Several very practical obstacles presently keeping both riblets and hybrid laminar flow control (HLFC) from inservice applications have been reviewed. For riblets, proving economic viability is a major challenge that will require an order of magnitude reduction in the time to apply them relative to the best application times demonstrated to date. And, this reduced application time must be achievable by non-specialized workers. In addition, the airlines must be convinced that riblets will retain their full performance benefit for five years with minimal additional attention required. Then, to make the total riblet drag reduction worth pursuing, it will be necessary that riblets can be used on wings and talls without adversely impacting flight characteristics which can effect operating economics. Practical challenges needing to be overcome for HLFC include the definition/development of a practical/effective wing ice protection system compatible with HLFC that does not adversely impact performance characteristics, and devising a means to overcome the performance deficiencies and risk incurred by the requirement to use a Krueger-type leading-edge device (in lieu of a slat). Further, it will be necessary to demonstrate that HLFC can be effectively used in the wing root area and in areas on the wing influenced by the installation of VHBPR engines, plus wind-tunnel-testing techniques must be developed for HLFC that reliably depict full-scale aerodynamic characteristics, and economics must be conclusively demonstrated to the airlines. The final challenge for HLFC is to overcome all of the foregoing obstacles without having to resort to a full-up prototype program prior to launching a production aircraft program.


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