If you look at a glider next to a conventional powered plane, you'll notice a significant difference in the wings. While the wings of both are similar in general shape and function, those on gliders are longer and narrower than those on conventional aircraft. The slenderness of a wing is expressed as the aspect ratio, which is calculated by dividing the square of the span of the wing by the area of the wing.
Glider wings have very high aspect ratios -- their span is very long compared to their width. This is because drag created during the production of lift (known as induced drag) can account for a significant portion of the total drag on a glider. One way to increase the efficiency of a wing is to increase its aspect ratio. Glider wings are very long and thin, which makes them efficient. They produce less drag for the amount of lift they generate.
Abstract from : http://www.4p8.com/eric.brasseur/glider2.html
Why such wide wings?
An airplane keeps flying because its wings constantly blow air downwards. Just like an helicopter does. Yet an helicopter stays at the same place and moves its wings/propeller through the air by rotating them, while an airplane passes its wing through the air by moving itself through the air. The air that an airplane's wings blow downwards does not travel all the way towards the ground. Instead it rotates sideways and you get two slow and huge turbulences/vortexes; one generated by each wing. This is kind of an invisible trail left by the airplane. Now, according to the laws of mechanics, the more volume of air the wings make rotate every second, the slower that air will move and the less energy the airplane will have to spend. That's one reason why gliders have very long wings; in order to span over a huge volume of air. The least the air moves once a glider went through it, the better that glider's yield is.
Check out this solar powered glider : http://www.dfrc.nasa.gov/gallery/photo/Helios/HTML/ED03-0152-4.html
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