Principles of Flight - Table of contents

Principles of flight

The modern light aircraft

The forces that act upon an aeroplane

Weight

Aerofoil-lift

Drag

Lift/Drag ratio

Thrust from the propeller

Stability

Neutral stability

Flaps

Straight and level

Climbing

Descending

Turning

Stalling

Spinning

 

 

The modern light aircraft

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  • Forces That Act Upon An Aircraft
  • Stability
  • The Control Surfaces

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The Four Forces that act upon an Aeroplane in Flight

Four Forces

Mass

The mass of the aircraft acting straight down towards the earth.


Thrust

Thrust, supplied by the engine turning the propellor.


Lift

Lift is generated by the airflow around the wings.


Drag

Drag is the resistance to the movement of the aircraft through the air.


Weight is balanced by the lift

Drag is balanced by the thrust

NB: in light aircraft lift will be approximately 10 times the drag

This is known as the lift drag ratio

 

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Weight

Centre of gravity - weight may be considered to act as a single force through the centre of gravity

Weight is the most reliable force always acting in the same direction, and gradually reducing as fuel is used

Wing loading = weight of aeroplane / wing area

The C of G moves as weight is redistributed

 

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Aerofoil - lift

Static Pressure: Ambient Pressure At The Same Level As The Aircraft

Total Pressure: Pressure in air which has been brought to rest from the free stream

Dynamic Pressure: Difference between total pressure and static pressure

Venturi - Bernoulli high flow low static pressure

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Total pressure = static pressure + dynamic pressure

Dynamic pressure = r v2 (r:Air density / V:speed)

Low static pressure on top of an aerofoil section

 

Aerofoil definitions

Leading Edge - Trailing Edge - Chord Line - Thickness

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Camber - Mean Camber Line

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Relative airflow - the airflow and its direction relative to the aircraft but undisturbed by the aircraft

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Angle of attack - angle between chord line and direction of relative airflow

Angle of incidence - longitudinal axis of aircraft and the chord line

Dihedral - wings at an angle to the lateral axis

Sweepback - angle between LE and lateral axis

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Centre of pressure - point where resultant lift acts

Centre of pressure changes with angle of attack

Moving forward with increase in AoA

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Centre of pressure changes with change of wing shape

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Lift = Clift r v2 s

Clift represents shape and angle

Rho (r) is air density

V is velocity (true air speed)

S is area

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Lift from symmetrical aerofoil - zero lift @ zero AoA

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Drag

Total drag is total resistance to motion

Made up from

Parasite drag

Induced drag

Parasite drag- skin friction, form drag, interference

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Ways of reducing parasite drag:

  • Streamlining
  • Fairing & filleting

Induced drag - lift

High static pressure under an aerofoil, leaks via the wing tips to the low static pressure ontop

This causes a spanwise flow outboard underneath and inboard ontop of the aerofoil

Spanwise flow causes induced lift

 

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there is an overall downwash of air behind the trailing edge

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Which leads to the lift force being inclined into the drag direction

 

Aspect ratio = span / mean chord

High aspect ratio give less induced drag for a given lift

Low aspect ratio give less profile drag for a given lift

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Other ways of reducing induced drag included

  • High aspect ratio
  • Tapering the wing
  • Washout
  • Wing tip modifications

 

Total drag versus airspeed

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Drag = Cdrag rv2 s

Cdrag represents shape and angle

Rho (r) is air density

V is velocity (true air speed)

S is area

 

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Lift / Drag ratio

The best lift drag ratio is the most efficient Speed (and AoA) for Maximum Range ie Minimum Drag Vmd

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Lowering gear (or flaps) changes the drag characteristics

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Thrust from the propeller

Propeller terminology

prop

The propeller blade angle is made progressively smaller from hub to tip to provide efficient angles of attack along its full length

NB: the propeller has "washout"

Propeller torque - is the resistance to motion in the plane of rotation

 

Fixed pitch propeller: angle of attack varies with forward speed and rpm

Variable pitch propeller: a constant speed unit retains an efficient angle of attack over a wide forward speed range by altering the blade angle automatically

Take off effects of propeller:

Slipstream, causes asymmetric flow over fin, resulting in yaw

Overcome with offset fin

Propeller torque reaction: with a clockwise rotating propeller (seen from behind), the aircraft will tend to want to roll left

Gyroscopic effect in tailwheel: pitch forward on take off run, yaw left

 

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Stability

For an aircraft to be in equilibrium the opposing forces must be and opposite, leaving no resultant force

Under normal circumstances c of g and c of p are not coincident:

Lift and weight produce a pitching couple

Under normal circumstances thrust line and drag line are not coincident:

Thrust and drag produce a pitching couple

The tailplane produces a stabilising force to counteract these pitching couples

 

Axes of motion

 

Angular motion:

Rolling about the longitudinal axis

Pitching about the lateral axis

Yawing about the normal axis

Stability:

Longitudinal stability is stability about the lateral axis

Lateral stability is stability about the longitudinal axis

Directional stability is stability about the normal axis

Stability is the natural ability of the aeroplane to return to its original condition after being disturbed without any action being taken by the pilot

 

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Neutral stability:

A Neutrally stable aircraft will, when disturbed remain in the new position, neither returning to its original attitude nor increasing its movement in the direction of disturbance

Unstable:

An unstable aircraft will continue to move away from original attitude after a disturbance

Stable:

A stable aircraft will return to its original attitude after a disturbance

 

 

Elevator: primary pitching control

To retain satisfactory handling characteristics and elevator effectiveness the position of the c of g must be kept within a limited range

The aft limit of the c of g is determined by the requirement for longitudinal stability

Aileron: primary rolling control

Adverse aileron yaw due to aileron drag:

Down going aileron causes drag (more lift), and hence yaw out of the turn

Overcome with:

  • Differential ailerons
  • Frize type ailerons
  • Rudder coupling

Rudder: primary yaw control

Note secondary effects

Roll causes yaw

Yaw causes roll

Note further effects of controls:

Spiral descent

Control effectiveness

Slipstream increases the effectiveness of rudder and tailplane

Desirable qualities:

Elevator and rudder authority at slow speed

Trim provides aerodynamic balance

Horn balance

Inset hinge line

Balance tab

 

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Flaps

Trailing edge flaps

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Leading edge devices

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*

 

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Straight and level

In steady straight and level flight the aeroplane is in equilibrium

The tailplane provides the final pitching moment for any pitching couples

ATTITUDE: IAS varies inversely with AoA

WEIGHT: For the same power a lighter aircraft has a lower AoA

 

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Climbing

Forces in the climb: Weight force acts vertically, but has a component the acts in the direction opposing flight

Thrust is greater than drag; lift is less than weight

 

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Descending

In a glide a component of weight balances drag

Estimation of gliding distance in still air

With a Lift/Drag ratio of 12:1 horizontally 12 times further in still air than the height it descends

 

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Turning

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During a properly executed horizontal turn, the inward force is provided by banking the aircraft to vector the lift

An increase in lift is obtained by increasing the AoA by pulling back on the control column

Load factor = lift / weight

Load factor will increase, so will stalling speed

Ex. At 60 degree banked load factor increases by 2, stall speed Vs increases by Öload factor = Ö2 = 1.4

 

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Stalling

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RECOGNITION OF STALL

  • Prestall buffet (turbulent flow over tailplane)
  • Sink (loss of lift)
  • Nose Drop (C of P moving rearwards)

NB Stalling is associated with a particular AoA

Usually this is 15-16 degrees in modern training aircraft

Stalling Speed varies with Ölift

Stalling speed therefore increases with increase in weight

Washout provides desirable stalling characteristics

 

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Spinning

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If an aircraft drops a wings at or near the stall the downgoing wing AoA increases, the upgoing wing AoA decreases.

This can cause a continuing rolling, yawing condition known as autorotation

Recovery opposite rudder to stop yaw and roll, then forward elevator to pitch the nose down, followed by a pull out of ensuing dive (if you have enough height!)

 

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