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| | MSN Encarta - Airplane |
 | | Airplanes also differ from other heavier-than-air craft, such as helicopters, because they have rigid wings; control surfaces, such as movable parts of the wings and tail, which make it possible to guide their flight; and power plants, or special engines that permit level or climbing flight. |
 | | The shock wave created by an airplane moving at supersonic and hypersonic speeds represents a rather abrupt change in air pressure and is perceived on the ground as a sonic boom, the exact nature of which varies depending upon how far away the aircraft is and the distance of the observer from the flight path. |
 | | Airplanes are heavier than air, in contrast to vehicles such as balloons and airships, which are lighter than air. |
| encarta.msn.com /encyclopedia_761556643/Airplane.html (1583 words) |
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