Recently, smoothly-deforming aircraft structures have been investigated for their ability to adapt to varying flight conditions. Researchers aim to achieve large changes in the shape of the wings: area changes of up to 50% and aspect-ratio changes of up to 200% are being pursued. The research described in this paper aims to develop a structural concept capable of achieving continuous stable deformations over a large range of aircraft shapes. The basic concept underlying the approach is a compliant cellular truss, with tendons used as active elements. The truss members of the unit cell are connected through compliant joints such that only modest bending moments may be transmitted from one member to another. Actuation is achieved by pulling on one set of cables while releasing another set. The tendonactuated compliant truss can be made to behave locally, and temporarily, as a nearmechanism, by releasing appropriate cables. As a result, in the absence of aerodynamic forces, the structure can be morphed using relatively low forces. The cables are reeled in or released in a controlled manner while the structure is loaded, hence, the stability of the structure can be maintained in any intermediate position. Highly-distributed actuation also enables the simultaneous achievement of global shape changes as the accumulation of local ones, while the use of compliant joints rather than true rotating joints eliminates binding as a significant concern. A six-noded octahedral cell with diagonal tendon actuation is developed for a bending type deformation in the wing. Initial cell geometry is determined by "strain matching" to the local morphing deformation required by the application. A finite element analysis is performed on a wing made of these unit cells and sized for a representative UAV weighing 3000 lbs. The areas of the individual truss members are sized so that they don't fail or buckle under the air loads, while deflection at the wing tip is reduced. The octahedral unit cell is capable of achieving smooth deformations of the truss structure. The cell size is dictated by the available space and the morphing strain. The cell sizes are reasonable for strains on the order of 10% to 15% and get smaller for larger strains. Additional cell shapes are being investigated for larger area changes through a process of topology optimization using genetic algorithms. Numerous other technical challenges remain, including the details of actuation and a robust skin. Nomenclature B= kinematic matrix d = vector of displacement of joints e = vector of elongation of members l = length of truss member SR = Stress Ratio, allowable stress/actual stress,

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The public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching eisting data sources, gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, Including suggestions for reducing the burden, to the Department of Defense, Executive Service Directorate (0704-0188). Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to any penalty for failing to comply with a collection of information if It does not display a currently valid OMB control number. PLEASE DO NOT RETURN YOUR PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES) 8. PERFORMING ORGANIZATION REPORT NUMBERThe Pennsylvania State University 229 Hammond AERSP-GAL-0228 University Park, PA 16802 SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES) 10. SPONSORIMONITOR'S ACRONYM(S)Department of the Air Force AFOSR Air Force Office of Scientific Research 11. SPONSORIMONITOR'S REPORT 875 North Randolph Street NUMBER(S) Arlington, VA 22203-1768 DISTRIBUTION/AVAILABILITY STATEMENTUnclassified -distribution is unlimited AFRL-SR-AR-TR-08-0164 13. SUPPLEMENTARY NOTES 14. ABSTRACT Reconfigurable ("morphing") aircraft offer the potential for efficient flight over a large range of airspeeds. The structural concept addressed herein is a tendon-actuated cellular compliant frame, with a flexible skin. 3-D cellular structures can perform well, but the need for active tendons to also carry passive lift loads limits their feasibility at larger scales. 2-D cellular structures can significantly change wing area and span, with lower actuation forces, and a system of parallel actuators is significantly lighter than a single actuator, especially at higher gross weights. Composite skins with cellular cores and flexible face sheets show promise for low-force actuation with reasonable lateral stiffness, while contact-aided cores offer additional benefits, including stress relief and increased lateral stiffness. Finally, while the potential benefits of morphing increase with aircraft gross weight, structural morphing capability decreases; this is accompanied by increasing structural and actuation weight fractions. This suggests that, for a given structural paradigm, there is a gross weight at which smooth morphing is most advantageous and practical. Continued research is needed to address the many challenges that remain before the promise of smoothly morphing aircraft can be realized. SUBJECT TERMS