Chapter 4 of the Principles of Flight manual examines subsonic airflow, focusing on how aerofoils interact with the air at low speeds. The manual emphasises that these notes complement an approved flight training programme and should not substitute for hands-on instruction.

Aerofoil Terminology

Before analysing airflow, it is important to understand the basic geometry of a wing section. The chapter defines several key terms for describing an aerofoil:

• Aerofoil: a cross-section designed to produce lift as air flows over it.
• Chord line: a straight line from the leading edge to the trailing edge.
• Chord: the length of the chord line.
• Angle of incidence: the angle between the chord line and the aircraft’s longitudinal axis.
• Camber line: the mean line equidistant from the upper and lower surfaces; defines camber.
• Maximum camber: expressed as a percentage of chord; positive for convex upper surfaces and negative for reflexed surfaces.
• Thickness/chord ratio: ratio of the maximum thickness to chord length.

Basics About Airflow

Airflow around a wing depends on relative velocity; it makes no difference whether the aircraft moves through still air or the air moves over a stationary wing. To simplify analysis, we often consider two‑dimensional flow, ignoring spanwise effects. In reality, three‑dimensional wings experience upwash ahead of the wing and downwash behind it, as air is accelerated into the low‑pressure region above and slowed beneath.

The following animation illustrates the concept of upwash ahead of the aerofoil and downwash behind it. Small particles rise in front of the wing and descend behind, while the wing itself remains stationary.

Influence of Dynamic Pressure

The dynamic pressure (½ ρ V²) increases as airspeed rises, increasing the pressure differential between the upper and lower surfaces. The manual notes that higher dynamic pressure (indicated airspeed) results in greater lift because the low-pressure region above the wing becomes even lower while the pressure beneath increases.

The table below compares dynamic pressure at different velocities (constant density):

Influence of Angle of Attack

Changing the angle between the chord line and the relative wind (the angle of attack or AOA) alters the shape of the streamtube over and under the wing. Increasing AOA moves the stagnation point downwards, reduces the cross-sectional area of the streamtube on the upper surface and increases velocity, thus lowering pressure. Up to about 16° the lift increases, but beyond this, the adverse pressure gradient becomes too steep and the flow separates, causing a stall.

Pressure Gradient, Flow Separation and Stall

Flow separation occurs when the boundary layer cannot overcome the adverse pressure gradient on the rear part of the wing. As AOA increases beyond about 16°, the stagnation point moves further down and the pressure difference becomes so great that the flow decelerates rapidly and reverses direction, leading to a separated wake and a sudden loss of lift (stall).

Centre of Pressure and Aerodynamic Centre

Lift acts as a distributed pressure load over the wing surface. The centre of pressure (CP) is the point along the chord at which the resultant lift force can be considered to act. For a cambered aerofoil, the CP moves forward as AOA increases until the stall, whereas for a symmetrical aerofoil the CP remains near the mid‑chord. The manual explains that the aerodynamic centre (AC) lies at about 25% of the chord for subsonic flows. The pitching moment about this point remains nearly constant regardless of AOA.

The coefficients of lift (C_L) and drag (C_D) are dimensionless ratios of the aerodynamic forces to dynamic pressure and wing area. They allow designers to compare aerofoil performance independent of size or speed.

Summary and Key Takeaways

• Aerofoil geometry includes chord, camber and thickness; these parameters determine how air is accelerated over the wing.
• Relative airflow and two‑dimensional assumptions simplify analysis; upwash and downwash occur due to pressure differences.
• Increasing dynamic pressure (via airspeed) amplifies lift by increasing the pressure difference across the wing.
• Lift increases with angle of attack until around 16°, when flow separation leads to stall.
• The centre of pressure moves forward with angle of attack on cambered aerofoils; the aerodynamic centre is at quarter‑chord with a nearly constant pitching moment.
• Lift and drag coefficients allow performance comparison across different wings and conditions.

This concludes Chapter 4’s exploration of subsonic airflow. By understanding how geometry, speed and angle of attack influence pressure distribution and lift, pilots and engineers can optimise performance and avoid stall conditions.