Design of an Internal Balance for a Wind Tunnel Ruddevator Model

A wind tunnel balance was designed to measure the aerodynamic loads on a ruddevator system during tests in the WSU 7′ x 10′ wind tunnel. The design goal for the system was to produce a balance that could accurately uncouple the components of the aerodynamic loads such that the lift, pitching moment and rolling moment relative to the balance reference point could be measured separately. This was a challenge because of the dimensional constraints and because of the accuracy requirement for a large range of possible load distributions during the tests. A simple flat plate idealization of the actual balance system was designed for laboratory structural tests to investigate some possible balance design strategies prior to the construction of the actual balance. Specific parameters of interests during these tests were that of choice of material, level of response of the structure to simulated service loads and strain gage selection and circuit design. These laboratory tests were routine verification of most of the parameters of the design with the exception of the design of the strain gage configuration and circuit design to produce optimum accuracy and uncoupled load information. This turned out to be a revealing experience and fortunate decision in that many of the subtle problems associated with the design of the strain gage system to measure the loads were encountered and rectified during these tests. Finite element analyses were also conducted for both the flat plate model and the actual balance design to verify the measured results over a broader range of parameters and to simulate candidate structural redesigns of the balance as well as redesigns of the strain gage configuration and circuitry. Structural tests were conducted with the final balance design to confirm the level of accuracy of the measured load data and to calibrate the balance for subsequent wind tunnel tests.