Principles of Flight - 1

First chapter of ATPL Principles of Flight

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  • Created by: Gul
  • Created on: 01-06-09 10:32

Newton's Laws of Motion

1) A body at rest will remain at rest & moving body will continue to move at same speed in same direction unless an external force is applied to it.

2) F = m * a

To change state of body, a force must be exerted on it.

3) F = m * g

For every action there is an equal and opposite reaction.

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Work Done + Power

Work Done = F * Distance Moved (Nm or J)

Power = Work Done/Time (J/s or W)

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Properties of Atmosphere

Properties of air mass defined by:





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Pressure (atmosphere)

1) Pressure decrease with altitude

2) Pascal's Law: In fluid at rest - pressure acts in all directions equally

3) Pressure = Force/Area (N/m^2)

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Temperature (atmosphere)

1) Temp of fluid = measure of it's molecular motion

2) Temp at which molecular motion occurs = ABSOLUTE ZERO (-273K)

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Density (atmosphere)

1) Density = ratio between mass of fluid and it's volume

2) Density = Mass/Volume (kg/m^3)

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Viscosity (atmosphere)

1) Viscosity of air = measure of resistance of one layer of air to movement over neighbouring layer

2) Heavy oil more viscose than water so takes longer to flow.

3) Air as fluid --> fluid free flowing + no definite shape but conforms to shape of container

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Relationship Between Presure/Temp/Density

Density of air mass changed by:

1) Changing volume occupied by air

2) Varying pressure/temp of air

ρ α P/T

( ρ = rho )

Density directly proportional to Pressure

Density inversely proportional to Temp

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Fluid Compressibility

1) Liquids --> incompressible --> nearly constant Density

2) Gases --> compressible --> variable Density

3) Air mostly incompressible at speeds below speed of sound

4) > 300 KTS air starts to behave like compressible fluid.

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Local Speed of Sound (LSS)

1) Speed at which small pressure disturbances propagated through air.

2) Solely a function of air temp

LSS = 38.94 * square root of Temp Kelvin

To predict compressibility the Mach Number must be known!


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Static Pressure

1) Exerted in all directions

2) Static P of atmosphere caused by weight of air acting on surface equally in all directions

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Dynamic Pressure - (q)


Dynamic pressure increases as Static pressure decreases.

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Bernoulli's Theorem

1) Pstat + ½ρTAS² = constant

2) Ptot = p + q

(Total Pressure = Static Pressure + Dynamic Pressure)

3) Increased speed (q) = decreased p

Decreased speed (q) = increased p

4) At zero speed Ptot = Ps

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Equation of Continuity

Principle of mass conservation = mass is constant (cannot be created or destroyed)

ρ A V = constant

Stationary subsonic streamline flow pattern, streamlines converge so Ps decreases and velocity (Pd) increases.

In convergent tube with incompressible subsonic flow:

Ps decreases

Pd increases

Ptot constant

ρ1 = ρ2 if cross-sectional area of tube changes

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Venturi Tube

1) Constriction in tube - speed of flow increased to maintain same mass flow

2) As V increased, q increased and p decreased

(Speed increased, Pd increased and Ps decreased)

3) As air flows through converging section of venturi tube:

Ps decreases -----> V increases -----> Mass flow constant

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Airspeed Measurement

Pitot Tube

Measures total pressure (stagnation pressure)

Air PRESSURE travels down pitot tube - NOT AIR.

Static Vent

Measures local static pressure on side of fuselage

Ps + Pd = Ptot therefore Ptot - Ps = Pd

Dynamic pressure is directly proportional to IAS (if IAS increased then Pd increased)

Pressure diff measuring device = capsule/diaphragm.

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IAS - Actual reading on instrument

CAS - IAS corrected for Position + Instrument error

EAS - CAS corrected for Compressibility

TAS - EAS corrected for Density

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