Remaining Technical Challenges

Without doubt, the most important technical issues in the application of dynamically scaled free-flight models are the effects of Reynolds num­ber. Although a few research agencies have attempted to minimize these effects by the use of pressurized wind tunnels, a practical approach to free-flight testing without concern for Reynolds number effects has not been identified.

In the author’s opinion, the challenge of eliminating Reynolds num­ber effects in spin studies is worthy of an investigation. In particular, the research community should seriously examine the possibilities of combining recent advances in cryogenic wind tunnel technology, magnetic suspension systems, and other relevant fields in a feasibility study of free-spinning tests at full-scale values of Reynolds number. The obvious issues of cost, operational efficiencies, and value added versus today’s testing would be critical factors in the study, although one would hope that the operational experiences gained in the U. S. and Europe with cryogenic tunnels in recent years might provide some optimism for success.

Other approaches to analyzing and correcting for Reynolds num­ber effects might involve the application of computational fluid dynam­ics (CFD) methods. Although applications of CFD methods to dynamic stability and control issues are in their infancy, one can visualize their use in evaluating the impact of Reynolds number on critical phenom­ena such as the effect of fuselage cross-sectional shape on spin damping.

In summary, the next major breakthroughs in dynamic free-flight model technology should come in the area of improving the prediction of Reynolds number effects. However, to make advances toward this goal will require programmatic commitments similar to the ones made during the past 80 years for the continued support of model testing in the specialty areas discussed herein.