Chapter 3: The Aerofoil and Subsonic Airflow

3.1 The Aerofoil 5555

• An aerofoil is a specially shaped body, like a wing or propeller blade, designed to generate an
• aerodynamic force (primarily lift) when it moves through the air. 56565656

•  How Lift is Generated: The curved upper surface of a typical aerofoil forces the air to travel
• a longer distance than the air flowing under the flatter bottom surface. According to the
• Principle of Continuity, the air on top must speed up. According to Bernoulli's Theorem, this
• increase in velocity leads to a decrease in static pressure. The pressure on the upper surface is
• therefore lower than the pressure on the bottom surface. This pressure differential results in
• an upward force, which we call
• lift.
•  Streamlines and Streamtubes:
• A
• streamline is the path traced by a single particle of air in a steady airflow.
• A streamtube is an imaginary tube made up of many streamlines. No air flows through the
• "walls" of a streamtube.

3.2 Key Aerofoil Terminology

•  Chord Line: A straight line joining the center of curvature of the leading and trailing edges.
•  Chord: The distance between the leading and trailing edges, measured along the chord line.
•  Angle of Incidence: The fixed angle between the wing's chord line and the aircraft's
• longitudinal axis.
•  Camber: The curvature of the aerofoil's surfaces.
• Maximum Camber is the greatest distance of the mean camber line from the chord line,
• expressed as a percentage of the chord.
• A
• symmetrical aerofoil has no camber; its mean camber line and chord line are the same.
•  Thickness/Chord Ratio: The maximum thickness of the aerofoil expressed as a percentage
• f the chord. This greatly influences airflow characteristics.
•  Leading Edge Radius: The radius of the leading edge's curvature. A smaller radius means a
• sharper leading edge.

3.3 Defining Aerodynamic Forces

•  Relative Air Flow (RAF): The direction of the airflow with respect to the wing. It is parallel
• and opposite to the aircraft's flight path. Its magnitude is the True Air Speed (TAS).
•  Total Reaction: The resultant of all aerodynamic forces acting on the aerofoil.
•  Centre of Pressure (CP): The point on the chord line through which the total lift force is
• considered to act.
•  Lift: The component of the Total Reaction that acts perpendicular to the Relative Air Flow.

•  Drag: The component of the Total Reaction that acts parallel to the Relative Air Flow.
•  Angle of Attack (α): Also called Aerodynamic Incidence, it is the crucial angle between the
• chord line and the Relative Air Flow.