Determining Aerodynamic Loads Based on Optical Deformation Measurements

Abstract

A preliminary study is described for determining aerodynamic loads based on optical elastic deformation measurementsusing a videogrammetricsystem. Data reduction methods are developed and used to extract the normal force and pitching moment from beam deformation data. The axial force is obtained by measuring the axial translational motion of a movable shaft in a spring/bearing device. Proof-of-concept calibration experiments are conducted to assess the feasibility of the optical technique for measuring aerodynamic loads. The uncertainties in optical force and moment measurements are discussed. I. Introduction I NTERNAL strain gauge balances have been used for years as a standard technique for measuring the integrated aerodynamic forces and moments on models in wind tunnels. A variety of internal strain gauge balances have been developed, and the technical aspects of various balances have been studied in detail. 1 Generally speaking, the structure of an internal strain gauge balance is complicated, and the cost of fabrication is high. This paper presents an exploratory study for remotely measuring aerodynamic loads using a videogrammetic system. Unlike strain gauges, this method optically measures beam deformation to determine the normal force and pitching moment. The axial force is obtained by measuring the translational motion of a movable shaft in a spring/bearing device. Mathematical models for data reduction are developed to extract the aerodynamic forces and moments from the deformation data. Uncertainty analysis is given to evaluate the contributions from the elemental error sources and correlation terms. At this stage, the normal force, pitching moment, and axial force are the primary quantities to be determined. In principle, the side force, rolling moment, and yawing moment can be determined in a similar manner. Proof-of-concept laboratory experiments have been conducted to validate the proposed methodology for measuring the aerodynamic loads. Potentially, this optical method can be used as an alternative to strain gauge balances. In addition, the technique described in this paper can be integrated with optical model attitude and deformation measurement techniques. 2;3