Worlds of David Darling > Children's
Encyclopedia of Science > Up, Up, and Away > Chapter 1
UP, UP, AND AWAY: The Science of Flight
a book in the eXperiment! series by David Darling
1. Amazing Air
A Boeing 747 "jumbo" jet taxis onto the runway, carrying over 350 passengers
and crew. Its main deck, 180 feet in length, is longer than the first flight
of the Wright brothers in 1908. Its weight is about 400 tons, including
enough fuel to drive a car nonstop for about three years. Yet, within seconds,
this monster of metal, fuel, and cargo is in the air. Soon it is flying
higher than Mount Everest at a speed of almost 600 miles per hour. How is
this possible? How could something so big and heavy fly so well?
Boeing 747 taking off
An albatross wheels lazily over the ocean, its five-foot-long wings hardly
ever beating. Forty thousand above it, a Concorde airliner tears through
the sky at twice the speed of sound. In just two and a half hours it whisks
its passengers from London to New York. All around the world, there are
things flying, hovering, gliding drifting, or floating in the air. Some
are living creatures; others are human-made machines. How does each of them
manage to stay up? How does flight work?
The answer has to do with air and the way air moves around objects.
Some Pressing News
Air is everywhere around us. It is invisible and, most of the time, unnoticeable.
But the power of moving air can make itself frighteningly clear. During
a tornado or hurricane, trees are ripped up, houses are wrecked, and cars
tossed around as if they were toys.
Many molecules bumping into a surface, such
as your hand, create pressure
Air seems weightless. But the air filling your bedroom weighs roughly the
same as you do. In fact, all the air on earth weighs about 11 quintillion
Like most matter, air is made up of tiny particles called MOLECULES. In
solids or liquids, the molecules are packed tightly together. But in gases
such as air the molecules are far apart and move very rapidly. The average
speed of air molecules in a warm room is about 1,130 miles per second.
When an air molecule crashes into something, such as your hand, it gives
it a tiny push before bouncing off again. The effect of just one molecule
is far too small to notice. But billions and billions of air molecules bump
into your hand every second. That gives rise to quite a strong pressing
force, or PRESSURE. Why, then, do you not feel it? The answer is that more
or less the same number of molecules are colliding with the other side of
your hand. So, the pressure on either side is balanced out.
Air on the Move
You will need:
- Two sheets of paper
- Two thick books
What to do:
Hold one of the sheets of paper close to your bottom lip and blow
hard across the upper surface. What happens?
Place the two books about 4 inches apart on a table. Lay a sheet of
paper over the books. Blow hard through the gap between the books.
Notice how the paper moves.
Hold the two sheets of paper upright, a few inches apart in front
of your face. Again, blow hard and watch what happens.
Try to explain your observations by thinking about the air pressure
on either side of the paper in each of the experiments. What did you
do to the air on each side when you blew on it? What effect do you
think this had on the pressure on that side?
More Speed, Less Pressure
In 1738, the Swiss mathematician Daniel Bernoulli made a surprising discovery.
It has become known as BERNOULLI'S PRINCIPLE.
This drawing shows how Bernoulli's principle
results in an upward-pressing force
Bernoulli found that as the speed of a gas or liquid increases, its pressure
drops. This means that air rushing over a surface, for example, pushes against
the surface less than if the air were still.
If air moves everywhere around an object at the same speed, then the pressure
on all sides will drop by the same amount. In this case, the pushing force
from every direction remains balanced. But what happens if the air only
moves over the top of an object and is still underneath? From Bernouilli's
principle it follows that the pressure on top will be less than that underneath.
Because of this, there will be an upward-pressing force.
Mapping Air Flows
You will need:
- A glass jar
- 20 or 30 pins
- Paper, scissors, and glue
- A large wood or cork board
- A large sheet of paper
- A pencil
- A blow dryer
What to do:
Cover the board with the sheet of paper. Draw a line down the middle
of the paper at right angles to the edge nearest you.
Cut a 1/2-inch wide, 2-inch long strip of paper and fold it around
one end of the pins. Glue the ends of the paper together so that the
result is a small flag, with the pin as the flagpole. The paper should
be able to spin easily around the pin when you blow on it. Make similar
flags using the other pins.
Place the glass jar at the near edge of the board on the middle of
the line. Stick the pins in at various points behind and to the side
of the jar. Some should be close behind the jar, others farther away.
Turn all the flags so that they are pointing inward at right angles
to the center line.
Point the blow dryer at the near side of the jar, about 6 inches away,
and directly along the center line. Switch it on to the lowest setting
for several seconds. Then turn it off. Look at the new position of
Under each flag draw a short line to show the direction in which it
now points. Then remove the flag. Observe the pattern of lines. What
does this tell you about the way the air flowed around the jar?
Taking it further:
Repeat the experiment with the pins in new positions. For example,
you could stick the pins at the ends (or beginnings) of the short
lines. By doing this a number of times, you could map out complete
lines of flow. Make sure that you hold the blow dryer in exactly the
same place in each time.
Do the whole experiment again with different shaped obstacles –
some with straight sides, others with smoothly curved sides. What
happens if you make the surface of he jar less smooth by wrapping
it with cloth? What happens if you increase the speed of the air flow?
Sometimes when you try to pour water from a jug the water runs down the
side of the jug. In fact, all liquids and gases behave this way. That is,
when flowing past a smooth surface, they tend to stay close to the surface.
The effect was first described by a Romanian engineer, Henri Coanda, in
1926, and is named after him.
The Coanda effect
Because of the COANDA EFFECT, moving air will try to follow the lines of
an object, even if it has to change direction to do so.