Worlds of David Darling > Children's
Encyclopedia of Science > Up, Up, and Away > Chapter 4
UP, UP, AND AWAY: The Science of Flight
a book in the eXperiment! series by David Darling
4. Flight with a Twist
One of the problems with most airplanes is that they need a lot of room
in which to take off and land. Also, they cannot simply hover in one place.
They have to be moving forward all the time so that the air rushing over
their wings can provide them with lift.
Helicopters can move in ways that a fixed wing airplane
A helicopter, however, does not suffer from these disadvantages. That is
because its "wings" are in the form of a giant horizontal propeller known
as a rotor that spins around very quickly. Each of the long, thin blades
of the rotor has the shape of an airfoil. As these blades ct through the
air at high speed, they provide the helicopter with lift, whether it is
going straight up, hovering, flying forward, or flying backward. To move
in these different ways, all the pilot has to do is tilt the rotor slightly
in the direction he or she wants to go.
Helicopters can perform tasks that would be impossible for an ordinary fixed-wing
plane. For example, they can hover above a rough sea of the side of a mountain
and help to rescue people who are injured or stranded. They can land at
special heliports or helipads, even in the middle of busy cities. And they
can rush victims of a road accident to the rooftop landing pad of a major
|The main forces acting on
a helicopter in flight
Going for a Spin
You will need:
- A square of stiff cardboard about 8" by 8"
- A light plastic thread spool
- Nylon thread or thin string
- A pencil
- A ruler
What to do:
Draw the shape shown here onto the cardboard using the measurements
Be sure to mark on the dotted lines as well. Cut out the shape. This
will be the rotor of your model helicopter.
Glue the plastic spool to the rotor, being careful to position it
exactly in the center. Allow the glue to dry. Bend the rotors down
slightly along the lines of the center square. Also bend the rotors
along diagonal lines as shown. When you look at them from the side,
the rotors should have a slight droop and twist that will allow them
to slice through the air as they spin.
Wind the thread or string about 15 times around the reel, making sure
that the loose end is securely held by the windings. Put the spool
onto the pencil, hold it upright, and pull hard on the thread.
What happens if you point the pencil at an angle during the launch?
What happens if you alter the amount of droop and twist of the rotor
Taking it further:
Try making a number of different helicopters, altering the size, number,
and folding of the rotors. Which design works best? Can you explain
An Extra Twist
If a helicopter had just a single rotor, it would be impossible to control.
The reason is that as the big rotor spins around in one direction it tends
to make the rest of the aircraft twist in the opposite direction. One way
to cancel out this effect is to mount a smaller rotor facing sideways on
the tail. This creates a twisting force that is exactly equal and opposite
to that produced by the main rotor.
Another way to keep a helicopter steady is to have two main rotors spinning
in opposite directions. This is the method often used on large helicopters
that are built to carry heavy loads. A twin rotor helicopter, such as the
Boeing Chinook, can generate more lift than an ordinary single rotor craft.
It also needs no tail rotor.
|Students at a California University
became the first ever to fly a human-powered helicopter. The whole
machine weighs only 97 pounds and had two pedal-driven rotors spanning
nearly 100 feet. Its first flight, in December 1989, lasted for 7
seconds and took it 8 inches off the ground.
Not all airplanes need runways. The British Aerospace Harrier, for example,
can rise straight up off the ground, hover, and even fly backward! Air,
taken in by the Harrier’s single jet engine, is compressed and then blasted
out of four nozzles, two on each side of the aircraft. For takeoff and landing,
these nozzles are angled straight down to provide vertical thrust. Smaller
air jets called "puffers," at the tips of the wings and at each end of the
aircraft’s body, allow the pilot to steady the Harrier when it is hovering.
Once the plane is off the ground, the nozzles can be gradually swiveled
back to push the Harrier forward up to a maximum speed of 720 miles per