11.1(a) — Aeroplane Aerodynamics and Flight Controls

This section covers the fundamental principles of aerodynamics as they apply to aeroplane flight. Understanding how air behaves around an aerofoil, how lift and drag are generated, and how an aircraft is controlled and stabilised is essential knowledge for every aircraft maintenance engineer. These principles underpin every maintenance task — from rigging flight controls to inspecting wing surfaces for contamination.

The Atmosphere

The atmosphere is the envelope of air surrounding the Earth. It is composed of approximately 78% nitrogen, 21% oxygen, and 1% other gases (argon, CO₂, water vapour, etc.). The atmosphere has several key properties relevant to flight:

• Pressure — decreases with altitude. At sea level, standard pressure is 1013.25 hPa (29.92 inHg, 14.7 psi). Pressure halves roughly every 18,000 ft.
• Temperature — decreases with altitude in the troposphere at approximately 1.98°C per 1,000 ft (lapse rate). ISA sea-level temperature is 15°C.
• Density — decreases with altitude and increasing temperature. Air density directly affects lift generation and engine performance.
• Humidity — water vapour displaces heavier nitrogen and oxygen molecules, reducing air density and therefore reducing lift.

International Standard Atmosphere (ISA)

The ISA is a theoretical model of atmospheric conditions used as a reference standard for aircraft performance calculations, altimeter calibration, and engineering design. ISA conditions at sea level:

ISA is used as a baseline. Actual conditions are expressed as deviations: "ISA +10" means the actual temperature is 10°C warmer than ISA standard at that altitude.

Bernoulli's Theorem and Subsonic Airflow

Bernoulli's Principle (Daniel Bernoulli, 1738) states that in a steady flow of an ideal fluid, an increase in velocity produces a decrease in static pressure, and vice versa. The total pressure (the sum of static and dynamic pressure) remains constant along a streamline:

When air flows over an aerofoil, the curved upper surface forces the air to travel a longer path and accelerate. By Bernoulli's principle, this increased velocity reduces the static pressure above the wing. The relatively slower airflow below the wing maintains higher pressure. This pressure difference creates an upward force — lift.

Boundary Layer

The boundary layer is the thin layer of air immediately adjacent to the aircraft surface where the air velocity transitions from zero (at the surface, due to viscosity) to the free-stream velocity. It is critical because drag, heat transfer, and surface contamination effects all occur within this layer.

Types of Boundary Layer Flow

Type	Characteristics	Drag
Laminar	Smooth, orderly layers of air sliding over each other; occurs near the leading edge; thin boundary layer	Low skin friction drag
Turbulent	Chaotic mixing of air; occurs further back along the surface; thicker boundary layer; better at resisting flow separation	Higher skin friction drag
Transition point	The location where laminar flow changes to turbulent flow; varies with Reynolds number, surface roughness, and pressure gradient	—

Free-stream flow is the undisturbed airflow outside the boundary layer. Relative airflow is the direction of airflow relative to the aerofoil — equal and opposite to the direction of flight.

Generation of Lift

Lift is the aerodynamic force generated perpendicular to the relative airflow. It is produced by the pressure difference between the upper and lower surfaces of the wing, combined with the deflection of airflow (Newton's Third Law — the wing pushes air down, and the air pushes the wing up).

Angle of Attack (AoA)

The angle of attack (α) is the angle between the chord line of the aerofoil and the relative airflow. As AoA increases, \( C_L \) increases (more lift is generated) — up to the stall angle (typically 15–18° for conventional aerofoils), beyond which \( C_L \) drops sharply as the airflow separates from the upper surface.

Centre of Pressure (CP)

The centre of pressure is the point on the chord line where the total aerodynamic force (resultant of lift and drag) effectively acts. As AoA increases, the CP moves forward; as AoA decreases, the CP moves aft. This movement affects the aircraft's pitching moment and longitudinal stability.

Aerofoil Types

Type	Characteristics	Typical Use
Symmetrical	Upper and lower surfaces are mirror images; zero lift at zero AoA; CP does not move	Tailplanes, aerobatic aircraft
Cambered (asymmetric)	Upper surface more curved than lower; generates lift at zero AoA; higher max \( C_L \)	Most transport wings
Supercritical	Flattened upper surface delays shock wave formation; higher critical Mach number	Modern transport aircraft (A320, B737)
Laminar flow	Maximum thickness further aft; maintains laminar flow over larger area; lower drag	High-performance gliders, some GA aircraft

Drag

Drag is the aerodynamic force that opposes the aircraft's motion through the air. Total drag is the sum of two main components:

Parasite Drag

Parasite drag (also called zero-lift drag) is drag that is not directly associated with lift production. It increases with speed squared. Components:

• Form drag (pressure drag) — caused by the shape of the aircraft; the pressure difference between front and rear surfaces. Streamlining reduces form drag.
• Skin friction drag — caused by air viscosity in the boundary layer rubbing against the aircraft surface. Smooth surfaces reduce skin friction.
• Interference drag — caused by the interaction of airflows at junctions (wing-fuselage, engine pylon-wing). Fairings reduce interference drag.

Induced Drag

Induced drag is drag that is a direct consequence of lift production. High-pressure air below the wing tip flows around to the low-pressure area above, creating wing-tip vortices. These vortices deflect the airflow downward (downwash), tilting the lift vector backward and creating an aft-acting component — induced drag. Key points:

• Induced drag decreases with speed (at higher speeds, less AoA is needed for the same lift, producing weaker vortices)
• Induced drag increases with AoA and weight
• High aspect ratio wings produce less induced drag (less tip vortex relative to wing area)
• Winglets reduce induced drag by limiting the spanwise flow at the wing tip

Ground Effect

When the aircraft is within approximately one wingspan of the ground, the ground disrupts the formation of wing-tip vortices, significantly reducing induced drag. This is called ground effect. It makes the aircraft feel like it is "floating" during the landing flare and can cause the aircraft to become airborne at a speed below normal take-off speed. Ground effect reduces induced drag by up to 50% when very close to the surface.

Lift/Drag Ratio and Aircraft Polar Diagram

The lift/drag ratio (L/D) is a measure of aerodynamic efficiency. It represents how many units of lift are generated for each unit of drag. The maximum L/D ratio occurs at a specific AoA and speed — this is the speed for maximum range and best glide performance.

• Typical L/D max for a modern transport aircraft: 15:1 to 20:1
• Typical L/D max for a glider: 30:1 to 60:1
• The aircraft polar diagram plots \( C_L \) against \( C_D \). A tangent line from the origin to the curve gives the angle of attack for maximum L/D ratio.

Stalling

A stall occurs when the angle of attack exceeds the critical angle (stall angle, typically 15–18°). The airflow can no longer follow the upper surface contour, separates from the wing, and lift decreases dramatically while drag increases sharply.

Key Stall Facts

• A stall is an angle of attack event, NOT a speed event — an aircraft can stall at ANY speed if the critical AoA is exceeded
• However, the stall speed is the minimum speed at which the aircraft can maintain level flight at 1g (with maximum \( C_L \))
• Stall speed increases with: weight (heavier = more lift needed = higher speed), load factor (in turns), altitude (lower density), forward CG (more tail down-force needed)
• Stall speed decreases with: use of flaps/slats (increase \( C_{L_{max}} \))

Stall Warning Systems

• Stall warning vane (angle of attack sensor) — a small vane on the fuselage side that measures the local AoA; triggers warnings before the stall angle is reached
• Stick shaker — vibrates the control column to give a tactile warning of approaching stall
• Stick pusher — automatically pushes the control column forward to reduce AoA and prevent a full stall (used on T-tail aircraft where a "deep stall" is irrecoverable)

Aerofoil Contamination

Contamination of the wing surface — even a very thin layer — disrupts the boundary layer and degrades aerodynamic performance. This is one of the most safety-critical maintenance considerations.

Contaminant	Effect
Frost	Even a thin layer of frost roughens the surface, causing early boundary layer transition. Can reduce lift by up to 30% and increase stall speed. Aircraft must be de-iced before departure — the "clean aircraft concept."
Ice	Changes the aerofoil shape, reducing lift and increasing drag. Can add significant weight. Rime ice (rough, opaque) is worse than glaze ice (smooth, clear) for drag increase.
Rain (heavy)	Water film increases surface roughness, adds weight, and can cause premature boundary layer transition. Typically 5–15% lift reduction in very heavy rain.
Insects	Insect residue near the leading edge disturbs the boundary layer. On laminar-flow wings, can cause significant drag increase. Aircraft operating in tropical regions require regular leading-edge cleaning.

Stability

Stability is the tendency of an aircraft to return to its original state after being disturbed (e.g., by a gust). There are two aspects:

• Static stability — the initial tendency after a disturbance:
  • Positive — tends to return to original state (nose drops after gust pitches it up)
  • Neutral — remains in the new state (no tendency to return or diverge)
  • Negative — tends to diverge further from original state (undesirable)
• Dynamic stability — describes the motion over time:
  • Positive — oscillations decrease in amplitude over time (returns to equilibrium)
  • Neutral — oscillations continue at constant amplitude (neither increasing nor decreasing)
  • Negative — oscillations increase in amplitude (divergent — dangerous)

Three Axes of Stability

Axis	Motion	Primary Stabilising Surface	Key Factors
Longitudinal (pitch)	Nose up/down about lateral axis	Horizontal stabiliser (tailplane)	CG position relative to CP; tailplane area and moment arm
Lateral (roll)	Wing up/down about longitudinal axis	Wing dihedral, swept wings	Dihedral angle, wing sweep, high/low wing position
Directional (yaw)	Nose left/right about vertical axis	Vertical stabiliser (fin)	Fin area and moment arm; fuselage side area

Wing Planforms

Feature	Definition	Effect
Aspect ratio	Span² ÷ wing area (or span ÷ mean chord)	High AR = less induced drag, better L/D (gliders); Low AR = better manoeuvrability (fighters)
Wing sweep	Angle of the leading edge from perpendicular to fuselage	Increases critical Mach number; provides directional stability; increases stall speed; tip stall tendency
Taper	Reduction in chord from root to tip (taper ratio = tip chord ÷ root chord)	Reduces structural weight; approximates elliptical lift distribution; reduces induced drag
Washout (twist)	Wing tip has lower incidence angle than wing root	Root stalls before tip — preserves aileron control during stall; prevents tip stall on swept wings
Mean Aerodynamic Chord (MAC)	Chord of a rectangular wing with the same area, same pitching moment and same lift	Used as reference for CG position (expressed as % MAC)

Control Around Three Axes

Primary Flight Controls

Control	Location	Axis	Movement	Cockpit Control
Ailerons	Wing trailing edge (outboard)	Longitudinal (roll)	Move differentially — one up, one down	Control wheel/stick left/right
Elevators	Horizontal stabiliser trailing edge	Lateral (pitch)	Move together — both up or both down	Control column forward/backward
Rudder	Vertical stabiliser trailing edge	Vertical (yaw)	Moves left or right	Rudder pedals left/right

Secondary Flight Controls

Trim Tabs and Tab Types

Tab Type	Purpose	Operation
Trim tab	Relieves control force in steady flight	Pilot-adjustable; moves opposite to control surface
Balance tab	Reduces hinge moment (lightens control feel)	Mechanically linked; moves opposite to control surface automatically
Anti-balance tab	Increases hinge moment (adds feel/resistance)	Moves in SAME direction as control surface
Servo tab	Pilot moves tab only; aerodynamic force moves main surface	Used on large control surfaces; reduces pilot effort significantly
Spring tab	Provides aerodynamic assistance at high speeds only	Spring preload means tab only activates when forces exceed threshold

High-Lift Devices

Leading edge devices and trailing edge devices are used to increase \( C_{L_{max}} \), allowing the aircraft to fly at lower speeds during take-off and landing.

Device	Location	Mechanism	Effect
Slats	Leading edge	Extend forward and down, creating a slot	Re-energise boundary layer; increase stall angle; increase \( C_{L_{max}} \)
Slots (fixed)	Leading edge	Permanent gap in the leading edge	Direct high-energy air to upper surface; delay stall
Plain flap	Trailing edge	Hinged portion deflects downward	Increases camber; increases \( C_L \) and drag
Split flap	Trailing edge (lower surface)	Only lower portion deflects down	Increases \( C_L \); very high drag increase
Slotted flap	Trailing edge	Creates slot between wing and flap when deployed	Re-energises boundary layer; higher \( C_{L_{max}} \) than plain flap
Fowler flap	Trailing edge	Extends aft AND deflects, increasing wing area	Highest \( C_{L_{max}} \) of all flap types; used on most transport aircraft

Drag-Inducing Devices

• Spoilers (flight spoilers) — panels on the upper wing surface that deploy upward into the airflow, reducing lift and increasing drag. Used in flight for roll assistance (differential spoilers) and descent control (speed brakes).
• Ground spoilers (lift dumpers) — deploy automatically or manually after landing to "dump" remaining lift, putting the full aircraft weight on the wheels for effective braking. Typically deploy to a higher angle than flight spoilers.
• Speed brakes — may be wing-mounted spoilers or fuselage-mounted devices. Increase drag to control airspeed during descent without reducing power excessively.

Boundary Layer Control Devices

• Vortex generators — small metal tabs on the wing upper surface that create small vortices, mixing high-energy free-stream air into the boundary layer to delay separation. Common on light aircraft and on engine nacelle surfaces.
• Wing fences — vertical plates on the upper wing surface that prevent spanwise flow (airflow migrating from root to tip on swept wings), delaying tip stall.
• Saw-tooth leading edge — a sharp notch in the wing leading edge that creates a controlled vortex at high AoA, delaying stall at the outboard wing section.
• Stall strips — small triangular strips on the inboard leading edge that cause the root to stall first, ensuring the ailerons remain effective during a stall approach.