Module 13 — Aircraft Aerodynamics, Structures and Systems
13.1(a) — Aeroplane Aerodynamics and Flight Controls
This section covers the fundamental principles governing how fixed-wing aircraft generate lift, produce drag, and are controlled in flight. Although B2 avionics engineers work primarily with electronic systems, understanding aerodynamic principles is essential for correctly interpreting flight data, troubleshooting sensor inputs, and appreciating why avionics systems behave as they do.
The Atmosphere and International Standard Atmosphere (ISA)
The atmosphere is composed of approximately 78% nitrogen, 21% oxygen, and 1% other gases (argon, CO₂, water vapour). Its properties — pressure, temperature, and density — decrease with altitude, directly affecting aircraft performance and the readings of pressure-based instruments.
ISA Sea-Level Values
- Temperature: 15 °C (288.15 K)
- Pressure: 1013.25 hPa (29.92 in Hg)
- Density: 1.225 kg/m³
- Lapse rate: −1.98 °C per 1 000 ft (up to 36 089 ft / tropopause)
- Tropopause temperature: −56.5 °C (constant above tropopause)
Aviation Context
Air data computers (ADCs) use ISA as the baseline model. They compute altitude, airspeed, and temperature deviations (ISA + / ISA −) by comparing measured static and total pressures to the ISA model. Accurate calibration of pitot-static probes is therefore critical for every computed air data parameter.
Bernoulli's Theorem and Subsonic Airflow
Bernoulli's theorem states that in a steady, incompressible airflow with no energy added or removed, the total energy (the sum of static pressure, dynamic pressure, and potential energy) remains constant along a streamline:
Bernoulli's Equation
\( P_{\text{static}} + \tfrac{1}{2}\rho V^{2} + \rho g h = \text{constant} \)
Where \( P_{\text{static}} \) is static pressure, \( \rho \) is air density, \( V \) is velocity, and \( h \) is height. In level flight, the height term is neglected, giving:
\( P_{\text{total}} = P_{\text{static}} + \tfrac{1}{2}\rho V^{2} \)
An aerofoil is shaped so air accelerates over the upper (cambered) surface. As velocity increases, static pressure decreases. The pressure difference between the lower and upper surfaces produces an upward net force — lift. This is also supported by Newton's Third Law: the wing deflects air downward (downwash), and the equal and opposite reaction acts upward on the wing.
Boundary Layer
The boundary layer is the thin layer of air in direct contact with the wing surface where friction slows the airflow from the free-stream velocity to zero at the surface itself.
| Type | Characteristics | Skin Friction |
|---|---|---|
| Laminar | Smooth, orderly layers; found near the leading edge | Low |
| Turbulent | Irregular, mixed flow; better energy exchange with free-stream | Higher |
The point where the laminar boundary layer transitions to turbulent is called the transition point. A turbulent boundary layer resists separation better but produces more skin-friction drag. The point where the boundary layer separates from the surface is the separation point — when this moves forward significantly, the wing stalls.
Generation of Lift
Lift Equation
\( L = C_L \times \tfrac{1}{2}\rho V^{2} \times S \)
Where \( C_L \) = lift coefficient (depends on angle of attack and aerofoil shape), \( \rho \) = air density, \( V \) = true airspeed, and \( S \) = wing area.
Angle of attack (AoA) is the angle between the chord line and the relative airflow. As AoA increases, \( C_L \) increases — up to a critical angle (typically 15–18°) beyond which the flow separates from the upper surface and lift drops abruptly: this is a stall.
Aerofoil Types
| Type | Features | Typical Use |
|---|---|---|
| Symmetrical | No camber; zero lift at zero AoA | Tail surfaces, aerobatic aircraft |
| Cambered (conventional) | Upper surface more curved; lift at zero AoA | General aviation, transport |
| Supercritical | Flat upper surface, reflex camber at trailing edge; delays shock wave | High-subsonic transport aircraft |
| Laminar flow | Maximum thickness far aft; low drag | High-performance gliders |
Drag
Total drag is the sum of parasite drag and induced drag.
Drag Equation
\( D = C_D \times \tfrac{1}{2}\rho V^{2} \times S \)
| Drag Type | Cause | Varies With Speed |
|---|---|---|
| Parasite drag | Form (shape), skin friction, interference | Increases with V² |
| Induced drag | By-product of lift; caused by wingtip vortices deflecting airflow downward | Decreases with V² (highest at low speed/high AoA) |
The lift/drag (L/D) ratio peaks at the speed where parasite drag equals induced drag. This is the most aerodynamically efficient speed and determines best glide range.
Stalling
A stall occurs when the critical angle of attack is exceeded — the boundary layer separates from the upper surface and lift decreases sharply. The stall speed depends on weight, load factor, configuration (flaps/slats), and bank angle:
Stall Speed in a Turn
\( V_{s,\text{turn}} = V_{s,\text{1g}} \times \sqrt{n} \)
Where \( n \) = load factor. In a 60° bank, \( n = 2 \), so stall speed increases by a factor of \( \sqrt{2} \approx 1.41 \) (41% increase).
Aerofoil Contamination
Ice, frost, or even rain on wing surfaces disrupts the boundary layer, reduces maximum \( C_L \), increases drag, and lowers the stall angle of attack. Just 1–2 mm of frost can reduce lift by up to 30% and increase stall speed significantly. This is why the avionics stall-warning system calibration must account for clean-wing assumptions, and ice-detection systems must trigger appropriate crew alerts.
Stability
Static stability — the initial tendency to return to the original state after a disturbance. Dynamic stability — whether the oscillations following a disturbance decrease (stable), remain constant (neutral), or increase (unstable) over time.
| Axis | Stability Type | Primary Contributor |
|---|---|---|
| Longitudinal (pitch) | Most important | Horizontal stabiliser, CG position |
| Lateral (roll) | Dihedral effect | Wing dihedral, sweepback, high wing |
| Directional (yaw) | Weathercock stability | Vertical fin (area and moment arm) |
Avionics Relevance
Stability augmentation is increasingly handled electronically. Yaw dampers suppress Dutch roll (coupled yaw-roll oscillation), and fly-by-wire flight control computers provide artificial stability. The B2 engineer maintains the sensors (rate gyros, accelerometers) and computers that make these systems work.
Flight Controls
Primary Controls
| Control | Axis | Cockpit Input | Movement |
|---|---|---|---|
| Ailerons | Lateral (roll) | Control wheel / sidestick lateral | Differential — one up, one down |
| Elevator | Longitudinal (pitch) | Control column fore/aft | Up for nose up, down for nose down |
| Rudder | Directional (yaw) | Rudder pedals | Deflects to yaw aircraft |
Secondary Controls
- Trim tabs — small surfaces on primary controls to relieve stick forces in steady flight
- Flaps — increase camber and wing area; lower stall speed for take-off and landing
- Slats — extend from the leading edge to energise boundary layer and increase stall angle
- Spoilers — rise from wing upper surface to destroy lift (in flight: speed brakes; on ground: lift dumpers)
- Speed brakes — increase drag without significantly changing pitch
High-Speed Flight
As an aircraft approaches the speed of sound (\( a = \sqrt{\gamma R T} \approx 340\,\text{m/s at sea level} \)), compressibility effects become significant. The Mach number is the ratio of TAS to the local speed of sound:
Mach Number
\( M = \frac{V_{\text{TAS}}}{a} \)
| Regime | Mach Range | Characteristics |
|---|---|---|
| Subsonic | M < 0.75 | No shock waves |
| Transonic | 0.75 – 1.2 | Mixed sub/supersonic flow; shock waves form |
| Supersonic | 1.2 – 5.0 | Entire flow supersonic; bow and oblique shocks |
Mcrit is the free-stream Mach number at which local airflow first reaches M = 1.0 (typically on the upper wing surface). Beyond Mcrit, shock waves produce wave drag and can cause shock-induced separation (Mach buffet), pitch changes (Mach tuck), and aileron reversal. Wing sweepback, thin aerofoils, and supercritical wing sections are all design features that raise Mcrit.
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