Have you ever watched birds in the sky and thought how easy it looks to fly? Centuries ago, people dreamed of joining birds in flight. Some went even further and flapped about, vainly, in wings made of feathers. But the human body is too heavy and does not have the muscles needed for flight. The pioneers of aviation soon realised that before they could join the birds, they needed to understand how birds flew. They discovered that the wings of a bird are specially curved surfaces, called aerofoils. When air flows over a bird's wings, a difference in air pressure is produced above and below the wings. This difference in pressure creates a force called "lift", which can overcome the weight of a bird or a plane. This is called heavier-than-air flying, and gliders and aeroplanes also fly this way. Balloons and airships are lighter-than-air fliers. They are filled with hot air (which always rises) or gases, such as helium or hydrogen that are lighter than the air around them.
Birds fly higher, farther, faster than any other flying animals. Many species can span entire oceans and continents on their migrations. Birds are also the only flying animals that regularly use the wind as as a source of lift. A bird's wings are powered by two sets of muscles on the breast, In most birds these muscles make about one-third of their total weight. Muscle power moves the long, stiff flight feathers. Feathers at the wing tips, called primaries, propel the bird forward, while the rest of the wing generates the lift. The wings change shape as they beat up and down. They are broad and extended on the down stroke, but tucked in tight on the upstroke. Wing shape and how a bird lives are closely related. Long wings are more efficient than short wings, but much harder to flap. Birds with long wings are usually soaring birds. Short-winged birds have less stamina, but can build up speed very quickly.
Four forces act on an aeroplane when its flying.
1) Lift: is an upward force that holds an aeroplane in the air.
2) Thrust: is the force that moves the aeroplane forward.
3) Drag: is air resistance that holds the aeroplane back.
4) Weight: is the force that pulls the aeroplane down.
The wings of an aeroplane are a a special shape, called an aerofoil. The top of the wing is straight.
Lift comes from an aeroplane's wings. Because the top of the wing is curved, air has further to go over the top than it does under the wing. This produces a lifting pressure over the wings that supports the weight of the aeroplane.
Gliders, hang gliders and birds use thermals to gain height during flight. Because they are light weight, they gain height when circling in thermals. A glider will only fly if it is moving fast enough to keep air flowing smoothly past its wings to create lift. Gliders gradually descend back to the ground unless they are flying in the air that is rising at a rate faster than the glider's normal rate of descent.
How Thermals are made:
Thermals occur when a column of warm air rises from heated parts of the ground. Air also rises when the wind meets rising ground such as hillsides, mountain ranges and coastal sand dunes.
A hot air-balloon is a huge envelope, or bag, made of light material that is filled with heated air. Passengers are carried in a basket or gondola suspended underneath. Hot-air balloons rise because the air inside the bag is warmer (and lighter) than the surrounding air.
Air that has been heated by the hot flames of a gas burner rises and collects in the balloon.
1) When the air inside the balloon is hot it gives enough lift to overcome the balloon's weight. The balloon rises from the ground and soars into the sky.
2) As it rises the hot air in the balloon begins to cool and the lift gets weaker. When the lift equals the balloon's weight, the balloon stops rising and floats at the same height in the air.
3) As the hot air cools further the lift becomes less than the balloon's weight. The balloon begins to sink.
4) To keep the balloon flying the pilot turns the burner on again. Short bursts of flame keep the air inside hot. The lift stays strong enough to keep the balloon from sinking.
5) To descend, a valve at the top of the balloon opens to release the hot air. Cold air replaces the hot air. The balloon becomes heavier and is able to descend and Iand.
Kites depend on wind to fly. They fly because they are held in the air by the force of the wind pushing against them. The wind rushing over the top of the kite goes faster than the wind flowing underneath it. This difference in air flow creates an upward lifting force on the kite.
Kites fly best in a good breeze. A kite's weight is very small in comparison to the strength of the forces of lift and drag. Some kites can fly in a very gentle breeze if they are light and their surfaces are wide. The wide surfaces give the breeze a large area from which to produce lift.
Propellers are driven by the aeroplane's engines. As a propeller turns, its blades force air backward at a much higher speed. This propels the aeroplane forward. The first aeroplanes were powered by piston engines that turned propels. Many aeroplanes are still powered this way today.
The way a propeller works is by having the propeller accelerate a large mass of air to a higher speed. It draws air in the front, accelerates it, and throws it backward. By doing this continuously, the propeller produces a continuous forward thrust.
The propeller has an aerofoil shape. The front of each blade is curved more than the back. As the blades spin, the air pressure is lower in front of than behind. The difference in air pressure pulls the aircraft forward.
To know more about the science behind flight and flying machines then click on:
http:www.allstar.fiu.edu/aero/princ1.htm
To find out how you can make your own cool paper aeroplanes then click on:
http://koolpaperairplanes.hypermart.net/
|
|
|
|
|
|
|
|
|
|
|
|
|
||
