CAA Part 66 Cat B1 Module 8 – Basic Aerodynamics Study Notes

CAA Part 66 Cat B1 Module 8 – Basic Aerodynamics

## Issue 3 14th January 2025

CONTENTS

• Physics of the Atmosphere
• Basic Aerodynamics

Physics of the Atmosphere

• Composition of the Atmosphere

  • The Earth's atmosphere is primarily composed of nitrogen (N2) at 78% and oxygen (O2) at 21%
  • The remaining 1% consists of other gases. The ratios remain fairly constant regardless of altitude, though moisture content decreases.
  • Figure 1-2: Composition of the Earth’s Atmosphere

• Pressure

  • Definition of Atmospheric Pressure: The force exerted against the earth's surface by the weight of the air above it.
  • The formula relating force, area, and pressure is: $F = A imes P$
  • To find atmospheric pressure, the formula rearranges to: $P = \frac{F}{A}$
  • Atmospheric pressure at sea level is approximately 14.7 psi (rounded), or equivalently 1013.25 hPa.
  • Instruments for Measurement: Barometer measured in inches of mercury (inHg) or millibars (mb).

• Density

  • Defined as weight per unit volume.
  • Air density varies directly with pressure and inversely with temperature:
    1. Density increases with pressure.
    2. Density decreases with temperature.
  • Changes in density affect aircraft’s aerodynamic performance.
  • Factors Influencing Density:
    • Hot days decrease density.
    • Humid conditions reduce density compared to dry days.
    • Increased altitude results in decreased density.

• Humidity

  • The amount of water vapor in the air; varies with temperature.
  • Types of Humidity:
    • Absolute Humidity: Weight of water vapor in a unit volume of air.
    • Relative Humidity: Ratio of current water vapor to the maximum it can hold at the same temperature, expressed as a percentage.
  • High humidity decreases air density, requiring longer takeoff runs due to decreased lift.

• Temperature and Altitude

  • Temperature affects density and subsequently lift and engine performance.
  • Within the troposphere (0 to 33,000 feet), temperature decreases at a rate of approximately -2°C per 1000 feet until reaching the tropopause at -57°C.
  • Higher altitudes experience lower temperatures, affecting aircraft performance negatively.
  • Figure 1-8: Atmospheric layers are depicted with their temperature changes.

International Standard Atmosphere

• Established values for performance comparisons and theoretical calculations.
• Air is assumed dry with:
  • Pressure: 1013.25 hPa at sea level, decreasing by approximately 1 hPa every 30 feet.
  • Temperature: Initially at 15°C with a lapse rate of 1.98°C per 1000 feet up to 36,090 feet; constant at -56.5°C up to 65,800 feet.
  • Density: At sea level density equals 1.225 kg/m³.

AERODYNAMICS

General Definitions for Fluid Flow / Airflow

• Fluid: A substance that can flow, including liquids and gases.
• Relative Airflow (RAF): The movement of air relative to the aircraft.
• Incompressible Flow: Flow treated as incompressible when velocity is less than 0.3 Ma (100 m/s at ISA standard sea level).
• Freestream Flow: Velocity, pressure, and temperature of air unaffected by the aircraft structure.
• Streamline: Path of fluid particle moving from one point to another.
• Boundary Layer: The layer of fluid in the immediate vicinity of a bounding surface; affected by viscosity.
• Laminar Flow: Fluid flow characterized by smooth, parallel streamlines.
• Turbulent Flow: Random, chaotic motion of fluid with eddies and vortices.
• Flow Separation: Detachment of flow around an aerofoil, typically at high incidence angles.

Velocity and Acceleration

• Speed vs. Velocity: Speed denotes magnitude only; velocity includes direction.
• Acceleration: Change in velocity over time.

Newton's Laws of Motion

1. First Law (Inertia): An object at rest stays at rest. The same applies to objects in motion unless acted upon by a force.
2. Second Law (F=ma): A body in motion under force will change speed proportional to the force applied.
3. Third Law (Action-Reaction): For every action, there is an equal and opposite reaction.

AIRFLOW AROUND A BODY

Bernoulli's Principle and Subsonic Flow

• Bernoulli's Principle: As a fluid moves through a constriction, velocity increases and pressure decreases (akin to airflow over an aerofoil).
• Visual illustration of air flowing through a constriction or across a cambered surface shows velocity increasing at the top and pressure decreasing, resulting in lift.

Upwash and Downwash

• Upwash: Air moving upward in front of the wing.
• Downwash: Air moving downward behind the wing. Upwash increases lift by lowering pressure above the wing.

AERODYNAMIC TERMS

Chord and Mean Aerodynamic Chord

• Chord: Width of the wing from leading edge to trailing edge.
• Mean Aerodynamic Chord (MAC): Average chord length of a wing, crucial for stability calculations.

DRAG

Types of Drag

1. Parasitic Drag: Not associated with lift (e.g., form drag, friction drag, interference drag).
2. Induced Drag: Generated by the creation of lift, associated with upwash and downwash.
3. Wave Drag: Occurs at transonic and supersonic speeds due to shock waves.

Drag Coefficients

• Drag Equation: $D = \frac{1}{2} \rho V^2 A C_D$, where every term relates to the conditions affecting drag.

FORCES IN FLIGHT

Key Forces

• Thrust: Forward force from engines.
• Weight: Downward force due to gravity.
• Lift: Upward force opposing weight.
• Drag: Force opposing thrust.

CONTAMINATION

• Effects of Aerodynamic Contamination: Icing, water, or frost can severely disrupt airflow and reduce lift.

FORCES IN THE GLIDE DESCENT

• Glide Ratio: Distance traveled versus altitude lost (e.g., 10:1 = 10,000 ft forward for 1,000 ft lost). Determined by lift to drag ratio.
• Best Glide Speed: Ensures maximum distance, adjusted for weight and other factors.

TURN THEORY AND LOAD FACTORS

• Load Factor: $n = \frac{L}{W}$, varies with bank angle and affects stall speeds.

FLIGHT STABILITY AND DYNAMICS

The Axes of an Aircraft

• Longitudinal, Lateral, and Vertical Axes: Each defines the motion dynamics of the aircraft through roll, pitch, and yaw.