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Assessment of Wingtip Modifications to Increase the Fuel Efficiency of Air Force Aircraft
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.
INTRODUCTION TO WINGTIP AERODYNAMICS
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
Richard Vogt, Twisted Wing Tip Fin for Airplanes, U.S. Patent 2,576,981, December 1951.
C.D. Cone, Minimum Induced Drag Airfoil Body, U.S. Patent 3,270,988, September 1966.
Richard T. Whitcomb, 1976, “A design approach and selected wind-tunnel results at high subsonic speeds for wing-tip mounted winglets,” NASA TN D-8260.
Ilan Kroo, 2005, “Nonplanar wing concepts for increased aircraft efficiency,” VKI Lecture Series on Innovative Configurations and Advanced Concepts for Future Civil Aircraft, June 6-10.