did not attain the potential savings suggested by the theory. This was partly due to simplistic design, which often included low-aspect-ratio, untwisted, flat-plate airfoils. Recognition of the importance of winglet location, twist, and aspect ratio was clear in the patent of Vogt in 1951⁸ and in a variety of other nonplanar wingtip geometries studied and patented by Cone.⁹ In the early 1970s, Whitcomb¹⁰ of the National Aeronautics and Space Administration (NASA) defined and tested high-aspect-ratio, carefully designed nonplanar wingtips, termed “winglets,” which were soon to appear on numerous aircraft, including Rutan’s VariEze in 1975 and the Learjet 28/29 in 1977. The winglet of the Boeing 747-400 has a much lower dihedral angle than the Whitcomb winglet, and since that time, numerous vertical, canted, and horizontal wingtip extensions have been put into commercial and military service, as shown in Figure 2-1.

Much of the drag of an aircraft is related to the lift generated by its wing. To create this lift, the wing pushes downward on the air it encounters and leaves behind a wake with a complex field of velocities. This air behind the wing moves downward then outward, while the air outboard of the wing tips moves upward, then inward, forming two large vortices, as shown in Figure 2-2.

The energy required to create this wake is reflected in the airplane’s “induced” or “vortex” drag. For most aircraft, induced drag constitutes a large fraction, typically 40 percent, of cruise drag. During takeoff, induced drag is even more significant, typically accounting for 80-90 percent of the aircraft’s climb drag. And while takeoff constitutes only a short portion of the flight, changes in aircraft performance at these conditions influence the overall design and so have an indirect, but powerful, effect on the aircraft’s cruise performance. Consequently, concepts that reduce induced drag can have significant effects on fuel consumption.¹¹