The tip of a wing generates drag due to the high-pressure air beneath the wing rolling up and around the tip into the area of low pressure on top of the wing. This persistence toward equilibrium between two unequal pressures produces wing-tip vortices and is considered one type of parasitic drag. One way to overcome this inefficiency is to build a barricade between the two pressure gradients. This can be accomplished in three ways. The old fashioned way is to simply increase the wing span a bit. All the old birds did this by simply putting a rounded tip on the wing. Bush pilots flying high wing aircraft accomplish the same thing by putting “droopy” tips on the wings that hang down. The more modern airplanes opt for the "cool” looking winglets that point up. All three approaches accomplish the same effect. They each reduce parasitic drag thus increasing power available for speed and performance. The main reason for winglets being that an increase of 15 foot per wing on say the new “stretch” 747 would cause considerable parking and hangering problems and a 15 foot droopy tip would wreck havoc on runway lighting as well as ground equipment. Logic would dictate going up with any additional wing tip. Just makes sense and of course it does look “way cool.” You can bet your bootstraps that the aeronautical engineers didn’t forget anything. Most people just don’t realize that tips are tips and bigger just isn’t always better. An additional 15-foot of wingtip per wing induces additional parasitic drag by just being there and one form of drag must be weighed against another.