Principles of Flight Overview: ATPL Subject 081 Complete Guide

Principles of Flight (Subject 081) is the study of aerodynamic forces, aircraft stability, control, and flight mechanics that enable aircraft to fly. As one of the most technically demanding ATPL theory subjects, it requires strong physics understanding and the ability to apply aerodynamic principles to real-world flight scenarios. This comprehensive guide covers all major topics in the Principles of Flight syllabus, from basic aerodynamics to high-speed transonic flight.

Introduction to Principles of Flight

Purpose and Scope

Why Principles of Flight Matters:

• Understand HOW and WHY aircraft fly
• Foundation for performance calculations
• Critical for flight safety
• Informs handling and maneuvering
• Career-long relevance

• Laws of physics applied to flight
• Forces acting on aircraft
• Airflow behavior
• Aircraft response to controls
• Performance limitations

• Stall recognition and recovery
• Maneuvering limitations
• Performance optimization
• High-speed operations
• Emergency handling

ATPL Subject 081 Exam

Examination Details:

• Questions: 60 multiple choice
• Time: 2 hours 15 minutes
• Pass Mark: 75% (45/60 correct)
• Difficulty: Hard
• Character: Understanding-based with calculations

• Basic aerodynamics (airflow, Bernoulli)
• Lift generation and factors
• Drag types and minimization
• Stall characteristics
• Boundary layer theory
• Flight controls and effects
• Stability (static and dynamic)
• Maneuvering
• High-speed flight (Mach effects)
• Propellers
• Helicopter basics

Basic Aerodynamics

The Atmosphere

Air Properties:

• Fluid (liquid or gas)
• Compressible
• Viscous
• Obeys physical laws

• 15°C and 1013 hPa at MSL
• Density decreases with altitude
• Performance affected by density

• Higher density altitude = Reduced performance
• Factors: Pressure, temperature, humidity
• Critical for lift and engine power

Newton's Laws

First Law (Inertia):

• Object at rest stays at rest
• Object in motion stays in motion
• Unless acted upon by force

• Force = Mass × Acceleration
• Greater mass requires more force
• Foundation of lift equation

• Every action has equal and opposite reaction
• Downwash creates lift
• Thrust propels aircraft forward

Bernoulli's Principle

Statement:

• In streamline flow, total energy is constant
• As velocity increases, pressure decreases
• Basis for lift generation

• Static pressure
• Dynamic pressure (1/2 ρ V²)
• Potential energy (height)
• Total = Constant

• Airflow faster over upper surface
• Lower pressure above
• Higher pressure below
• Net upward force = Lift

• Ideal fluid (inviscid, incompressible)
• Streamline flow
• Real world has viscosity and compressibility

Continuity Equation

Principle:

• Mass flow rate is constant
• ρ₁A₁V₁ = ρ₂A₂V₂

• Narrowing duct increases velocity
• Decreases static pressure
• Carburetor, wing design

Airfoils and Wings

Airfoil Geometry

Parts:

• Leading Edge: Front of airfoil
• Trailing Edge: Rear of airfoil
• Chord Line: Straight line from LE to TE
• Mean Camber Line: Midpoint between upper and lower surfaces
• Thickness: Maximum distance between surfaces

• Camber: Curvature of mean line
• Positive camber: Curved upward (typical)
• Symmetrical: No camber
• Thickness Ratio: Max thickness / chord
• Chord: Length from LE to TE

• Angle of Attack (AoA/α): Angle between chord line and relative airflow
• Angle of Incidence: Angle between chord and fuselage reference line (fixed)
• Pitch Attitude: Angle of fuselage relative to horizon

Lift Generation

Two Explanations (Complementary):

1. Pressure Difference (Bernoulli):

• Faster flow over upper surface
• Lower pressure above
• Higher pressure below
• Net upward force

• Wing deflects air downward (downwash)
• Air pushes wing upward (reaction)
• Circulation around wing
• Equally valid explanation

L = ½ ρ V² S C_L

• L: Lift force
• ρ: Air density
• V: Velocity
• S: Wing area
• C_L: Coefficient of lift

• Depends on angle of attack
• Airfoil shape
• Varies from 0 to ~1.8 (max)
• Increases linearly with AoA (until stall)

Factors Affecting Lift

Angle of Attack:

• Most significant factor pilot controls
• Increase AoA → Increase lift
• Up to critical angle (~15-16°)
• Beyond: Stall

• Squared relationship (V²)
• Double speed = 4× lift
• Most efficient way to increase lift

• Higher density = More lift
• Altitude, temperature, humidity
• Cannot control (except altitude selection)

• Larger area = More lift
• Fixed for given aircraft
• Flaps effectively increase area

• Camber
• Thickness
• Planform
• Design feature, cannot change

Center of Pressure (CP)

Definition:

• Point where total lift force acts
• Along chord line
• Moves with angle of attack

• Low AoA: CP aft (~30-40% chord)
• Increasing AoA: CP moves forward
• Near stall: CP at ~25% chord
• After stall: CP moves aft (nose drop)

• Point where pitching moment doesn't change with AoA
• ~25% chord for subsonic airfoil
• Used for stability calculations

Drag

Drag Types

Total Drag = Parasite Drag + Induced Drag

Parasite Drag (Profile Drag):

• Increases with V²
• Composed of:

• Due to shape
• Separation and wake
• Streamlining reduces
• Blunt shapes have high form drag

• Air friction on surface
• Depends on surface roughness
• Laminar vs. turbulent boundary layer

• Where components meet
• Wing-fuselage junction
• Fillets reduce

• Byproduct of lift generation
• Wingtip vortices
• Downwash angles airflow
• Increases with CL² (decreases with V²)
• Dominant at low speed/high AoA

D = ½ ρ V² S C_D

Total Drag vs. Speed:

• Low speed: Induced drag dominates
• High speed: Parasite drag dominates
• Minimum total drag: V_MD (best L/D)

Lift/Drag Ratio (L/D)

Definition:

• Efficiency of wing
• L/D = Lift / Drag

• Occurs at specific AoA (~4-6° for typical aircraft)
• Best glide angle
• Best range (jet)
• Typically 15:1 to 20:1 for transport aircraft

• Glide performance
• Range optimization
• Efficiency measure

Wingtip Vortices

Formation:

• High pressure below wing flows around tip
• Meets low pressure above
• Creates rotating vortex
• Trails behind aircraft

• Induced drag
• Wake turbulence hazard
• Energy loss

• Winglets
• High aspect ratio wings
• Wing fences
• Shaping

• AR = (Wingspan)² / Wing Area
• Higher AR = Less induced drag
• Gliders: Very high AR
• Fighters: Low AR (maneuverability)

The Stall

Stall Physics

Definition:

• Airflow separation from upper surface
• Loss of lift
• Occurs at critical angle of attack

• ~15-16° for most aircraft
• Stalls at specific AoA, NOT specific speed
• Can stall at any speed if AoA exceeded

• Low AoA: Smooth flow (attached)
• Increasing AoA: Separation starts at TE
• Critical AoA: Massive separation
• Lift drops, drag increases

• Speed at which stall occurs in 1g level flight
• From lift equation:

VS = √(2W / ρSCL max)

• Increases with weight
• Increases with load factor (turns, maneuvers)

Stall Warning

Buffet:

• Airframe vibration
• Separated flow hitting tail
• 5-10% above stall speed

• Mushy controls
• Less effective
• Heavy feel

• Stick shaker
• Warning horn
• Visual warning
• Typically 5-10 kt above stall

• Nose-up trim change
• Reduced control authority

Factors Affecting Stall Speed

Weight:

• Heavier = Higher V_S
• √Weight relationship
• Double weight = 1.41× V_S

• n = Lift / Weight
• VSnew = V_S × √n
• 60° bank (n=2): V_S × 1.41
• 4g maneuver: V_S × 2

• Flaps down: Lower VS (higher CL max)
• Gear down: Slight increase (drag, weight)
• Ice/frost: Increases VS (reduced CL max)

• Forward CG: Higher V_S (tail download)
• Aft CG: Lower V_S (but unstable)

• TAS increases with altitude for same IAS
• V_S (IAS) constant
• V_S (TAS) increases

Stall Recovery

Standard Recovery:

1. Reduce AoA (lower nose)
2. Increase power (if available)
3. Level wings (if in bank)
4. Recover to level flight (smoothly)

• 50-300 ft typical for transport aircraft
• More for larger aircraft
• Consider altitude AGL

• Wing drop (asymmetric stall)
• Potential spin entry
• Recovery: Standard technique

Boundary Layer

Boundary Layer Theory

Definition:

• Thin layer of air in contact with surface
• Velocity changes from 0 (at surface) to free stream
• Typically inches thick

Laminar:

• Smooth, layered flow
• Low skin friction drag
• Unstable
• Leading edge of wing

• Chaotic, mixing flow
• Higher skin friction drag
• More stable (resists separation)
• Most of wing at high Re

• Laminar → Turbulent
• Occurs at specific Reynolds number
• Affected by surface roughness
• Transition point moves forward with speed

Reynolds Number

Definition:

• Ratio of inertial to viscous forces
• Re = ρVL / μ
• ρ: Density
• V: Velocity
• L: Characteristic length (chord)
• μ: Dynamic viscosity

• Low Re: Laminar flow
• High Re: Turbulent flow
• Critical Re: Transition

• Small aircraft: Low Re, more laminar
• Large aircraft: High Re, mostly turbulent
• Model aircraft behave differently

Adverse Pressure Gradient

Favorable Gradient:

• Pressure decreasing (accelerating flow)
• Boundary layer healthy
• Leading edge to max camber

• Pressure increasing (decelerating flow)
• Boundary layer stressed
• Can cause separation
• Aft portion of wing

• Boundary layer lifts off surface
• Airflow no longer follows contour
• High drag
• Lift loss
• Stall

Boundary Layer Control

Turbulence Promotion:

• Vortex generators
• Force boundary layer turbulent
• More energy, resists separation
• Small drag penalty

• Remove slow-moving air
• Keep boundary layer thin
• Laminar flow control

• Energize boundary layer
• Powered lift systems
• STOL aircraft

Flight Controls

Primary Control Surfaces

Ailerons (Roll Control):

• Outer trailing edge of wings
• Deflect differentially (opposite directions)
• Create rolling moment
• Left aileron up + Right down = Roll right

• Down aileron creates more drag
• Aircraft yaws opposite to roll
• Solutions: Differential ailerons, coupled rudder (Frise ailerons)

• Trailing edge of horizontal stabilizer
• Change tail lift
• Nose up/down
• Longitudinal control

• Entire horizontal tail pivots
• More effective
• Used on some aircraft

• Trailing edge of vertical stabilizer
• Yaw left/right
• Coordinated turns
• Crosswind corrections

Secondary Control Surfaces

Flaps:

• Increase lift (C_L max)
• Increase drag
• Lower stall speed
• Steeper approaches

• Plain flaps
• Split flaps
• Slotted flaps
• Fowler flaps (increase area + camber)

• Slats: Gap allows airflow, prevents separation
• Krueger flaps: Fold down from LE
• Increase C_L max
• Delay stall

• Disrupt airflow
• Reduce lift, increase drag
• Speed control
• Roll assist (flight spoilers)
• Ground spoilers (dump lift on landing)

• Small surfaces on control surfaces
• Hold control deflection
• Reduce pilot workload
• Trim for specific condition

• Increases control forces
• Prevents over-control
• Feel augmentation

• Moves opposite to control surface
• Provides aerodynamic assist
• Reduces control forces

Control Surface Effectiveness

Speed:

• Higher speed = More effective
• Pressure force proportional to V²

• Higher density = More effective

• Larger = More effective

• More deflection = More force
• Limits to prevent stall or structural damage

• Distance from pivot to surface
• Longer arm = More moment

Stability

Static Stability

Definition:

• Initial tendency after disturbance
• Returns toward equilibrium or not

• Positive: Returns to equilibrium (stable)
• Neutral: Remains in new position
• Negative: Diverges from equilibrium (unstable)

• Pitch stability
• Disturbance in pitch
• Requirement for certification

• CG Location: Most critical
• Forward CG: Stable
• Aft CG: Less stable or unstable
• Horizontal Stabilizer: Provides restoring moment
• Wing Position: Affects stability

• Pull force increases with speed
• Positive gradient
• Pilot feedback

• Roll about longitudinal axis
• Wing drop → Returns to level
• Dihedral: Upward wing angle
• Positive dihedral: Stable
• Anhedral: Downward angle
• Negative stability (fighters for agility)

• Yaw stability
• Disturbance in yaw → Returns to original heading
• Vertical Stabilizer/Fin: Provides stability
• Fuselage: Contributes
• Like weather vane

Dynamic Stability

Definition:

• Tendency over time
• Oscillatory motion damping

• Positive static, positive dynamic: Returns, damped
• Positive static, neutral dynamic: Returns, oscillates
• Positive static, negative dynamic: Divergent oscillation

Short Period Oscillation:

• Rapid pitch changes
• 1-2 seconds period
• Heavily damped (typically)
• Pilot usually doesn't notice

• Long period pitch/speed oscillation
• 30-60 seconds cycle
• Lightly damped
• Altitude and speed trade
• Pilot easily corrects

Dutch Roll:

• Combined roll and yaw oscillation
• Lightly damped
• Uncomfortable
• Yaw damper suppresses

• Slow divergence into spiral descent
• Weak directional stability + Strong lateral stability
• Pilot corrects easily

• Resistance to roll rate
• Upgoing wing: More AoA
• Downgoing wing: Less AoA
• Opposes roll

Turning Flight

Forces in Turn

Load Factor (n):

• n = Lift / Weight
• In level turn: n = 1 / cos(bank angle)
• 60° bank: n = 2g

• Required to turn
• Horizontal component of lift
• F = mV² / r

• r = V² / (g × tan(bank angle))
• Higher speed = Larger radius
• Steeper bank = Smaller radius

• ω = g × tan(bank angle) / V
• Lower speed = Higher rate
• Steeper bank = Higher rate

Stall Speed in Turns

Increased Stall Speed:

• VS turn = VS level × √n
• 60° bank (n=2): V_S × 1.41
• 70° bank (n=2.92): V_S × 1.71

• Stall at higher speed due to load factor
• Common in steep turns
• High-G maneuvers

Limitations

Structural:

• Load factor limits
• Maneuver speed (V_A)
• Below V_A: Will stall before structural damage
• Above V_A: Can over-stress

• Stall
• Buffet boundary
• High-speed buffet (Mach effects)

• Steeper bank requires more power (level turn)
• May not have sufficient thrust

High-Speed Flight

Speed of Sound

Mach Number:

• M = V / a
• V: True airspeed
• a: Speed of sound (~661 kt at MSL ISA)

• Depends on temperature only (ideal gas)
• a = √(γRT)
• Decreases with altitude (until stratosphere)
• ISA: ~661 kt MSL, ~573 kt at 36,000 ft

• Subsonic: M < 0.75
• Transonic: M = 0.75 - 1.2
• Supersonic: M > 1.2

Critical Mach Number (M_CRIT)

Definition:

• Mach number at which local airflow first reaches M = 1.0
• Over wing upper surface
• Before aircraft reaches M = 1.0

• Airflow over wing accelerates
• Can reach M = 1.0 before aircraft
• Typical M_CRIT: 0.70-0.85 for transports

• At M_CRIT: Normal shock forms on wing
• Sudden pressure rise
• Airflow separation
• Drag increase ("drag divergence")

Transonic Effects

Wave Drag:

• Sudden drag increase near M_CRIT
• Shock wave formation
• Can double total drag

• Shock-induced separation
• Airframe vibration
• "High-speed buffet"
• Limit: Buffet onset speed

• Shock Stall: Separation behind shock
• Mach Tuck: Nose-down pitching moment
• CP moves aft
• Trim change
• Aileron Reversal: Loss of effectiveness

Swept Wings

Purpose:

• Delay M_CRIT
• Effective component of airflow reduced
• Wing "sees" lower Mach number

• Typical: 25-35° for jets
• Reduces effective airflow velocity
• Delays transonic effects

• Lower C_L max (higher stall speeds)
• Tip stall tendency
• Complex structure

Mach Buffet Boundary

Coffin Corner:

• High altitude
• Low-speed buffet (stall)
• High-speed buffet (shock)
• Narrow margin
• Requires careful speed control

• Below low-speed buffet
• Below high-speed buffet (MMO)
• "Buffet-free envelope"

Propellers

Propeller Theory

Blade Element Theory:

• Propeller blade is rotating wing
• Generates lift (thrust)
• Also generates drag

• Resultant of rotation and forward motion
• Changes along blade
• Twist compensates

• Reaction to accelerating air rearward
• T = mass flow × velocity change

• Reaction to thrust generation
• Countered by engine

Propeller Pitch

Geometric Pitch:

• Angle of blade
• Distance advanced per revolution (theoretical)

• Actual distance advanced
• Less than geometric (slip)

• Fixed Pitch: Cannot change (light aircraft)
• Variable Pitch: Ground adjustable
• Constant Speed: Automatically adjusts to maintain RPM

Constant Speed Propeller

Operation:

• Governor maintains RPM
• Pilot selects RPM (power lever separate from throttle)
• Blade angle changes with conditions

• Takeoff, climb
• Low blade angle
• High RPM
• High thrust, low speed

• Cruise
• High blade angle
• Lower RPM
• Efficient at high speed

• Optimal efficiency at all conditions
• Engine operates at best RPM

• Blade edge into wind
• Minimum drag (engine failure)
• Multi-engine aircraft

• Negative blade angle
• Thrust forward
• Landing deceleration

Propeller Effects

Torque Reaction:

• Propeller torque rolls aircraft opposite
• Single-engine: Compensate with rudder/aileron trim

• Rotating airflow from propeller
• Hits vertical stabilizer
• Yaw effect

• Descending blade (right side, clockwise rotation) has higher AoA
• More thrust on right
• Yaws left
• High power, high AoA (takeoff)

• Propeller is gyroscope
• Force applied 90° ahead in rotation direction
• Pitch/yaw coupling during rotation

EASA Learning Objectives

LO 081.01: Basic Aerodynamics

Knowledge Requirements:

• Bernoulli's principle
• Continuity equation
• Boundary layer theory
• Reynolds number
• Newton's laws

• Pressure/velocity relationship
• Boundary layer types
• Separation causes

LO 081.02: Lift and Drag

Knowledge Requirements:

• Lift generation
• Lift equation and factors
• Drag types
• L/D ratio
• Induced vs. parasite drag

• Angle of attack effects
• Drag variation with speed
• Minimum drag speed

LO 081.03: Stall

Knowledge Requirements:

• Stall mechanism
• Critical angle of attack
• Stall speed factors
• Stall recovery
• Warning signs

• Load factor effects on V_S
• Stall recognition
• Recovery technique

LO 081.04: Controls and Stability

Knowledge Requirements:

• Primary and secondary controls
• Stability types
• CG effects
• Dynamic stability modes

• Control functions
• CG position importance
• Phugoid, Dutch roll

LO 081.05: High-Speed Flight

Knowledge Requirements:

• Mach number
• Critical Mach number
• Shock waves
• Swept wings
• Transonic effects

• M_CRIT definition
• Shock wave effects
• Mach buffet boundary

Exam Tips & Common Questions

Frequently Tested Topics

1. Stall:

• Critical AoA ~15-16°
• V_S increases with √(load factor)
• Recovery: Reduce AoA

• Induced drag ∝ 1/V²
• Parasite drag ∝ V²
• Minimum total drag at best L/D

• n = 1/cos(bank)
• 60° = 2g
• V_S × √n

• M_CRIT: First local M=1.0
• Shock wave at M_CRIT
• Wave drag increases

• Forward CG: Stable, higher V_S
• Aft CG: Less stable, lower V_S

• Laminar: Low drag
• Turbulent: Resists separation
• Transition depends on Re

Common Pitfalls

Stall Speed Confusion:

• Stall is AoA, not speed
• V_S varies with weight, load factor

• Confusing induced and parasite
• Variation with speed

• M_CRIT vs. MMO
• Shock wave vs. sound barrier

• Static vs. dynamic
• Positive vs. negative

Memory Aids

Stall Recovery:

• "PARE" - Power, Ailerons neutral, Rudder opposite, Elevator forward

• "60 degrees = 2g"

• "Slow = Induced, Fast = Parasite"

• "Forward = Safe but Slow" (stable, high stall speed)

• "Critical is First Sonic" (M_CRIT = first M=1.0 locally)

Study Strategy

Recommended Approach

Phase 1: Fundamentals (Week 1-3)

• Basic aerodynamics
• Lift and drag
• Build physics foundation

• Stall mechanics
• Factors affecting stall
• Load factors

• Flight controls
• Stability types
• CG effects

• High-speed flight
• Propellers
• Helicopters (basic)

• Question bank intensive
• Mock exams
• Weak areas

Study Materials

Essential:

• School textbook (Oxford, BGS)
• Question bank (800+ questions)
• Diagrams and animations
• Physics refresher if needed

• Reading: 70-90 hours
• Question practice: 50-60 hours
• Mock exams: 8-12 hours
• Review: 15-25 hours
• Total: 143-187 hours

Practical Application

Flight Operations:

• Stall awareness and avoidance
• Maneuvering limitations
• High-altitude operations
• Performance optimization

• Understanding aircraft behavior
• Emergency handling
• Efficiency and safety
• Type rating foundation

Conclusion

Principles of Flight provides the aerodynamic foundation for understanding aircraft behavior, performance, and limitations. This knowledge enables safe, efficient operations and informed decision-making throughout your aviation career.

Success requires:

• Strong physics understanding
• Conceptual grasp of aerodynamics
• Ability to apply principles to scenarios
• Extensive practice
• Patience and persistence (challenging subject)

Related Articles:

• Aircraft General Knowledge ^[?]
• Performance ^[?]
• Mass and Balance ^[?]
• ATPL Theory Exams Overview