TIP PLATES AND TIP BODIES

When a wing is mounted so that it completely bridges the walls of a wind tunnel, no wing tip vortices form. The bound vortex extends from wall to wall and is cut off cleanly. The wing then behaves as if it had infinite aspect ratio and vortex-induced drag is nil. Attempts have been made many times in the past to get the benefits of an infinite aspect ratio by fitting end plates or large, streamlined tip bodies which, on some full-sized aircraft, have been used as fuel tanks. The plates or bodies are intended to act like the tunnel walls and prevent the formation of tip vortices. They do succeed in this to some extent, but to be completely successful they have to be very large. (They also steepen the slope of the lift curve.) This is not achieved without penalty. The tip plates themselves cause form and skin friction drag, and this parasitic drag in total may easily be larger than the saving. This depends very much on the Cl at which the model flies. Since at high speeds vortex drag is small in any case, while form and parasite drag are large, tip plates and bodies have a very bad effect and their use cannot be justified at all for aircraft that commonly fly faster than their speed for best L/D. (See Fig. 4.10). At lower speeds, a model with a low aspect ratio wing may be improved by fitting end plates. The best size is about twice the wing root chord in length, according to tests carried out by A. Raspet of Mississippi State College. Such plates would be a considerable nuisance on any practical model aircraft. If the wing is already of high aspect ratio, large end plates of this type will have little effect. Since the a. r. is already high, the gain in drag from the end plate is proportionately smaller, and parasite drag no less. Tip vulnerability is greater. Plates, or tip bodies, smaller than the recommended size do not inhibit the tip vortices enough to make much difference, and still add their quota of parasite drag. They should be avoided. The few full-sized aircraft and sailplanes which do have tip bodies usually do so for non-aerodynamic reasons: for example, a sailplane wing and aileron end may be protected by a small tip body such as

that of the Blanik two-seater, or the powered aircraft may lack internal space for large – capacity fuel tanks, and the wing tips may be the best place for mounting external ones.