Principles of Flight

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Principles of Flight

• Thrust and Drag

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Principles of Flight So Far

• You already know that
• Every action has an equal and opposite reaction
• Every object has weight
• To generate lift, you need airflow over the
  wings.
• Lift varies with airspeed, angle of attack, wing
  shape and air density
• For straight and level flight, lift and weight
  must be equal

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Aim of the Lesson

• By the end of the lesson, you should have learnt
• What thrust and drag are, and how they are
  produced
• How to minimise drag
• How drag varies with the airspeed
• How the levels of thrust and drag affect an
  aircrafts flight
• What is necessary for an aircraft to be in balance

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An Aircraft In Flight

• Besides lift and weight, there are two other
  forces that act on an aircraft in flight
• These are thrust and drag

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Thrust

• The airflow over the wings that is needed to
  generate lift is caused by thrusting the aircraft
  forwards through the air.
• Thrust is therefore the forward force acting on
  an aircraft
• It is produced by the engines by throwing air
  backwards
• Either a propeller can do this, or air can be
  expelled from the rear (in the case of a jet
  engine)

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Thrust

• In both cases, throwing air backwards thrusts the
  aircraft forces
• This is because every action has an equal and
  opposite reaction
• The size of the thrust depends on the amount of
  engine power selected by the pilot

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Drag

• When youre on a bicycle, the faster you go the
  more resistance you encounter
• The force which hinders your progress is called
  drag
• The same can be said for aircraft.
• Every part of the aircraft over which air flows
  produces drag which resists forward motion
• Thus the more drag there is, the more thrust is
  needed to over come it

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Drag

• However to get more thrust you need a bigger
  engine, more fuel and more weight, and therefore
  more expense
• The aircrafts designer wants to make the
  aircraft fly at the best possible speed for the
  available thrust
• Therefore he will try and reduce drag
• This makes the aircraft more efficient and
  economical

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Causes of Drag

• The shape of the aircraft causes a great deal of
  drag
• When any object moves through the air it is
  accompanied by a wake of complicated eddies and
  vortices
• In flight, engine power which should be used for
  forward power is wasted in making these vortices

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Drag

• Here is an extreme example a flat plate in an
  airflow
• The drag is greatest at 90 to the airflow
• The drag is least when it is parallel to the
  airflow

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Minimising Drag

• Drag can be minimised by

streamlining
This means smoothing all the parts of the
aircraft which are in the airflow, thus making
the air flow as smoothly as possible.
It also means minimising the number of protruding
external parts.
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Minimising Drag
How much drag would this object produce?
A flat circular plate
Answer Maximum Drag
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Minimising Drag
How much drag would this object produce?
A ball of the same diameter as the plate
Answer 50 Drag (less!)
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Minimising Drag
How much drag would this object produce?
A streamlined shape (same diameter)
Answer 5 Drag (a lot less!)
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Minimising Drag
An effective way to streamline shapes is to have
a fineness ratio (length compared to breadth)
of 4(or 3)1
width one unit
length four units
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Variation of Drag with Airspeed
The amount of drag is proportional to the square
of the airspeed That is, when an aircraft is
going at twice the airspeed there is four times
as much drag.
So, when an aircraft is going at three times the
airspeed, how many times more drag will there be?
Answer 9 times as much drag
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Thrust and Drag in Straight and Level Flight
You have already learnt that for straight and
level flight, lift must equal weight.
Now we must learn that for the aircraft to travel
at a constant speed, the thrust must be equal to
the drag.
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Thrust and Drag in Straight and Level Flight
drag
thrust
When thrust drag, the aircraft travels a
constant speed.
When thrust is greater than drag, the aircraft
accelerates.
When drag is greater than thrust, the aircraft
slows down.
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The Aircraft in Balance
lift
To summarise
thrust
drag
weight
When thrustdrag and liftweight, the aircraft
flies straight and level at a constant speed.
The aircraft is in balance.
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Questions
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