Structural Locking in a Nastic Actuated Shaped-Changing Beam
This thesis endeavors to develop a new locking method for a twisted morphing wing spar. The conventional wing has to have hinges and a discontinuous surface. These cause air separation that decreases aerodynamic performance. Unlike this old concept, the new airfoil comprises a square cross section spar into the wing blade. Twisting the spar changes the airfoil?s angle of attack to control lifting and thrust force without a discontinuous surface. A nastic actuator generates shear stress for twisting the spar. A thermoplastic polymer locks the twisted shape. Applying heat and solidifying the polymer makes the beam lock into the twisted position even after removing the shear stress. This concept was evaluated by computer simulation and an experiment with a prototype construction. The analysis with 5m long spar shows that +450Pa shear stress generated +2 degrees twist and maximum 1.49MN/m spring constant at the spar tip. This spring constant helps a designer select the locking material, Ultem. The analysis proves that the Ultem film?s shear spring constant is high enough to hold the aluminum spar?s spring back. Physical experiment conditions might differ from computer simulation because environmental limitations might be present. The prototype spar has to be less than 300mm long to fit in an electric oven. Tension made the beam twist and baked it with locking material. When the polymer softened, the beam was taken from the oven and cooled. The solidified locking material held the spar at twisted status. The observation shows no detectable spring back after removing tension. Analytic solution also presents no spring back in twisting the prototype section spar. The FEA of the section spar verifies the physical experiment results. As a normal polymer, the Ultem shows stress relaxation. The load drop affects deceasing elastic modulus. Subsequently, the Ultem is able to lock the twisted spar even after the relaxation.
Cha, Gene (2010). Structural Locking in a Nastic Actuated Shaped-Changing Beam. Master's thesis, Texas A&M University. Available electronically from