THE PRINCIPLES OF FLIGHT
The Air Around You
The 6 properties of air:
1. Air takes up space.
2. Air has weight.
3. Air can be compressed by applying pressure or squeezing.
4. Air can insulate.
5. Air can expand and create pressure when heated.
6. Air can contract when cooled.
Air is a mixture of 78% nitrogen, 21% oxygen and 1% a combination of carbon dioxide,
helium, water vapour and other gases.
When air is heated, the molecules speed up and move farther apart. This heated air takes
up more space. As the air is cooled, the molecules slow down and move closer together,
causing the air to take up less space. Heated air, which has greater volume than cooler air
with the same mass (the number of atoms that holds or clings together in an object), will
naturally rise, since the pull of gravity workers harder on the denser air.
Think about and jot down examples for each property of air.
Gravity
Gravity is a force.
For an object to fly, there must be enough lift (a force that causes something to rise) to
overcome the force of gravity acting on the mass of an object. In a hot-air balloon,
unbalanced forces work such that the force of lift must overcome the force of gravity.
The lighter the object, the less lift is required to overcome the force of gravity. Thus a
very light object, such as a dandelion seed, can use the wind to give it enough lit to be
carried away. Gravity is a force that results from the masses of objects being attracted to
each other. Your mass is attracted to Earth’s surface. It is important to know that the
force of gravity does not depend on air, it depends only on mass.
Bernoulli’s Principle
Bernoulli’s Principle states that moving fluids (air considered a fluid) creates less
pressure than non-moving fluids. TRY THIS EXPERIEMENT: To show how to
generate lift, hold one end of a strip of paper (5 by 20 cm) against your lower lip so that
the strip sags down over your chin. Blow hard across the top of the paper and watch the
paper strip rise. Lift is being created. The question is – how was the lift created?
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Moving air pushes on objects.
Moving air can move other objects.
Moving air can cause changes in pressure.
When you blew air across the strip of paper, you lowered the air pressure by making the
air above the strip move faster than the air below the strip. With lower pressure on top
compared to below, lift was created and caused the paper to rise.
Bernoulli’s Principle is used to explain properties of aerodynamics (The study of air in
motion). This same principle explains how airplanes work. As a jet plane is thrust
through the air by the hot exhaust gases expelled from its engines (or air being moved by
propeller) the shape of the wing forces the air traveling over the top of the wing to move
faster. This lowers the air pressure and gives it lift to the wing, allowing the plane to take
off from the ground and increase altitude as it flies through the air. (Refer to attached
diagrams).
The Four Forces of Flight
The four forces of flight are: Lift, Gravity (weight), Thrust and Drag. All flying devices
must use thrust and lift to overcome the forces of drag and gravity (weight). When there
is a balance of the four forces, a stable flight can occur.
Thrust is the force that moves the plane forward. Thrust comes from the propellers or jet
engine. And when you have thrust or moving air across a surface you get lift. This is how
an airplane can fly. The thrust moves the plane forward and generates lift to keep the
plane in the air.
Drag slows down or prevents the motion of an object in air or water. It is difficult to run
in water because the water slows you down. You could say that the water is creating a
drag on your movement. Air behaves in a similar way. Air creates drag on a plane and
slows it down. The force has advantages and disadvantages. Advantage: It helps the pilot
to steer the plane. Disadvantage: It needs more thrust (energy) than drag to fly.
Three terms describe the movements of an airplane. (Refer to attached diagrams). Pitch
is the up-and-down movement of the nose of a plane. Roll is a motion where one wing tip
moves up while the other wing tip moves down, causing the body of the plane to roll.
Yaw is the side-to-side movement of the nose of a plane. Pilots maneuver planes through
the air using combinations of pitch, roll, and yaw.
HOMEWORK:
In your group, design, construct and test a simple flying device (e.g., paper airplane, hot
air balloon). Review the principles of flight and the properties of air. Be prepared to fly
your structure in class, and explain how the principles of flight and the property of air
affected your flying device.