ppl pof ch 2

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)