Worlds of David Darling > Children's
Encyclopedia of Science > Up, Up, and Away > Chapter 2
UP, UP, AND AWAY: The Science of Flight
a book in the eXperiment! series by David Darling
2. Wings and Lifting Things
Birds and airplanes have one very important thing in common: They both make
use of wings to fly.
Bald eagle flying
An eagle and an airliner may not look alike, but their wings are shaped
in a similar way. The upper surface of the wing is curved. It rises steeply
and smoothly from the front, or leading edge, and then drops away gently
to the rear, or trailing edge. The lower surface of the wing, by comparison,
is much flatter.
Why should this particular shape be so important for flight?
A Wind Tunnel
You will need:
- 16 large milk cartons
- A powerful fan, preferably with several speed settings
- A large wooden board
- Glue or tape
- A sheet of paper (about 8" by 11")
- A knitting needle
- Cotton thread
What to do:
Cut out the front and back of the milk cartons. Glue or tape the cartons
together to make a 4-by-4 square, as shown. Fix this arrangement to
the wooden board. Place the fan behind cartons so that it is exactly
in line with them. Position the center of the fan opposite the center
of the square. The basic wind tunnel is now complete.
Fold the sheet of paper in half. Tape the top half to the bottom half
about 1 inch from the edge. The paper now looks like a stubby wing.
Slide the knitting needle into the fold.
Turn the fan to its lowest speed. Hold the knitting needle level in
the center of the air stream with the curved side of the wing uppermost.
What happens? Turn the wing upside down and try again. What effect
does this have?
Taking it further:
Tape several pieces of thread along the front edge of the wing so
that they trail back over the upper wing surface. Replace the wing
in the wind tunnel. What happens to the thread when the air stream
is turned back on? What happens if you switch the fan to a higher
Make wings of various different shapes and test them. Records your
results and try to explain them.
Many improvements to the basic wind tunnel are possible. It would
be very useful, for example, to install a fan with a continuously
variable speed. This should only be attempted by an adult with a good
knowledge of electrical equipment. Similarly a device for measuring
wind speed (an anemometer) would be a valuable addition for more advanced
experiments. The milk cartons could be replaced by a more permanent
array made from thin panels of wood.
Some sort of arrangement will also be needed if model planes are to
be tested properly in the wind tunnel. A framework could be built
from which the model could be suspended by threads. The threads would
need to be slack enough that the plane could rise or fall, or bank
to the left or right, but not so loose as to let it go out of control.
In practice, this is not easy. An alternative method is to hold the
plane steady and tape cotton thread to the areas of interest on the
model. The way in which the thread is blown indicates the behavior
of the air flow.
Wings and Lift
Think what happens as a wing passes through air. At the leading edge of
the wing, the airflow divides. Some air goes over the top of the wings;
the rest goes underneath. Because of the Coanda effect, the airflow tends
to "hug" the outline of the surface over which it is moving.
The air passing under the wing continues on more or less a straight line.
This is because the lower surface of the wing is almost flat. However, the
air flowing over the wing has farther to go, because the upper surface is
curved. To keep up with the air underneath, the air on top has to move faster.
But by Bernoulli's principle, higher speed means lower pressure. As a result,
the air on top of the wing must press less strongly against the wing than
does the air underneath.
How the shape of a wing, or airfoil, creates
Since the downward pressure on the top of the moving wing is less than the
upward pressure on the bottom of the wing, there is an overall upward force.
This upward force is called LIFT.
A wing shape, or AIRFOIL, happens to be the best shape for producing lift.
That is why nature uses airfoils in birds, flying insects, and bats, and
why human engineers use them in various types of aircraft.
You will need:
- The wind tunnel
- A paper wing
- A knitting needle
- A stiff piece of cardboard
What to do:
Set up the wing in the wind tunnel as before. Observe its position
in the moving airstream. Now block off the air from the top of the
wing with the piece of cardboard. To do this place the edge of the
cardboard along the center of the leading edge of the wing so that
the cardboard shields the upper surface of the wing. What happens?
Repeat the test with the cardboard blocking off the wind from the
lower surface of the wing. What do you observe?
Try to explain your results in terms of what you have already learned
The Flight of the Frisbee
Scientific discoveries and inventions are often made completely by accident.
Take Frisbees, for instance. In 1871, William Russell Frisbie opened a pie
factory close to the college that, sixteen years later became Yale University.
Hungry students of the college, who were regular customers, soon found that
the empty pie tins flew extremely well when thrown upside down! It was not
until the 1940s, however, that someone took full advantage of the idea.
Then Frederick Morrison, a keen pie-tin thrower, made a similar disk out
of a new type of plastic. Later, he sold his invention to the Wham-O Corporation
of California, which coined the named "Frisbee" for the toy.
A Frisbee produces lift in the same way as a wing. That is, the top of the
disk is curved while the bottom is flat. During flight, this results in
a lower air pressure on top than on the bottom. But the similarity between
the Frisbee and a wing ends there. Because of its spin, the Frisbee behaves
in a much more complicated way.
Looking at Feathers
You will need:
|Close-up diagram of a bird's
- A feather (Find one of the long narrow feathers lost from the
end of a bird's wing)
- A magnifying glass
What to do:
Look at the feather from the side. What shape does it have? Examine
the feather under the magnifying glass or microscope. What do you
Identify the central shaft and the barbs projecting from each side.
Focus on one of the barbs. Attached to each barb are tiny branches
called barbules. These lock together – but how? Examine them
carefully to find out.
Hold the feather straight out by the end of its shaft. Tilt it so
that the front edge is slightly higher than the back. Now move it
quickly through the air. What do you notice?
A bird's wings, and each of its feathers, are shaped much like an aircraft's
wing – curved on top and flatter underneath. This allows them to generate
lift in the same way.
The main problem facing a bird – especially a heavy bird – is
taking off. To do this, the bird has to beat its wings very hard to force
air quickly over the wings' upper surfaces and so produce a lot of lift.
Once it has climbed high enough, the rate of beating can be reduced to that
needed for level flight.
Several factors help to give birds mastery of the air. They have bones that
contain many hollow spaces, making them lightweight yet strong. A bird's
chest muscles, which operate the wings, are extremely large and powerful.
And, finally, the wings themselves are covered in tightly fitting feathers
that trap the air.