Sub-Sonic Aerodynamics – Comprehensive Bullet-Point Study Notes

Course Overview & Learning Objectives

  • Full module title: Sub-Sonic Aerodynamics (Ethiopian Aviation Academy)

  • Global aims (across 400+ slides):

    • Identify every structural component, axis, and primary control surface of an aircraft.

    • Explain fundamental physical laws (Bernoulli, Newton I-II-III, conservation laws).

    • Define & relate atmospheric variables (pressure, temperature, density, humidity, viscosity) to flight.

    • Analyse density–altitude, velocity–dynamic-pressure relationships.

    • Classify all airspeeds (IAS, CAS, EAS, TAS, GS) and wind effects.

    • Describe forces & moments on aircraft (lift, weight, thrust, drag) including CG, CP & AC.

    • Distinguish AoI vs AoA, flight-path vs relative wind, zero-lift AoA, absolute AoA.

    • Describe airfoil geometry, boundary-layer types, pressure distribution, upwash/downwash.

    • Track CP travel, define AC (a specific point on the chord), compare to CP.

    • Compute coefficients; show variation with angle of attack; derive lift equation.

    • Identify stall concepts (critical AoA, flow separation, pre-stall buffet) & warning/control devices.

    • List design features for favourable stall pattern (geometric twist, aerodynamic twist, stall strip).

    • Analyse drag (parasite
      skin-friction, form, interference; induced; total) & reduction devices.

    • Discuss 3-D flow, aspect ratio, sweep, dihedral/anhedral, taper.

    • Explain ground effect, wake turbulence, vortex strength factors.

    • Correlate 4 forces in steady level, climb, descent, glide.

    • Survey high-lift & drag-augmentation devices (TE/LE flaps, spoilers, speed brakes).

    • Diagnose left-turning tendency (torque, slipstream, P-factor, gyroscopic precession) & corrections.

    • Define minimum control airspeeds, emergency-descent causes.

    • Provide exhaustive propeller fundamentals: nomenclature, aerodynamics, forces/stresses, fixed & variable pitch (incl. constant-speed, full-feather, reverse, contra/counter), governors, synchro-phasing, ice-control systems, auto-feather.

    • Historical references, bibliographic sources supplied (Jeppesen, Kermode, Hurt, FAA HBKs).

1. Atmospheric Fundamentals

  • Atmosphere: mass of air extending hundreds of miles; flight possible only within.

  • Static pressure: weight of column above; equal in all directions (Pascal).

  • Dynamic pressure.

  • Total pressure: sum of static and dynamic pressure (measured by pitot tube).

  • Temperature lapse: ISA standard temperature at MSL, decreasing with altitude up to 36,000 ft.

  • Density varies directly with pressure, inversely with temperature & humidity.

  • Humidity: water-vapour lighter than dry air
    lower density
    longer take-off/landing.

  • Viscosity: internal resistance causing boundary-layer phenomena.

2. Density Altitude & Dynamic Pressure

  • Density altitude = pressure altitude corrected for non-ISA temperature.

  • High temp/humidity
    “HIGH density ALTITUDE”
    higher true airspeed needed for same dynamic pressure
    longer take-off.

  • “At constant lift, constant dynamic pressure is required”: so if density decreases then true airspeed must increase.

3. Airspeed Regime

  • IAS: direct ASI reading.

  • CAS: IAS corrected for instrument/position error.

  • EAS: CAS corrected for compressibility (important at higher speeds).

  • TAS: true airspeed, related to equivalent airspeed by density ratio.

  • GS: TAS plus or minus wind component; ground speed is airspeed plus wind.

4. Aircraft Components & Axes

  • Cockpit (flight-deck)

  • Fuselage (joins components, houses payload).

  • Wing (stores fuel; primary lift).

  • Empennage: Horizontal stabiliser (nose-down force), vertical stabiliser.

  • Control surfaces: elevator (pitch about lateral axis), aileron (roll about longitudinal), rudder (yaw about vertical).

5. Fundamental Laws

  • Bernoulli: states that changes in fluid velocity are inversely related to changes in pressure (steady incompressible flow).

  • Continuity (mass): for incompressible flow, the product of area and velocity is constant
    Venturi effects.

  • Newton I–III: inertia, force equals mass times acceleration, action–reaction
    wing deflects air downwards
    equal opposite lift upwards.

6. Airfoil Geometry & Flow

  • Key terms: chord, mean camber line, camber, thickness, leading edge radius.

  • Cambered vs symmetric vs negative camber; zero-lift angle of attack varies.

  • Angle of incidence (AOI): chord vs longitudinal axis (fixed). Angle of attack (AOA): chord vs relative wind (variable).

  • Upwash (ahead), downwash (aft) created by low-pressure region.

  • Boundary layer: laminar (no cross-flow) vs turbulent (energy-rich, resists separation).

  • Flow separation due to adverse pressure gradient; separation leads to stall.

7. Coefficient Curves & Pitching Moments

  • Lift coefficient rises linearly to a certain angle of attack then drops (stall).

  • Center of pressure moves forward as angle of attack increases until stall; symmetric airfoil center of pressure is relatively fixed.

  • Aerodynamic center at 25% chord: pitching moment about aerodynamic center is constant through normal angle of attack range.

8. Stall Mechanics & Mitigation

  • Critical AoA: the angle of attack at which stall occurs.

  • Types: tip stall (swept wings), deep/super-stall (T-tails), dynamic stall (rotorcraft).

  • Pre-stall buffet, stick-shaker, stick-pusher, alpha vane sensors.

  • Design fixes: geometric twist, aerodynamic twist (airfoil variation), stall strips, wing fences.

9. Drag Breakdown

  • Parasite drag includes skin-friction, form, and interference drag; varies with the square of velocity.

  • Induced drag is inversely proportional to aspect ratio; predominates at low velocity.

  • Total drag polar: total drag is the sum of parasite and induced drag.

  • Mitigation:

    • High aspect ratio, elliptical planform, end-plates, winglets, raked tips, droop/Horner tips.

    • Streamlining (fineness ratio), flush riveting, retractable gears.

10. 3-D Flow, Aspect Ratio, Sweep, Dihedral

  • Span-wise flow around tip
    vortices.

  • Higher Aspect Ratio
    less induced drag but structural limits.

  • Swept wing delays compressibility, prone to tip stall; requires fences.

  • Dihedral (positive) enhances roll stability; anhedral on high-wing jets to prevent excess stability.

11. Ground Effect & Wake Turbulence

  • Within a certain height: downwash decreases, induced drag decreases up to 48 %
    lift increases
    float during landing.

  • On take-off premature liftoff may sink when leaving ground cushion.

  • Vortex strength increases with weight, angle of attack, and decreases with speed.

  • Separation minima (ICAO): standard separation distances between aircraft based on weight categories.

12. Four-Force Equilibrium Scenarios

  • Level: thrust equals drag, lift equals weight.

  • Steady climb: thrust is greater than drag, lift is less than the cosine of weight, and the sum of weight's sine component and drag equals thrust.

  • Descent: reverse relations.

  • Glide (no thrust): resultant of lift and drag balances weight; optimal at maximum lift-to-drag ratio.

13. High-Lift & Drag Devices

  • Trailing-edge flaps:

    • Plain (increases lift coefficient)
      Split (increases lift coefficient)
      Slotted (increases lift coefficient)
      Fowler (increases lift coefficient and wing area).

  • Leading-edge: Krueger, droop nose, slats (automatic or manual) increase the stall angle of attack.

  • Spoilers: ground, flight, roll; speed-brakes in fuselage.

14. Left-Turning Tendencies (Clockwise Prop)

  • Torque roll.

  • Slipstream corkscrew striking fin.

  • P-factor (asymmetric blade AoA) greatest at high AoA/high power.

  • Gyroscopic precession during pitch/yaw.

  • Corrections: engine cant, fin/tab offset, aileron rig, bank angle, rudder trim.

15. Minimum Control Airspeeds & Weight Influence

  • Accelerated stall speed is related to normal stall speed and load factor.

  • Heavier aircraft
    higher AoA for lift
    higher asymmetric effects.

  • Minimum control speed in air; minimum control speed on ground (rudder effectiveness threshold).

16. Propeller Fundamentals
16.1 Nomenclature

  • Blade parts: LE, TE, chord, camber, back (suction), face (thrust side), shank, cuff, root (butt), tip.

  • Hub: transmits torque & thrust; houses pitch gear.

  • Pitch definitions:

    • Geometric pitch (theoretical forward distance per revolution).

    • Effective pitch (actual advance).

    • Slip = geometric – effective.

  • Blade angle measured at 75 % radius; twist provides constant AoA along span.

16.2 Aerodynamics

  • Relative airflow on blade = rotational component + axial inflow.

  • Blade AoA depends on forward speed, RPM, blade angle.

  • Tip-speed limited to avoid compressibility.

  • Forces/stresses: centrifugal, thrust bending, torque bending, aerodynamic twisting moment (increases pitch), centrifugal twisting moment (decreases pitch), vibration & resonance.

16.3 Materials

  • Laminated wood (birch, mahogany) with metal tipping.

  • Aluminium alloy, steel insert designs.

  • Composite (graphite/kevlar/epoxy): light, vibration-resistant.

16.4 Propeller Configurations

  • Fixed-pitch (simple trainers).

  • Ground-adjustable (change pitch on ground, no in-flight control).

  • Two-position (low/high pitch in flight).

  • Constant-speed (governor maintains RPM, pilot sets prop lever)
    can feather.

  • Full-feathering (blades nearly perpendicular to airflow on engine failure).

  • Reverse-pitch (ground braking).

  • Counter-rotating (left/right opposite) vs contra-rotating (co-axial, sequential discs).

  • Tractor vs pusher installations; mixed (Cessna 337).

16.5 Governor Operation

  • Flyweights + speeder spring balance oil-pressure pilot valve.

  • Underspeed: weights in
    valve down
    oil to decrease pitch (RPM increases).

  • Overspeed: weights out
    valve up
    oil dump / pitch increases (RPM decreases).

  • On-speed: forces balanced.

16.6 Auxiliary Systems

  • Synchronizer: matches slave RPM to master.

  • Synchro-phaser: aligns blade phase to cut noise/vibration.

  • Ice control:

    • Anti-ice fluid (isopropyl) weeps; must activate pre-icing.

    • Electro-thermal de-ice boots (cyclic heating) melt & centrifuge ice.

  • Auto-feather: engine torque falls below a certain threshold (armed)
    automatic feather.

17. Emergencies & Operational Notes

  • Emergency descent: smooth, rapid loss of altitude with minimal discomfort.

  • Icing raises stall speed, adds weight, disturbs boundary layer.

18. Historical & Reference Context

  • First powered flight: Wright Brothers, 17 Dec 1903.

  • Aircraft categories: lighter-than-air (balloons, airships) per Archimedes; heavier-than-air (airplane, glider, rotorcraft) via Bernoulli/Newton.

  • Bibliography: Jeppesen Private Pilot HBK, Hurt’s “Aerodynamics for Naval Aviators”, Kermode’s “Mechanics of Flight”, FAA Pilot’s Handbook (AC-61-23C).

These notes consolidate every concept, definition, example, design feature, operational implication and system description presented through the entire slide deck, structured for quick study while preserving full technical depth.