Aerodynamic test specimens are secured within a wind tunnel by a variety of mounting structures. Longer mounts, known as stings, reach downstream to reduce aerodynamic disturbances near the test specimen. Traditional stings are made from steels or other lossless materials. Consequently, structural modes of the fixture/model system have little or no inherent damping. For larger, more massive models, aerodynamic loads may excite these structures into resonance. Even if such physical conditions do not exist, test regimes may require changes in the model attitude. Inertial impulses between each movement result in long ringdown times between new positions. The presence of either condition can have detrimental effects on data acquisition and may dramatically increase the duration of the test, at increased cost. This project examined several passive damping methods for existing sting hardware. An effective damper design was tested under realistic conditions in a low speed wind tunnel.
Specifications dictated that our design could not deviate from existing hardware dimensions. Furthermore, a final design would need to be retrofitable to a number of different stings. These requirements presented the greatest challenge as all damping had to be implemented within the relatively small physical envelope. Several damping techniques were investigated including particle damping and a multilayer composite sting with higher inherent loss. Lower than expected tip displacements precluded the use of the particle damper while the prohibitive cost of manufacturing a composite structure ruled out its use under the research budget. Given the design constraints, a viscoelastic hub damper mounted at the base of the sting was the most efficient design.
The design incorporated a central flexure with viscoelastic shear lap dampers located at a moment arm away from the central hub. The flexure provided the necessary compliance that targeted a significant amount of strain energy into the viscoelastic material. It is there that the damping was added to the system.
Theoretical conditions can be difficult to achieve in any hardware implementation and this assembly was no different. Critical damping performance required a solid mechanical bond between the shear laps and the viscoelastic. Furthermore, adherence of this bond layer to the surrounding structure would shunt the strain path around the viscoelastic, reducing the effective loss. A significant effort was required to address these challenges and we developed careful assembly techniques to minimize these errors and optimize the system performance.
Laboratory testing with a full sized sting verified a system damping of 2.09\% ($\eta$). Subsequent wind tunnel testing illustrated the enhanced dynamic performance in an operational environment. A simple airfoil produced the necessary aerodynamic excitation. We collected ringdown data for several pitch angle changes with and without the hub damper. These steps were intended to excite the fundamental mode of the structure. The step-test regimes were repeated for various wind speeds. Limitations in the wind tunnel test setup, specifically, mechanical play and flexibility in the wind tunnel mount, led to performance that was less than might have been expected from the fixed base testing.
Utilizing the shear lap design shows tremendous promise, especially if deviations from the initial sting form factor are allowed. Increasing the moment arm of the shear lap will allow a greater amount of modal strain energy to be transferred to the viscoelastic, thereby increasing total damping.