Sep 2012Lesson 2.4

Theory of Flight

Flight Performance

Reference

From the Ground UpChapters 2.1.5, 2.1.6, 2.1.7:Flight Performance Factors, AirspeedLimitations, Mach NumberPages 26 - 33

Introduction• There are many factors that affect an

aircraft’s flight performance. As well, the four forces are manipulated to be able to maneuver an aircraft.

Outline• Flight Performance Factors• Climbing, Gliding & Turns• Stalls, Spins & Spiral Dives• Load Factor & Airspeed

Torque

In nose-engine aircraft, propellerrotates clockwise (as seen by pilot)

Result: Roll to left (counterclockwise rotation from equal and opposite reaction)Correction: Slight right-turning tendency built-in to aircraft

Asymmetric ThrustAt high angles of attack and high power setting (i.e. take-off), descendingpropeller blade has greater angle of attack than ascending blade

Right side of prop produces more thrust then left side

Result: Yaw to leftCorrection: Use right rudder

PrecessionSpinning propeller acts like a gyroscope: When force applied to spinninggyro, force acts as if it was 90° in direction of rotation

Result:Quick Nose-Up = Sharp yaw rightQuick Nose-Down = Sharp yaw leftCorrection: Use opposite rudder

Tail-wheel aircraft prone to precession when nose pushed forward on take-off

SlipstreamPropeller pushes air back in corkscrew motion which hits left side offin (pushing it right)

Result: Constant yaw to left (depending on power setting)Correction: Offset fin, trim, right rudder

Climbing

Weight

Drag

LiftThrustAngle of AttackIncrease: More lift, less speedDecrease: Less lift, more speed

Ability to climb dependent on thrust: More thrust needed at higher altitudes

Climbing

Best Rate of Climb (Vy)Most altitude in least time(used on normal take-off)

Best Angle of Climb (Vx)Most altitude in leasthorizontal distance(used for obstacles)

- Longer Time- Shorter Distance

- Shorter Time- Longer Distance

Normal ClimbUsed during cruise

GlidingGliding = 3 forces (Weight, Lift, Drag)

Weight

Drag

Lift

Thrust = Horizontalcomponent of weight

Glide Reaction= Resultant of liftand drag, opposesweight

Gliding

Best Range SpeedFurthest distance per altitude lost

Best Endurance SpeedMost time in air peraltitude lost

- Longer Time- Shorter Distance

- Shorter Time- Longer Distance

Turns

Weight

Lift

Angle of Bank

Centripetal ForceHorizontal component of lift,pulls aircraft into turn

Vertical Component of LiftKeeps aircraft in air (opposes weight)

Centrifugal ForceImaginary force thatpulls aircraft outside ofturn (is really inertia)

TurnsShallow Bank- Lesser turn rate- Larger turn radius- Lower Stall Speed- Less Wing Loading

Steep Bank- Greater turn rate- Smaller turn radius- Higher Stall Speed- More Wing Loading

Turns

Faster Airspeed- Lesser turn rate- Larger turn radius

Slower Airspeed- Greater turn rate- Smaller turn radius

Same bank angle

Turns

Load Factors in TurnsAngle of bank increase= Load factor increase

60° bank = 2 G's

DangersHigh load factor= Possible structural failure(overload)

Increased load factor= Increased stall speed

Stalls• Definition: Wing can’t create enough lift to support

weight

• When Critical Angle of Attack (Stall Angle) reached, turbulent airflow surpasses laminar airflow on wing

• C of P rapidly moves towards trailing edge

• Aircraft can stall at any airspeed or attitude if critical angle of attack is exceeded

• Aircraft will stall at same indicated airspeed regardless of altitude

Factors Affecting Stall• Weight

– More weight = higher angle of attack (closer to stall angle)

• C of G– Forward = higher stall speed– Rearward = lower stall speed

• Turbulence– Upward vertical gust could cause aircraft to exceed stall angle

• Turns– Angle of bank increase = Stall speed increase (load factor/weight)

• Flaps– Increasing lifting potential of wing = Stall speed decrease

• Aircraft Condition– Snow, Frost, Ice, Dents = Disrupted laminar flow (increases stall speed)

Spins• Definition: Auto-rotation which develops after

aggravated stall

• When wing drops in stall:– Down-going wing has greater angle of attack– Wing receives less lift, drops more rapidly– Drag on down-going wing increases, further

increasing angle of attack– Wing stalls further, nose drops, auto-rotation starts

Spins

Spiral Dives• Definition: Steep descending turn in which

airplane has excessive nose down attitude

• Characteristics:– Excessive angle of bank– Rapidly increasing airspeed– Rapidly increasing rate of descent

• Structural damage can occur if airspeed increases beyond limits

Spiral Dives

Spins vs Spiral Dives• Spin:

– Aircraft stalled– Airspeed constant and low

• Spiral Dive:– Aircraft not stalled– Airspeed increasing rapidly

Airspeed Limits• Never Exceed Speed (VNE)

– Max speed airplane can be operated in smooth air

• Normal Operating Speed (VNO)– Design cruise speed, should not be intentionally exceeded

• Maneuvering Speed (VA)– Max speed at which flight controls can be fully deflected

without damage to structure

• Maximum Flaps Extended Speed (VFE)– Max speed at which full flaps can be used

Mach Number• Ratio of speed of body to speed of

sound (in air surrounding body)

• Mach 1 = Speed of sound

• Varies with air temperature, pressure and density

Next Lesson

2.5 - Theory of FlightFlight Instruments

From the Ground UpChapter 2.2:Flight InstrumentsPages 33 - 44