| Abstract | The 30 ft. tunnel is the largest of a new generation of low-speed wind tunnels that have been built or initiated since 1960. The large size is mainly the result of a need to minimize working section wall effects on models. The nature of this problem is illustrated graphically in Figure 1. The nomogram at the bottom of the figure relates aircraft span loading and airspeed to the momentum deflection angle of the wake. It is this latter quantity that mainly determines the severity of tunnel wall effects on a model of any given size. Two of these effects are illustrated in the graph; the approximate value of the wall-induced angle-of-attack correction, and an estimated boundary of the region of complete flow breakdown in the working section. The actual values shown are not to be taken too literally since they have been calculated for the special case of equilibrium level light. Wall effects tend to become more severe when simulating decelerating or descending flight. The figure illustrates that for many V/STOL flight cases, the permissible model span may be limited to, say, 30 or 40 percent of the tunnel height. Because of the complexity of the models required for this type of wind tunnel work, and also to obtain reasonable Reynolds numbers, their span generally will be of the order of 10 feet or more. Hence the need for large test section dimensions. On the other hand, the figure also indicates that large-span models are permissible for typical STOL aircraft of modest span loading. |
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