Principles of Flight – Thrust and Drag
Principles
of Flight
Thrust and Drag
Principles of Flight – Thrust and Drag
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
Principles of Flight – Thrust and Drag
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
aircraft’s flight
• What is necessary for an aircraft to be in
balance
Principles of Flight – Thrust and Drag
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
Principles of Flight – Thrust and Drag
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)
Principles of Flight – Thrust and Drag
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
Principles of Flight – Thrust and Drag
Drag
• When you’re 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
Principles of Flight – Thrust and Drag
Drag
• However to get more thrust you need a
bigger engine, more fuel and more weight,
and therefore more expense
• The aircraft’s 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
Principles of Flight – Thrust and Drag
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
Principles of Flight – Thrust and Drag
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
Principles of Flight – Thrust and Drag
Minimising Drag
Drag can be minimised by:
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.
Principles of Flight – Thrust and Drag
Minimising Drag
How much drag would this object produce?
A flat circular plate
Answer = Maximum Drag
Principles of Flight – Thrust and Drag
Minimising Drag
How much drag would this object produce?
A ball of the same diameter as the plate
Answer = 50% Drag (less!)
Principles of Flight – Thrust and Drag
Minimising Drag
How much drag would this object produce?
A streamlined shape (same diameter)
Answer = 5% Drag (a lot less!)
Principles of Flight – Thrust and Drag
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
Principles of Flight – Thrust and Drag
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
Principles of Flight – Thrust and Drag
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.
Principles of Flight – Thrust and Drag
Thrust and Drag
in Straight and Level Flight
thrust
drag
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.
Principles of Flight – Thrust and Drag
The Aircraft in Balance
To summarise:
lift
thrust
drag
weight
When thrust=drag and lift=weight, the aircraft
flies straight and level at a constant speed.
The aircraft is in balance.
Principles of Flight – Thrust and Drag
Questions
Principles of Flight – Thrust and Drag