air molecules are held in contact with the Earth's surface?

The Earth's force of gravity Signup and view all the answers

What is the relationship between air pressure and air density?

Air pressure is directly proportional to air density Signup and view all the answers

What happens to the volume of air when it is heated at constant pressure?

It increases Signup and view all the answers

What is the unit of measurement for air density?

Kilograms per cubic meter Signup and view all the answers

What is the effect of the weight of air above on the air near the surface?

It increases the air pressure Signup and view all the answers

What is the main factor that determines the air density?

Air pressure Signup and view all the answers

What is the result of the gravitational force acting between the Earth and air molecules?

Air molecules are held in contact with the Earth's surface Signup and view all the answers

What is the analogy used to describe the variation of pressure and density with altitude?

A stack of foam rubber blocks Signup and view all the answers

What happens to air density when temperature increases?

It decreases Signup and view all the answers

What is the effect of decreasing pressure on air density?

It decreases air density Signup and view all the answers

Why do pilots need to be careful when operating from airfields that are 'hot and high'?

Because air density is low Signup and view all the answers

What is the standard unit of pressure?

Newton per square meter Signup and view all the answers

What is the direction of the force exerted by atmospheric pressure?

in all directions Signup and view all the answers

What is the reason for the atmosphere exerting pressure on objects?

The force of gravity acting on the mass of air Signup and view all the answers

What happens to the lift generated by the wings when air density is low?

It decreases Signup and view all the answers

What is the effect of decreasing air density on the power output of the engine?

It decreases the power output Signup and view all the answers

What is the result of low air density on take-off runs?

Longer take-off runs Signup and view all the answers

What happens to atmospheric pressure with increasing altitude?

It decreases Signup and view all the answers

What is the primary source of heat for the atmospheric air?

The Earth's surface Signup and view all the answers

What is the name of the lowest layer of the atmosphere?

Troposphere Signup and view all the answers

At what altitude does the temperature begin to rise again?

18 miles Signup and view all the answers

What percentage of the total mass of the atmosphere is in the Troposphere?

75% Signup and view all the answers

What is the term for the boundary between the Troposphere and the Stratosphere?

Tropopause Signup and view all the answers

At what altitude is the temperature around -56.5°C (-69°F) in the Troposphere?

18 miles Signup and view all the answers

What is the altitude of the Troposphere over the Equator?

50 000 feet Signup and view all the answers

Why does the temperature decrease with increasing altitude in the Troposphere?

Because the Earth's surface heats up the air Signup and view all the answers

What is a measure of air's resistance to flow?

Viscosity Signup and view all the answers

What is true about the viscosity of air?

It is very low Signup and view all the answers

What is affected by the viscosity of air in flight?

The movement of air molecules Signup and view all the answers

What is a characteristic of fluids with high viscosity?

They are thick Signup and view all the answers

What is true about air compared to treacle and tar?

It has a lower viscosity Signup and view all the answers

What is the purpose of understanding ISA deviations in flight?

To understand the effect of atmosphere on instrument readings Signup and view all the answers

What is the standard sea level pressure in ISA conditions?

1013.2 millibars Signup and view all the answers

What is the term for the altitude read from an altimeter with the subscale setting of 1013.2 millibars?

Flight Level Signup and view all the answers

Why is it important to consider ISA deviations when reading aircraft performance data?

To ensure accurate performance data Signup and view all the answers

Where can more information on ISA deviations be found?

Meteorology and Aircraft (General) volumes Signup and view all the answers

What is the primary purpose of the ICAO Standard Atmosphere (ISA)?

To provide a standard reference for aerodynamicists and engineers Signup and view all the answers

What is the air pressure at Mean Sea Level in the ICAO Standard Atmosphere (ISA)?

1013.2 millibars Signup and view all the answers

What is the temperature at Mean Sea Level in the ICAO Standard Atmosphere (ISA)?

15º Celsius Signup and view all the answers

How does the temperature change with altitude in the ICAO Standard Atmosphere (ISA)?

It decreases by 2º Celsius for every 1,000 ft Signup and view all the answers

Why is it important to remember that actual atmospheric values can differ from ISA values?

Because actual values can affect aircraft performance and instrument readings Signup and view all the answers

What is the purpose of the ICAO Standard Atmosphere (ISA) in relation to aircraft instruments?

To calibrate instruments and measure aircraft performance Signup and view all the answers

What is the significance of the ICAO Standard Atmosphere (ISA) in the field of aviation?

It provides a standard reference for measuring aircraft performance and calibrating instruments Signup and view all the answers

What does the ICAO Standard Atmosphere (ISA) illustrate?

The standard variation of pressure, temperature, density, and viscosity with altitude Signup and view all the answers

What is the standard unit of temperature measurement in the aviation world outside the United States?

Degrees Celsius Signup and view all the answers

At what temperature does water freeze in degrees Celsius?

0°C Signup and view all the answers

What is the formula to convert from Fahrenheit to Celsius?

ºC = (ºF - 32) × 5/9 Signup and view all the answers

What is the ratio of a Fahrenheit degree to a Celsius degree?

5/9 Signup and view all the answers

At what temperature does water boil in degrees Fahrenheit?

212°F Signup and view all the answers

What is the formula to convert from Celsius to Fahrenheit?

ºF = (ºC × 9/5) + 32 Signup and view all the answers

Study Notes

Definition of Aircraft

Aircraft can only fly when immersed in air.
Lighter-than-air craft are called aerostats (e.g. hot-air balloons).
Heavier-than-air craft that require relative movement between air and their lifting surfaces are called aerodynes.

Types of Aerodynes

Fixed-wing craft are called generically aeroplanes.
Aeroplanes include mainplanes (wings), tail-plane (horizontal stabiliser), and fin (vertical stabiliser).
Rotary-wing craft are collectively known as helicopters.

Etymology of Helicopter

The word "helicopter" comes from the Greek "pterón" (meaning wing) and "heliko" (from "helix", meaning spiral).
Inspired by Leonardo da Vinci's 15th-century design for a vertical take-off flying machine with a rotating spiral wing.

Scope of the Book

This book focuses on the principles of flight for aeroplanes only.
The terms "aeroplane" and "aircraft" will be used interchangeably throughout the book.

The Composition of the Atmosphere

The atmosphere is a gaseous mixture that surrounds the Earth, similar to a giant ocean of air.
The lower 10,000 feet of the atmosphere is where most private-pilot licence-holders operate their light aircraft.
Jet airliners regularly fly at altitudes up to about 40,000 feet.
The total depth of the atmosphere is approximately 500 miles (800 km).
About 90% of the mass of air lies in the lower 50,000 feet (9 miles or 15 km) of the atmosphere.

Principal Gases in the Atmosphere

Nitrogen makes up 78% of the atmosphere.
Oxygen makes up 21% of the atmosphere.
The remaining 1% consists mainly of Argon and Carbon Dioxide, with traces of:
- Carbon Monoxide
- Helium
- Methane
- Hydrogen
- Ozone

Water Vapour and Humidity

Atmospheric air contains a small amount of water vapour, which varies in quantity.
Humidity is the measure of the amount of water vapour in an air mass.
Meteorologists measure humidity in terms of mass of water vapour per unit volume of air (e.g., 5 gm/m³) or mass of water vapour per unit mass of air (e.g., 3 gm/kg).
Water vapour in the air is usually invisible, but its influence on weather is significant.
The air can hold more water vapour as temperature increases, and less as temperature decreases.

Saturation Point and Dew Point

When air can no longer hold any more water vapour as gas, it reaches its saturation point.
The temperature of air at its saturation point is called the dew point.
The more water vapour in the air, the higher the dew point will be.
When the dew point is close to the actual air temperature, the danger of condensation and mist or fog increases.

Cloud Formation and Condensation

When unsaturated air rises and cools, it eventually reaches its saturation point, forming cloud.
Mist and fog are low-level clouds that form when the air reaches its saturation point.
If visibility is less than 1000 m, it is termed fog, and if 1000 m or more, it is termed mist.

Relative Humidity

Relative humidity is the ratio of actual water vapour in the air to the amount the air can hold at a given temperature.
When the air temperature falls to the dew point, relative humidity becomes 100%, and the air becomes saturated.

Properties of Water Vapour

Water vapour is lighter than dry air at equal pressure and temperature.
A mixture of air and water vapour is less dense if the water vapour content is high than if it is low.

Formation of the Atmosphere

The atmosphere was formed over 4 billion years ago when gases were released from the Earth during its formation.
The Earth's gravity prevented the gases from escaping into space, causing them to spread out and cover the entire surface.

Air Pressure and Air Density

Air pressure and air density are greatest near the Earth's surface and decrease with increasing altitude.
The weight of the air above compresses the air near the surface, leading to higher pressure and density.
Air density refers to the number of air molecules contained within a given volume of air and is measured in kilograms per cubic meter.
Air density is directly proportional to pressure and inversely proportional to temperature.

Effects of Air Density on Aircraft Performance

Air density affects lift, service ceiling, and the relationship between true and indicated airspeed.
Low air density reduces lift, engine power output, and requires longer take-off runs.

Pressure

Pressure is a description of the way in which a force is spread over a contact area.
Pressure is defined as "force per unit area".
Atmospheric pressure or static pressure is the pressure exerted by the atmosphere on objects immersed in it when neither the air nor the object is in motion.
Standard units of pressure include Newtons per square meter, pounds per square inch, bars, millibars, Pascals, and hectopascals.

Properties of Air

Air possesses mass, and the force of gravity acting on that mass gives air weight.
The pressure and density of air decrease with altitude due to the weight of the air.
Variations in atmospheric pressure and density, along with humidity, affect aircraft performance and flight instruments.

Temperature of the Atmosphere

The temperature of atmospheric air decreases with increasing altitude.
The air is not heated directly by the sun, but rather by the Earth's surface through conduction, convection, and long-wave radiation.

The Troposphere

The lowest layer of atmospheric air is the Troposphere, which is heated through its proximity to the Earth's surface.
The Troposphere is where temperature and pressure changes cause the mixing of air, resulting in weather.
Almost all of the Earth's weather occurs in the Troposphere.
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.

The Tropopause

The boundary between the Troposphere and the Stratosphere is called the Tropopause.
At the Tropopause, the temperature is around -56.5°C (-69°F) and remains constant to an altitude of about 18 miles (35 km).

Temperature Variation with Altitude

The temperature begins to rise again at altitudes greater than 18 miles (35 km).
The temperature variation with altitude ends at around 95,000 feet.

Physical Properties of the Atmosphere

Air has mass, which is an essential fact in understanding the Principles of Flight.
The atmosphere's physical properties, including pressure, density, temperature, and relative humidity, change under certain circumstances.

Viscosity of Air

Viscosity measures the air's resistance to flow due to internal friction between air molecules moving relative to each other.
Air has a low viscosity, making it a "thin" fluid, similar to water.
In contrast, fluids like treacle and tar have high viscosity, making them "thick" fluids.
Despite being low, air's measurable viscosity has significant consequences for an aircraft in flight.

ICAO Standard Atmosphere (ISA)

The ICAO Standard Atmosphere (ISA) is a set of standard values for air pressure, density, temperature, and humidity defined by the International Civil Aviation Organisation (ICAO) in 1964.

Purpose of ISA

The ISA serves as an international standard reference for measuring aircraft performance and calibrating instruments.
It provides a common set of atmospheric conditions for aerodynamicists, aircraft manufacturers, and engineers to work with.

Characteristics of ISA

The ISA shows a standard variation of pressure, temperature, density, and viscosity with altitude.
The Mean Sea Level air pressure in the ISA is 1013.2 millibars (1013.2 hectopascals) or 29.92 inches of Mercury.
The ISA temperature at Mean Sea Level is 15°C.
Temperature decreases with altitude at approximately 2°C for every 1,000 ft.

Importance of ISA

ISA values are used as a reference for calibrating flight instruments, such as the altimeter and air speed indicator, and for manufacturers' figures for aircraft performance.
It is essential to understand the effect of the atmosphere's deviation from ISA conditions on instrument readings and aircraft performance.

ISA and Flight Levels

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

Measurement of Temperature

The standard unit of temperature measurement in the aviation world, outside the United States, is degrees Celsius (formerly Centigrade).
Until the 1960s, the Fahrenheit scale was the primary scale of temperature measurement for non-scientific purposes in most English-speaking countries.

Fahrenheit Scale

In the Fahrenheit scale, water freezes at 32ºF and boils at 212ºF.
There are 180º between the boiling points of water in the Fahrenheit scale.

Celsius Scale

In the Celsius scale, water freezes at 0ºC and boils at 100ºC.
There are 100º between the boiling points of water in the Celsius scale.

Conversion between Scales

One Fahrenheit degree is only 5/9 the value of a Celsius degree.
The formulae for converting between the two scales are:
- ºC = (ºF - 32) × 5/9 (Fahrenheit to Celsius)
- ºF = (ºC × 9/5) + 32 (Celsius to Fahrenheit)

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Learn about the different types of aircraft, including aerostats, aerodynes, and aeroplanes, and their characteristics.