CAR-66 Category A: Module 8 Aerodynamics

Physics of the Atmosphere

• Aerodynamics studies the action and reaction of bodies in a moving stream of air
• Composition of the air:
  • 78% Nitrogen
  • 21% Oxygen
  • 0.9% Argon
  • 0.03% Carbon Dioxide
  • Traces of hydrogen, helium, and neon
• Atmosphere divided into layers:
  • Troposphere (closest to earth)
  • Tropopause (upper limit of Troposphere, temperature stops decreasing)
• Troposphere:
  • Contains virtually all water in the atmosphere
  • Weather patterns exist here
  • Temperature decreases with an increase in height
• Tropopause:
  • Isothermal region
  • Ranges in height between 20,000 ft (poles) to 60,000 ft (equator)
  • Temperature remains at -56.5°C

Properties of the Atmosphere

• Physical properties:
  • Pressure
  • Density
  • Temperature
• Variations in properties affect aircraft performance
• International Standard Atmosphere (ISA):
  • Reference for aerodynamic computations
  • Standard variation of properties for mid-latitudes (45°N)
  • Assumes set of conditions for performance comparison and instrument calibration
• ISA based on Sea Level (SL) criteria

Important Concepts

• Atmosphere layers: Troposphere, Tropopause
• Properties of the atmosphere: pressure, density, temperature
• International Standard Atmosphere (ISA) for standardization### Atmosphere and Pressure
• The standard atmospheric pressure at sea level is 1013.2 millibars or 101.3 kN/m².
• The International Standard Atmosphere (ISA) is a theoretical model of the atmosphere, used as a standard in aerospace engineering and aviation.
• ISA values vary with altitude, with temperature, pressure, and density decreasing as altitude increases.
• The lapse rate is the rate of decrease of temperature with altitude, approximately 1.98°C per 1000 feet.

Temperature

• The standard temperature at sea level is 15°C (288 K).
• The lapse rate continues up to an altitude of 38,000 feet, where the temperature remains at a constant -56.5°C up to 65,000 feet.

Density

• Density is defined as mass per unit volume of a substance, at specified temperature and pressure.
• The density of air is a property of great importance in aerodynamics, and it decreases with altitude.

Pressure Measurement

• There are three common systems of measuring pressure in aviation:
  • Pounds per square inch (psi)
  • Inches of Mercury (inHg)
  • Millibars (mb)

Humidity

• Humidity is the condition of moisture or dampness in the air.
• The maximum amount of water vapour that the air can hold depends on the temperature of the air.

Viscosity

• Viscosity is a measure of the resistance of fluid to an applied stress.
• Viscosity essentially describes the air's internal resistance to flow and may be thought of as a measure of its internal friction.

Aerodynamics

• Aerodynamics is the study of air in motion, which includes changes in the physical characteristics, such as pressure and density.
• The study of aerodynamics involves the study of forces being generated, such as lift and drag.

Airflow Around a Body

• When an object is placed in the path of a uniform airflow, the airflow is disturbed, creating forces in all directions.
• The force produced is a result of pressure and viscous forces.

Lift and Drag

• Lift is the component of total reaction force acting perpendicular to the free stream airflow.
• Drag is the component of total reaction force acting parallel to the free stream airflow.

Aerofoils

• An aerofoil is any surface that produces an aerodynamic force when it is passed through a stream of air.
• The cross-sectional shape of an aerofoil is curved, with a leading edge, trailing edge, chord line, and mean camber line.
• The angle of attack (α) is the angle formed between the chord-line and relative airflow.

Flow Characteristics

• Streamlines are imaginary lines that indicate the instantaneous direction of flow, and if flow is steady, they also show the path that a particle of air would follow.
• The closeness of the streamlines gives an indication of flow speed, with closer lines indicating faster air flow.Here are the study notes in detailed bullet points:

Laminar and Turbulent Flow

• Laminar flow is characterized by a "layered" appearance and remains unchanged unless a change is deliberately introduced
• Laminar flow is present at the leading edge of an aerofoil, where there is an "upwash" of air
• At the trailing edge, there is a "downwash" of air
• The point where the velocity of the free stream air has decreased to zero is called the "Stagnation" point
• At this point, the full force and pressure is felt, and it is the combination of both static and dynamic pressure on the aerofoil

Generation of Lift

• Lift is generated in accordance with the fundamental principles of physics:
  • Conservation of Energy
  • Conservation of Mass
  • Conservation of Momentum
• The law of conservation of energy states that energy cannot be created or destroyed, only converted from one form to another
• The continuity equation states that mass in = mass out, and is used to relate the values of density, velocity, and area at one section of a stream tube to the same quantities at another section

Bernoulli's Equation

• Bernoulli's equation describes the relationship between speed and pressure
• The equation is: P + ½ ρV² = constant
• This equation states that as the velocity of the air increases, the pressure decreases, and vice versa
• This principle can be applied to aerodynamics, as the flow through a venturi has similar characteristics to the flow over an aerofoil

Venturi Tube

• A Venturi tube is a tube that gradually narrows to a throat, and then expands at the exit
• The Venturi tube demonstrates an approximate relationship between pressure and velocity for low flow speeds
• As air flows into the constriction, the air is squeezed, and the streamlines move closer together, resulting in an increase in velocity
• According to Bernoulli's equation, this increase in velocity results in a decrease in static pressure

Newton's Laws

• Lift can also be generated by the upper and lower surfaces of the wing when air flows around the wing and is deflected downwards
• The force exerted on the wing is equal to the rate of change of momentum of the air
• According to Newton's Third Law of Motion, for every action, there is an equal and opposite reaction
• Therefore, the air that is deflected downwards will produce an upward or lifting reaction on the wing

Aerodynamic Forces

• The lift force results from a difference in pressure between the upper and lower surfaces of the aerofoil
• The pressure distribution can be determined by experimentation using manometers
• The pressure coefficient (Cp) is an important quantity that leads directly to the value of the coefficient of lift (CL)

Centre of Pressure

• The centre of pressure (CofP) is the point at which the resultant force of pressure acts
• The CofP is located less than halfway back along the chord
• The vector representing lift through the centre of pressure passes through the point of minimum pressure on the upper surface

Aerodynamic Centre

• The aerodynamic centre (AC) is the point about which there is no change in pitching moment as the angle of attack is changed
• The AC is located at a fixed point along the chord, regardless of the angle of attack