analysis only. The project staff is a talented group of investigators with expertise in this area, and they are focused on a topic of significance to the community—performance—but at this early stage, in some respects, they are playing catch-up to others in the field. This work would benefit from the identification of a path that will distinguish this effort from others in this area. One opportunity that does exist is to seek collaboration in the upcoming NASA Ames Research Center testing on Boeing and Sikorsky active rotors (which may be in advance of the next VTD active twist rotor test in late 2009) and to see if those data can be the catalyst to the VTD work.

A second example is the structural dynamics for rotorcraft activity, which is an effort to address one building-block component of comprehensive rotorcraft analysis—that being the dynamic modeling of redundant, nonlinear airframe structures. In reality, this is one element of a very broad and robust rotorcraft community of existing and past efforts along these lines. The focus on addressing fastener/bolted joints has been the subject of prior work (e.g., by the National Rotorcraft Technology Center and Rotorcraft Industry Technology Association). This effort will benefit from detailed discussions within the structural dynamics community to best define an approach that can leverage past work.

A third example has to do with mesoscale flapping wing structures. The scope of the VTD work, to design and construct mesoscale flapping wings capable of generating forces similar to those generated by a fruit fly, is impressive and applauded. The study combines experimental and modeling work on millimeter-scale flapping wings. The modeling so far is limited to two-dimensional modeling using corrections (history integrals, added mass) to the quasi-steady formulas for lift and drag. The Reynolds number is larger than 1 but still low so that viscous effects are significant. The Strouhal number for the flapping action is of order 1 so that the unsteady effects are significant. Because of the values of these similarity parameters, classical wing theory does not apply, as the investigators recognize. A CFD solution rather than the modeling with corrections should be pursued. Clear objectives and a systematic approach could result in a considerable contribution to the greater community, because this is a research area of very broad activity with support coming from a variety of agencies and organizations. Well-defined goals and specific concentrations will help to ensure success in this context.

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