Measuring Wing Profile Drag using an Integrating Wake Rake

Abstract

An integrating wake rake concept is discussed here as very suitable for in-flight profile-drag measurements on gliders. The experimental setup is in essence the same as used with other integrating wake rakes, which uses a series of Pitot tubes that feed into a single manifold to measure the averaged total pressure deficit in the wake of an airfoil. The difference compared to those conventional integrating wake rakes lies in the post-measurement data reduction. In an iterative approach, the measured pressures are corrected in order to account for the unknown shape
of the wake-deficit region and other unidentified parameters. With this approach, the possible accuracies in profile drag are similar to those obtained using a traversing wake survey system. Thus, in contrast with conventional wake rakes, it is possible to measure accurate absolute drags of an actual section of a wing in flight that can be compared with previous wind-tunnel results and/or theoretical predictions. The general theory of this drag measurement device is discussed in this paper. Furthermore, the experimental apparatus developed at Saint Louis University is introduced. Preliminary results of a comparison study demonstrate the capability of the system and the approach to measure profile drag during flight.