Download Flight – Background Information

Flying High
HHJS Science Week 2013
Background Information
Forces and Flight
Flight – Background Information
Flying is defined as controlled movement through the air. Many things can
become airborne but this does not necessarily mean they are actually flying and
under control.
FOUR PRINCIPAL FORCES AFFECT THINGS THAT FLY:
1.
Weight is a force due to gravity. It acts in a downward direction, i.e.
towards the centre of the Earth.
2.
Lift is the force that acts at a right angle to the direction of motion through
the air. If the plane/bird is level (i.e. parallel to the earth’s surface) the lift
will be acting in the opposite direction to gravity.
3.
Thrust is the force that propels the plane/bird in the direction of motion.
4.
Drag is the force that acts in the opposite direction to the direction of
motion through the air. Drag is caused by friction (friction drag) and
differences in air pressure (pressure drag).
Balanced forces
When an aeroplane is flying straight, and level, and at a constant speed, the lift
it produces balances its weight, and the thrust it produces balances its drag.
The forces are said to be balanced. These forces will become unbalanced as
soon as the airplane rises and descends, or speeds up and slows down, or
changes direction.
The principles are the same with birds, although there will rarely be moments
when the forces are actually balanced, i.e. when the bird is moving at a constant
speed and in a constant direction.
Air
Air is not just empty space—it has substance, or mass. Air has molecules that
are constantly moving. Because air has mass, Earth’s gravity attracts it and
gives it weight. Because it has weight, and the air molecules are constantly
bumping into things, air exerts pressure.
As you go up (e.g. climbing a mountain), air pressure goes down, because the
higher you go, the less air there is pressing down on you from above. The
pressure of the air doesn’t squash you because the pressure from in your lungs
and in other spaces inside your body balances the pressure of the air around
you.
We live at the bottom of a “sea of air”
Sounds strange but effectively this is true - our atmosphere and our oceans
have much in common. Both consist of matter that flows and both have currents
that circulate throughout them. Both create pressure that changes with depth
and provides buoyancy—e.g. for fish in water and for balloons in air. The forces
that apply to movement through air apply to movement through water as well.
We do indeed live at the bottom of a “sea of air.”
Drag
Drag (or air resistance) on an aircraft or bird is caused by a combination of two
things: (1) friction and (2) differences in air pressure.
Friction
The force of friction is the resistance that occurs when two things rub together.
A familiar example might be car tyres gripping a road. The same thing happens
in air. When an object moves through the air, e.g. the wings of an aeroplane or
bird, the air closest to the surface of the object is dragged along with the object.
This layer of air which is ‘stuck’ to the object, rubs against the air that passes it
in the opposite direction. The resulting force is called friction drag.
Differences in air pressure
Imagine an object that is stationary in air. Air will be pushing equally around the
object. Now imagine the object starts moving through the air. When this
happens, air flows past the object. The air flowing past it will be pushing harder
against the front of the object than against the back of it. This creates a
difference in air pressure and the resultant force is air resistance. It is also
called pressure drag.
The size of the air resistance on an object depends on the shape of the object
and on the speed at which it is travelling.
Shape
All four of the shapes in this diagram have the same
width/diameter. The first flat shape ‘blocks’ airflow and
causes more turbulence behind it, creating an area of
lower pressure there. This pressure difference results
in a bigger force against the front of the object, i.e. a
pushing force backwards.
A sphere is a better shape for airflow around the
object as the area of lower pressure behind it is not as
great as the flat shape.
Better still, the object tapers at the back, reducing the
lowering in pressure behind it. Best of all, the object is
also tapered at the front, giving airflow with minimal
turbulence and so reducing the pressure difference
between front and back as much as possible. This
shape is what is known as an aerodynamic shape.
Speed
As an aircraft's speed increases, the faster the flow of air in the opposite
direction. This means that the pressure difference between the front and back
gets bigger and so the drag on the aircraft increases. The relationship between
speed and air resistance is not linear, e.g. doubling the speed makes the
airplane encounter twice as much air, moving twice as fast, causing drag to
quadruple. Drag, therefore, sets practical limits on the speed of an aircraft.
Aerodynamics (or streamlining) is the term used to describe the shaping of
objects such as aircraft or fast cars to help them speed up and travel faster
through the air. Imagine putting your hand out of a car window while the car is
moving. If you have your palm turned towards the front of the car you will feel
the drag of the wind on your hand. If your palm is down or flat, the wind goes
over it more easily and you don’t feel the same amount of resistance.
Lift and Bernouilli’s Principle
Bernoulli’s principle helps explain that an aircraft or bird can achieve lift because
of the shape of its wings – often referred to as aerofoils. The wings are shaped
so that as the object moves forwards, air flows faster over the top of them than it
does underneath them. Faster moving air has lower air pressure than slower
moving air. The more slowly moving air underneath the wings will have higher
pressure than the air over the wings. Therefore the air underneath the wings
will push the aircraft/bird upwards.
Free Falling Objects
There are two main forces acting on free falling objects – weight, due to gravity
and air resistance. As an object falls it accelerates due to gravity. The faster it
falls the greater the air resistance becomes until it reaches the point where it
equals the object’s weight (force due to gravity). At this point, the forces are
said to be balanced, and from this moment, the object will stop accelerating and
continue to fall at a constant speed; this speed is called terminal velocity.
Parachutes
A parachute works by increasing air resistance. This means that the forces of
air resistance will balance the object’s weight will more quickly, resulting in
terminal velocity being lower. In other words, the object will continue to fall at a
slower constant speed than it would have done without the parachute. The
larger the surface area of the parachute, the greater the increase in air
resistance and the lower the terminal velocity will be.
The Paper Spinner
The paper spinner spins as it falls. When it starts its fall, the air pressure under
the wings increases (air resistance). This causes an upward force underneath
the wings. The increased pressure also causes a sideways push on the vertical
part at the top of the spinner (where the red dot is). The same thing will be
happening diagonally opposite, under the other wing, which causes the spinner
to spin.
The faster the spinner falls, the greater the sideways push, so the more it spins.
Balloons
Balloons can float in the air because of buoyancy, an upward force that the air
exerts on them. If a balloon rises, it means that it and its contents weigh less
than the air it displaces (that is the air that would occupy the same space if the
balloon wasn’t there).
Since the balloon itself is denser than air, it must be filled with a large volume of
something much less dense - either hot air or a very low density gas, such as
helium. Because the combined weight of the balloon and the gas is less than
the weight of an equal volume of surrounding air, the balloon rises.
Helium balloon
Blue dots = air particles
Green dots = helium particles
Helium is seven times less dense than air. The balloon and the helium together
weigh less than the same volume of air, which creates a lift force (or upthrust)
greater than the downward force (due to gravity). This means the balloon rises
through the air. Remember that as you go up, the density and pressure of the
air decreases, so the balloon will continue to rise until the lift force equals its
weight.
Hot air balloon
A hot air balloon is filled with air. This air inside is heated up, and so it expands,
or takes up more space; some of it will escape but the balloon will still be filled.
The molecules of air inside the balloon have spread out and are further apart
than before. In other words the air that remains in the balloon has become less
dense than the air around it, so a lift force is created.
Useful website - How Things Fly