Scaling of Performance,Weight,and Actuation of a 2-D Compliant Cellular Frame Structure for a Morphing Wing

Abstract

A typical mission profile for a fixed-wing aircraft consists of several idealized phases: takeoff, climb, cruise, dash, loiter, descent, and landing. Corresponding to each phase of flight and associated weight is a specific wing configuration that yields optimal performance. Conventional aircraft employ a single fixed structure (with movable control surfaces), one that is necessarily a compromise. Morphing structures technology provides the possibility of making large global changes to the wing shape, ideally enabling optimum performance over a larger range of flight conditions. Although structural and actuator weights necessarily increase with morphing capability, overall performance gains can offset these subsystem weight increases with fuel savings or by enabling new aircraft missions. A 2-D compliant cellular truss structure was designed to replace the fixed internal structure of a wing; this structure is able to achieve large changes in span and thus planform area and aspect ratio. The capability of this design was determined for aircraft of varying scale. At RC model scale, approximately 0.5—5 kg (1—10 lb), the wing structure is capable of an 85% decrease in the planform area with a structural weight of 2.9% of the gross weight. As the gross weight of the aircraft increases, the achievable span reduction decreases while the structural weight fraction increases. For 50 and 500 kg (100 and 1000 lb) aircraft, the decrease in planform area is 74% and 48%, respectively. At 50 kg (100 lb), the wing structure comprises 7.4% of the gross weight, while at 500 kg (1000 lb), this increases to 8.9%. The difference between the weight of the morphing structure and a similar passive structure is a weight penalty: the trade-off required for the ability to morph. Actuation systems, including a single actuator and a system of parallel actuators, were also considered for their ability to deform the structure. The weight fraction of the actuators required to deform the structure was found to increase with increasing gross weight. A system of parallel actuators was found to have a significant advantage over a single actuator, especially at higher gross weights. The benefit of morphing increases with gross weight but structural morphing capability decreases with gross weight. This suggests, for a given structural paradigm, that there is a gross weight for which morphing is most advantageous and practical.