Comprehensive experimental and analytical studies have been conducted to assess the integration of a force balance in a low speed wind tunnel and to measure the performance of a new type of aeroplane winglet called a winggrid. The winggrid is an extension to a wing that utilises a cascade of wings to decrease the induced drag on a wing (La Roche, 2005).
Calibration tests were performed on the wind tunnel force balance that showed inaccuracy in the measurement of drag forces. The inaccuracy was attributed to the low sensitivity of the drag-measuring bar and the associated difficulty in the calibration of the drag-measuring bar.
The investigation into the force balance revealed a number of problems with the integration of the force balance into the wind tunnel. The problems were based on mechanical interference and vibrations, affecting the signal to noise ratio of the force balance. A set of solutions which show how the force balance can be integrated into the wind tunnel to produce the most accurate results was produced. Methods for improving the accuracy of the force balance, for further wind tunnel testing of model wings, were also produced in this investigation.
Wind tunnel testing revealed low scatter and good repeatability in the data obtained from the force balance. The absolute uncertainty, however, was found to be fairly large, up to 20%. The low scatter and good repeatability enabled a comparison to be made of the different models tested in the wind tunnel. The high absolute uncertainty prevented an accurate comparison of the experimental and theoretical performance of the wind tunnel models.
Wind tunnel testing of the winggrid revealed the winggrid had a more rectangular lift distribution than the conventional wing, but the drag on the winggrid model wing was greater than the drag on the conventional model wing. It was not determined whether the winggrid drag increase was parasitic or induced. Several winggrid parameters still need to be investigated to enable a more conclusive determination of its performance.