Fundamentals of the Atmosphere CASA 8.1

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Questions and Answers

What is the primary gas composition of the atmosphere?

78% nitrogen, 21% oxygen, 1% noble gases

78% nitrogen, 21% oxygen, 1% other gases (correct)

78% oxygen, 21% nitrogen, 1% carbon dioxide

78% hydrogen, 21% carbon dioxide, 1% oxygen

In which atmospheric region do most aircraft operate?

Stratosphere

Troposphere (correct)

Mesosphere

Thermosphere

What occurs for every 1000 ft increase in altitude in the troposphere?

The temperature drops approximately 2 ⁰C (correct)

The pressure increases by 1 atm

The temperature remains constant

The temperature increases by 2 ⁰C

What is the temperature at the tropopause?

-57 ⁰C Signup and view all the answers

What defines the tropopause?

The boundary between the troposphere and stratosphere Signup and view all the answers

Which atmospheric effect can significantly impact aircraft performance?

Humidity variations Signup and view all the answers

At what approximate altitude does the tropopause occur over the poles?

20,000 ft Signup and view all the answers

Which layer is directly above the troposphere?

Stratosphere Signup and view all the answers

What is the primary reason there is no weather in the stratosphere?

There is no water vapour. Signup and view all the answers

What is the average atmospheric pressure at sea level in inches of mercury?

29.92 in Hg Signup and view all the answers

Which statement about air density is true?

Air density decreases as temperature increases. Signup and view all the answers

What happens to temperature as altitude increases in the troposphere?

Temperature decreases at a steady rate. Signup and view all the answers

How does humidity affect air density?

Air becomes less dense with higher humidity. Signup and view all the answers

What is the definition of absolute humidity?

The total amount of moisture in the air. Signup and view all the answers

What does the dew point represent?

The temperature at which air becomes saturated. Signup and view all the answers

Which factor does NOT affect air density?

Wind speed Signup and view all the answers

Which organization administers the International Standard Atmosphere (ISA)?

International Civil Aviation Organisation (ICAO) Signup and view all the answers

What effect does increased temperature have on the air's ability to hold water vapour?

It increases water vapour holding capability. Signup and view all the answers

How is atmospheric pressure primarily measured?

Using a barometer. Signup and view all the answers

What is the rate of temperature decrease in the troposphere known as?

Lapse rate. Signup and view all the answers

What happens to air density as altitude increases?

It decreases due to lower pressure. Signup and view all the answers

How does water vapour weigh compared to dry air?

Approximately 5/8 as much as dry air. Signup and view all the answers

What does the fineness ratio measure in an aerofoil?

The thickness ratio of length to breadth Signup and view all the answers

What happens to lift when the angle of attack increases up to the stalling point?

Lift increases Signup and view all the answers

What is the acute angle between the wing chord and the aircraft's longitudinal axis called?

Angle of Incidence Signup and view all the answers

Which type of aerofoil has the same shape on both sides of its center line?

Symmetrical Aerofoil Signup and view all the answers

Where does the Centre of Pressure generally fall relative to the chord position?

25% chord position Signup and view all the answers

What does the angle of attack measure?

Angle between chord line and free-stream flow Signup and view all the answers

What occurs when the angle of attack exceeds the stalling point?

Lift drops to zero Signup and view all the answers

What does the term 'Total Air Reaction' refer to?

The lift and drag forces experienced by the aerofoil Signup and view all the answers

How does the Centre of Pressure move with changes in angle of attack?

It moves forward with increased angle of attack Signup and view all the answers

What is the term for the resultant force acting on an aerofoil?

Total Air Reaction Signup and view all the answers

What is the lapse rate for ISA Standard Conditions?

2 °C/1000 feet Signup and view all the answers

What does an ISA+ model account for?

Temperature exceeding the ISA standard day temperature Signup and view all the answers

What is the primary use of pressure altitude in aviation?

Aircraft performance calculations Signup and view all the answers

What happens to aircraft performance when density altitude is high?

Decreased performance Signup and view all the answers

What does laminar flow in fluid dynamics indicate?

Streamlined, parallel flow with no turbulence Signup and view all the answers

What is the typical structure of streamlines during turbulent flow?

Chaotic with unpredictable fluctuations Signup and view all the answers

What percentage resistance to airflow does a streamlined shape (aerofoil) typically have?

5% Signup and view all the answers

Where does the boundary layer originate on an aerofoil?

At the stagnation point Signup and view all the answers

What characterizes the boundary layer on a surface?

It has zero velocity at the surface. Signup and view all the answers

What is the primary factor that can cause a transition from laminar to turbulent flow?

Critical velocity Signup and view all the answers

What are the characteristics of free stream airflow?

Predictable and smooth flow Signup and view all the answers

Which shape has the highest resistance to airflow?

Flat plate Signup and view all the answers

What does the term 'camber' refer to in aerodynamics?

The curvature of the aerofoil Signup and view all the answers

What is meant by the term 'fineness ratio' in aircraft design?

The ratio of length to width of the aircraft Signup and view all the answers

What characterizes the airflow in the boundary layer compared to the free air stream?

Particles of air are rotating Signup and view all the answers

What happens to the transition point as the speed of an aircraft increases?

It moves further forward on the wing Signup and view all the answers

What is the relative airflow when an aircraft is moving forward and upward?

Backwards and downwards Signup and view all the answers

What is the effect of upwash on airflow in relation to an aerofoil?

Airflow moves upwards Signup and view all the answers

How do wing-tip vortices form?

From airflow over the top meeting with lower surface airflow Signup and view all the answers

What primarily causes wake turbulence behind an aircraft?

Wing-tip vortices Signup and view all the answers

What does Bernoulli’s principle explain regarding aircraft lift?

Lift results from a pressure difference between upper and lower wing surfaces Signup and view all the answers

What shape is referred to when discussing the cross-section of an aircraft wing?

Aerofoil shape Signup and view all the answers

What is the chord line of an aerofoil?

The straight line from leading edge to trailing edge Signup and view all the answers

What does camber refer to in the context of an aerofoil?

The curvature of an aerofoil section Signup and view all the answers

When is camber considered positive?

When the curvature is upward Signup and view all the answers

What is meant by maximum camber?

The greatest distance between the chord and mean camber line Signup and view all the answers

What role does the venturi tube play in understanding Bernoulli’s principle?

It demonstrates changes in pressure with airflow speed Signup and view all the answers

What occurs at the separation points on a wing?

Boundary layers break away from the surface Signup and view all the answers

Study Notes

The Atmosphere

The atmosphere is composed of the following gases:
- 78% nitrogen
- 21% oxygen
- 1% other gases such as carbon dioxide
All gases have physical properties like pressure, density, and temperature
These properties affect aircraft performance
- Changes in atmospheric pressure, temperature, or humidity can affect aircraft performance, such as lift, drag, and engine power

Atmospheric Regions

Troposphere:
- This is where we live and where most aircraft fly
- Extends from the Earth's surface to the tropopause
- Contains water vapor which causes clouds and weather
- Temperature decreases approximately 2°C for every 1000 ft increase in altitude (lapse rate)
Tropopause:
- Marks the boundary between the troposphere and stratosphere
- Has a consistent temperature regardless of altitude, typically around -57°C
- Located at approximately 20,000 ft over the poles and 60,000 ft above the equator (average height is assumed to be 36,000 ft)
Stratosphere:
- Extends above the tropopause
- No water vapor so there's no weather in this layer
- Contains the ozone layer which absorbs harmful UV radiation

Atmospheric Conditions

Air Pressure:
- The weight of the air above any surface causes pressure at that surface
- Average pressure at sea level is 14.7 PSI, 29.92 inches of mercury, 760 mm of mercury, or 1013.25 millibars
- Measured using a barometer
Air Density:
- The mass per unit volume of air, making flight possible.
- Density is lower at higher altitudes due to decreased pressure and temperature.
Air Temperature:
- Temperature decreases with altitude.
- Lapse rate of 2°C for every 1000 ft increase in altitude applies within the troposphere.
Viscosity:
- Air tends to "stick" to surfaces slowing down airflow.
- This reduces aircraft efficiency.
Humidity:
- The amount of water vapor in the air, influenced by temperature.
- Humid air is less dense than dry air.
- Dew point is the temperature air needs to be cooled to for water vapor to condense.

International Standard Atmosphere (ISA)

A standardized model of atmospheric conditions for aircraft performance calculations.
Uses set values:
- Lapse rate of 2°C/1000 ft
- Tropopause height of 36,000 ft
- Sea level pressure of 1013.25 hPa
- Sea level temperature of 15°C
- Gravity of 32.174 ft/sec² or 9.81 m/s²
ISA+ model is used on days with ground temperatures above 15°C, incrementing temperatures at all levels by the difference from the standard value.

Pressure and Density Altitude

Pressure Altitude:
- The altitude indicated when the altimeter is set to 29.92 in.Hg (1013 hPa in other parts of the world)
- Used in aircraft performance calculations and high-altitude flight.
Density Altitude:
- An indicator of aircraft performance.
- Higher density altitude means decreased performance.

Airflow Characteristics

Free Stream Flow:
- Airflow at a distance away from the aircraft's surface.
- Used as a reference point to understand airflow.
Laminar Flow:
- Smooth, parallel streamlines.
- Minimal drag.
Turbulent Flow:
- Chaotic, random motion with unpredictable fluctuations and vortices.
- Increased drag.
Boundary Layer:
- Very thin layer of air adhering to the aircraft's surface.
- Influences lift and drag.
Transition Point:
- Where laminar flow changes to turbulent flow.
Relative Airflow:
- The movement of air relative to the aircraft.
- Regardless of wind conditions.
Upwash and Downwash:
- Upwash: upward airflow in front of the wing.
- Downwash: downward airflow behind the wing.
Vortices:
- Rotating eddies of air formed by the wing tips.
- Result from pressure differences.
Wake Turbulence:
- Disturbance in the atmosphere behind an aircraft caused by wing-tip vortices.

Aerofoils

Chordline:
- A straight line connecting the leading edge to the trailing edge of an aerofoil.
- Used as a reference.
Camber:
- The curvature of the aerofoil's upper and lower surfaces.
- Influences lift and drag.
Mean Camber Line:
- The line connecting the mid-points between the upper and lower camber lines.
Fineness Ratio:
- Ratio of the chord to the thickness of the aerofoil.
- Determines overall shape.
Angle of Attack (AoA):
- The angle between the chord line and the direction of relative airflow.
- Influences lift and drag.
Center of Pressure (CP):
- The point on the aerofoil where the total aerodynamic force is considered to act.
- Moves with changes in AoA.
Resultant Force:
- The combination of lift and drag forces.
Bernoulli's Principle:
- Explains lift generation.
- Faster airflow over the upper surface creates low pressure.
- Slower airflow under the wing creates higher pressure.
- This pressure difference generates lift.

Camber

Positive camber occurs when the aerofoil curves upwards
Negative camber occurs when the aerofoil curves downwards
Symmetrical aerofoils have equal upper and lower camber

Maximum Camber

This represents the maximum distance between the chord line and the mean camber line

Fineness Ratio

This metric measures the thickness of an aerofoil by dividing its length by its width

Angle of Attack

Represents the angle between the chord line and the free-stream flow
Increased angle of attack generally results in increased lift up to the stalling point
Stalling occurs when the angle of attack is too great causing a rapid decrease in lift
This applies to wings, propeller blades, helicopter blades, and jet engine components

Angle of Incidence

The angle between the wing chord and the longitudinal axis of the aircraft
Predetermined during manufacturing and does not change

Aerofoil Shapes

Symmetrical aerofoils have identical shapes on either side of their center line
Non-symmetrical aerofoils have differing shapes on either side

Center of Pressure

The point where the total air reaction acts on an aerofoil
The component of resultant force perpendicular to airflow is lift
The component of resultant force parallel to airflow is drag
The center of pressure is often referred to as the center of lift
The location moves forward as the angle of attack increases and moves aft as the angle of attack decreases
Generally located at approximately 25% of the chord position for most aerofoils

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Description

Explore the composition and properties of the atmosphere and their impact on aircraft performance. Learn about different atmospheric regions, including the troposphere and tropopause, and how altitude affects temperature and other factors critical to aviation. This quiz is designed to enhance your understanding of atmospheric science.