I have watched the discussion about winglets and thought a little information might be helpful.
The following was scanned from the textbook Aircraft Design: A Conceptual Approach by Daniel P. Raymer, American Institute of Aeronautics and Astronautics and run through an OCR program: Wing Tips Wing-tip shape has two effects upon subsonic aerodynamic performance. The tip shape affects the aircraft wetted area, but only to a small extent. A far more important effect is the influence the tip shape has upon the lateral spacing of the tip vortices. This is largely determined by the ease with which the higher-pressure air on the bottom of the wing can "escape" around the tip to the top of the wing. A smoothly-rounded tip (when seen nose-on) easily permits the air to flow around the tip. A tip with a sharp edge (when seen nose-on) makes it more difficult, thus reducing the induced drag. Most of the new low-drag wing tips use some form of sharp edge. In fact, even a simple cut-off tip offers less drag than a rounded-off tip, due to the sharp edges where the upper and lower surfaces end. (Fig. 4.27). The most widely used low-drag wing tip is the Hoerner wingtip (developed by S. Hoerner, Ref. 8). This is a sharp-edged wing tip with the upper surface continuing the upper surface of the wing. The lower surface is "undercut" and canted approximately 30 deg to the horizontal. The lower surface may also be "undercambered" (i.e., concave). The "drooped" and "upswept" wing tips are similar to the Hoerner wing tip except that the tip is curved upwards or downwards to increase the effective span without increasing the actual span. This effect is similar to that employed by endplates, as discussed below. The sweep of the wing tip also affects the drag. The tip vortex tends to be located approximately at the trailing-edge of the wing tip, so an aft-swept wing tip, with a greater trailing edge span, tends to have lower drag. However, the aft-swept wing tip tends to increase the wing torsional loads. A cut-off, forward-swept wing tip is sometimes used for supersonic aircraft. The tip is cut off at an angle equal to the supersonic Mach-cone angle, because the area of the Wing within the shock cone formed at the wing tip will contribute little to the lift. Also, this tip shape will reduce the torsional loads applied to the wing. The F-15 fighter uses such a cut-off tip for both wings and horizontal tails. Induced drag is caused by the higher-pressure air at the bottom of the wing escaping around the wing tip to the top of the wing. An obvious way to prevent this would be to mount a vertical plate at the wing tip. The endplate effect has been known almost since the dawn of flight, but has been seen rarely. The wetted area of the endplate itself creates drag. Also, an endplated wing has an effective span increase of only about 80% of the actual span increase caused by adding the endplates' height to the wing span. However, endplates can be useful when span must be limited. An advanced version of the endplate can offer lower drag than an equal area increase in wing span. The "winglet," designed by NASA's R. Whitcomb, gets an additional drag reduction by using the energy available in the tip vortex. The winglet is cambered and twisted so that the rotating vortex flow at the wing tip creates a lift force on the winglet that has a forward component. This forward lift component acts as a "negative" drag, reducing the total wing drag. A properly designed winglet can potentially provide an effective span increase up to double that bought by adding the winglets' height to the wing span. Winglets provide the greatest benefit when the wing tip vortex is strong, so a low-aspect-ratio wing will see more advantage from the use of winglets than an already-efficient high-aspect-ratio wing. One problem with winglets is that they add weight behind the elastic axis of the wing, which can aggravate flutter tendencies. Also, the twist and camber of a winglet must be optimized for one velocity. At other than design speed, the winglet will provide less benefit. For these and other reasons, winglets tend to be used more as add-on devices for existing wings requiring a little more efficiency without major redesign. When an all-new wing is being designed it is usually better to rely upon increased aspect ratio to improve aerody~iamic efficiency. This is not always true so a trade study should be conducted sometime during the conceptual design effort. Steve Eberhart mailto:[email protected]
