Theoretical and measured airflow about the twin otter wing

Abstract

The NAE Twin Otter atmospheric research aircraft carries instruments mounted under the wings to count and image cloud and precipitation particles for cloud physics studies. As part of an investigation on the influence of the aircraft presence on these measurements, airflow velocities about the wing have been calculated. These theoretical results have been compared with in-flight measurements taken by pressure probes mounted in a cannister suspended from a long and a short pylon below the wing at two spanwise locations. The Twin Otter wing has been represented by a Joukowski aerofoil of the same maximum thickness but with a cusped trailing edge. The wing camber has been neglected but aspect ratio and planform effects have been included. The effect of the cannister was included in the comparison of theory with experiment. Use of the theory aft of the quarter chord will require experimental verification. Both experimental and theoretical results for the local flow velo city ratio are linear functions of aircraft lift coefficient (CL) in the range of CL between 0.35 to 1.06. The worst discrepancy between theory and experiment in the CL range of 0.4 to 0.6 is -2.8% to 1.1%. The results for the inboard/long pylon and outboard/short pylon have considerably less deviation. The theory has been experimentally verified to the above accuracy and can be used to describe the spatial variation of the flow velocity ahead of the wing. Applications of the theory to the study of aircraft floweffects on cloud physics measurements can proceed with a quantifiable level of confidence.