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Why does a higher aspect ratio generate less drag? (Don't understand, jepp book doesn't go into detail)
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The wing tip vortice is caused by the higher pressure air below the wing spilling over the wing tip to the lower pressure region above the wing. This destroys the pressure differential (lift) at the wing tip.
To see why a high aspect ratio has less drag, we will compare 2 wings of the same total wing area, but of different aspect ratios. Our low aspect ratio wing has a short span and a wide chord. It has a relatively large area at the wing tip that is affected by the wingtip vortices. Since the wing tip vortices destroy the lift created at the wing tip, the wing must be flown at a higher angle of attack to create enough lift. This creates more induced drag.
The high aspect ratio wing has less area concentrated at the wing tip since it is relatively long and narrow. The wingtip vortices affect a smaller area and less lift is lost. This allows the wing to fly at a lower angle of attack compared to the low aspect ratio wing. Since the wing needs less angle of attack to create the same amount of lift, it has less induced drag.
We know that at low speeds induced drag predominates, and as speed increases induced drag is reduced. At higher speeds parasite drag greatly increases. This is why you usually only see high aspect ratio wings on gliders or other low speed aircraft. High aspect ratio wings can also have a weight penalty. It is hard to make a long, thin wing light and strong.
Winglets can be used to increase the efficiency of lower aspect ratio wings, by lessening the effect of lift destroying wing tip vortices. They allow the use of a lower aspect ratio wing for better high speed performance (shorter span = less frontal area = less parasite drag) while helping to reduce induced drag.Tip tanks also have much the same effect.