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    Posts : 143
    Join date : 2010-09-23


    Post  meodingu on Wed Oct 27, 2010 2:22 am


    RAF Exeter airfield on 20 May 1944, showing the layout of the runways that allow aircraft to take off and land into the wind

    There are many different forms of sailing ships, but they all have certain basic things in common. Except for rotor ships using the Magnus effect, every sailing ship has a hull, rigging and at least one mast to hold up the sails that use the wind to power the ship.[77] Ocean journeys by sailing ship can take many months,[78] and a common hazard is becoming becalmed because of lack of wind,[79] or being blown off course by severe storms or winds that do not allow progress in the desired direction.[80] A severe storm could lead to shipwreck, and the loss of all hands.[81] Sailing ships can only carry a certain quantity of supplies in their hold, so they have to plan long voyages carefully to include appropriate provisions, including fresh water.[82]

    While aircraft usually travel under an internal power source, tail winds affect groundspeed,[83] and in the case of hot air balloons and other lighter-than-air vehicles, wind may play a significant role in their movement and ground track.[84] In addition, the direction of wind plays a role in the takeoff and landing of fixed-wing aircraft and airfield runways are usually aligned to take the direction of wind into account. Of all factors affecting the direction of flight operations at an airport, wind direction is considered the primary governing factor. While taking off with a tailwind may be permissible under certain circumstances, it is generally considered the least desirable choice because of performance and safety considerations, with a headwind the desirable choice. A tailwind will increase takeoff distance and decrease climb gradient such that runway length and obstacle clearance may become limiting factors.[85] An airship, or dirigible, is a lighter-than-air aircraft that can be steered and propelled through the air using rudders and propellers or other thrust.[86] Unlike other aerodynamic aircraft such as fixed-wing aircraft and helicopters, which produce lift by moving a wing, or airfoil, through the air, aerostatic aircraft, such as airships and hot air balloons, stay aloft by filling a large cavity, such as a balloon, with a lifting gas.[87] The main types of airship are non-rigid (or blimps), semi-rigid and rigid. Blimps are small airships without internal skeletons. Semi-rigid airships are slightly larger and have some form of internal support such as a fixed keel. Rigid airships with full skeletons, such as the massive Zeppelin transoceanic models,[88] all but disappeared after several high-profile catastrophic accidents during the mid-20th century.[89]

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