Sunday, July 17, 2011

Aerostats and Aerodynes

The two main types of aircraft are lighter than air and heavier than air aircraft. 

Lighter than air – aerostats

Image result for hot airballoonAerostats were among the first man-made objects to fly successfully. They use buoyancy to float in the air in much the same way that ships float on water. They are characterized by one or more large bags of gas or canopies, filled with low density gases such as helium, hydrogen or hot air (heated by burners), which are all less dense than cold air. Thus, the less dense air is pushed upwards by the cold air around it, and pulls the canopy and anything else attached to it upward.

Small hot air balloons called sky lanterns date back to the 3rd century BC, and were only the second type of aircraft to fly, the first being kites.

(Right): A modern hot-air balloon.

Image result for zeppelinOriginally, a balloon was any aerostat, while the term airship was used for large, powered aircraft designs – usually fixed-wing – though none had yet been built. The advent of powered balloons, called dirigible balloons, and later of rigid hulls allowing a great increase in size, began to change the way these words were used. Huge powered aerostats, characterized by a rigid outer framework and separate skin surrounding the gas bags were produced, the Zeppelins being the largest and most famous. There were still no fixed-wing aircraft or non-rigid balloons large enough to be called airships, so "airship" came to be synonymous with these aircraft. However, several accidents, such as the Hindenburg disaster in 1937, led to the demise of these crafts. Nowadays, a "balloon" is an unpowered aerostat, while an "airship" is a powered one. (Above-Left): A Zeppelin airship.

Heavier than air – aerodynes

Image result for fixed wing aircraftHeavier-than-air aircraft must find some way to push air or gas downwards, so that a reaction occurs (by Newton's laws of motion) to push the aircraft upwards. This dynamic movement through the air is the origin of the term aerodyne. There are two ways to produce dynamic upthrust: aerodynamic lift, and powered lift in the form of engine thrust.

(Right): A modern heavier-than-air aircraft.

Aerodynamic lift involving wings is the most common, with
fixed-wing aircraft being kept in the air by the forward movement of wings, and helicopters by spinning wing-shaped rotors. A wing is a flat, horizontal surface, usually shaped in cross-section as an aerofoil. To fly, air must flow over the wing and generate lift. A flexible wing is a wing made of fabric or thin sheet material, often stretched over a rigid frame. A kite is tethered to the ground and relies on the speed of the wind over its wings, which may be flexible or rigid, fixed or rotary. To learn more about how wings generate aerodynamic lift, click here:

With powered lift, the aircraft directs its engine thrust vertically downwards. V/STOL aircraft, such as the Harrier Jump Jet and F-35 take off and land vertically using powered lift and transfer to normal lift in steady flight. To learn more about the Harrier Jump Jet, click here:

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