Principles of Flight

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Principles of Flight
Spitfire
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Chapter 1 Weight and Lift

• How is an aircraft, which is much heavier than
  the air it flies in, supported by the air?

C130
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• Sir Isaac Newton formulated laws to explain the
  movement of objects.
• Newtons Third Law states that to every action
  there is an equal and opposite reaction.
• For example Imagine a car weighing 10,000N
  parked on the road. The cars weight presses down
  on the road with a force of 10,000N and from
  Newtons third law the road must be supporting
  the car and pushing up with an equal and opposite
  force of 10,000N.

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• How does this apply to an aircraft flying?
• How can an aircraft which might weigh many tonnes
  be supported by such a flimsy substance such as
  air?
• One clue is that an aircraft can only stay
  airborne as long as it is moving. (except a
  Harrier of course!)

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Air is a substance

• Moving air can exert forces on objects e.g. in
  hurricanes
• The greater the speed, the greater the force it
  exerts.
• Conclusion Air is undoubtedly a very real
  substance, invisible as it is!

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Moving objects through air

• Is there a force if an object is moving through
  stationary air?
• Imagine cycling along a flat road on a perfectly
  calm day. As your speed increases, you will
  begin to feel the air on your face. The faster
  you pedal the greater the force of this apparent
  wind.
• Whether you have air moving past a stationary
  object or an object moving through stationary
  air, similar forces are experienced.

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• Every object has weight.
• The weight of an aircraft must be supported or
  lifted by a force.
• How is this lifting force to be given to an
  aircraft?

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Try this Hold two sheets of paper with the edges
vertical and about finger width apart. What will
happen if you blow between the two pieces of
paper?
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What causes this to happen?

• Wind Tunnel Test
• The air goes in at point A, passes the
  constriction at point B, and comes out at point
  C.
• What are the speeds and pressures at points A, B
  and C?

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Bernoullis Principle

• The same amount of air enters B as is leaving,
  therefore the speed of air at B must increase to
  pass through the narrowest part of the tube.
• The increased speed of the air at B, creates a
  streamlined flow (non-turbulent) which exerts
  less pressure on the tube.
• Conclusion Air moving a increased speeds exerts
  less pressure or creates an area of lower
  pressure.

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• Can you now explain why the two pieces of paper
  move together when you blow air between them?

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Proving the theory

• Try this Hold a half sheet of A4 (halved
  lengthways) such that the end between you fingers
  is horizontal, but allow the rest of the paper to
  curve downwards under its own weight. Then blow
  along the curved top of the paper only.
• What happens?

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• Observation The paper rises in line with the
  air flow.
• By speeding up the air over top you have reduced
  the pressure above the paper, so the air pressure
  under the paper pushes it up.
• What happens when you blow harder?
• The harder you blow the more it rises.

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Lift

• The top surface of an aircrafts wing is shaped
  such that air which flows between it and the
  undisturbed air a little way above the wing is,
  in effect, being forced through a constrictions.
• The air flows over the wing at an increased
  speed-and therefore at a reduced
  pressure-compared with the surrounding atmosphere

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• The resulting pressure difference between the air
  above and below the wing tends to lift it up.
• This is not the only thing that contributes to
  lift. In real life, the airflow rarely approaches
  a wing as shown above.
• In most flight conditions, the wing is inclined
  to the airflow at a slight positive angle so that
  it deflects some of the airflow downwards.
• This results in an upward force, though not
  normally as much as the lift generated by the top
  surface of the wing

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Distribution of lift

• There are pressure forces acting all over the
  wing- and there can be lift forces all over the
  wing
• The length of each arrow indicates the amount of
  lift at that point on the wings surface.

Pressure patterns near A wing
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• Note that
• Lift is not distributed evenly around the wing
• The top surface generates more lift than the
  bottom surface (sometimes as much as 80 of the
  total!)
• The greatest amount of lift on the top surface
  occurs where the surface is curved the most.
• The greatest effect, on both top and bottom
  surfaces, is nearer the front edge of the wing
  than the rear (1/3 of the way from the front)
• All lift forces act at 90 to the direction of
  the airflow.

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Centre of Pressure

• We normally add all the forces together and
  represent them by a single straight line.
• This force is drawn from the point at which all
  the forces balance.
• This is the point which all the lift can be said
  to act and it is called the centre of pressure.

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How Lift Varies

• Several factors affect the amount of lift
  produced by a wing
• A) Air speed greater airspeed creates greater
  lift (doubling the airspeed, quadruples the
  lift!)

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• B) Angle of attack- This is the angle between the
  chord line of the wing and the oncoming air. A
  pilot can alter this by altering the pitch
  attitude of the aircraft- by easing forward or
  pulling back on the controls. The lift will
  increase until the angle reaches about 15.
  Beyond this point, the lift rapidly decreases and
  the wing has stalled

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• C) Air Density If air becomes thinner or less
  dense ( at increased height temperature or
  humidity), the amount of lift is reduced. To
  calculate the take off run the pilot would use
  tables which allow for engine performance,
  weight, wind speed and direction, and of course
  the air temperature, humidity and density.

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• D) Wing shape and area- The shape of the wing
  section and the plan area of the wing are
  calculated by the aircraft designer to suit the
  aircrafts role and required performance.
• A high-lift section would be used where good lift
  at low speeds is all-important

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• General Purpose section would suit a moderately
  fast aircraft, not intended to approach the speed
  of sound.
• More lift for any given wing section and
  airspeed can be found by increasing the plan area
  of the wing- that is, by making the wing bigger.

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• Unfortunately a bigger wing is heavier and also
  more resistant to rapid manoeuvres- not
  acceptable for air combat aircraft.
• Hinged flaps fitted to most light aircraft have
  the effect of increasing the camber of the wing
  when they are lowered.
• More advanced Fowler flaps slide backwards to
  increase the wing area, as well as hinging
  downwards

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Lift and Weight in Straight and Level Flight

• The lift force equals the force of gravity acting
  on the aircraft (its weight)
• If the lift is greater than the weight, an
  aircraft will climb
• If the weight is more than the lift, it will
  descend.

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Questions
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