An experiment has been performed to demonstrate the use of a new technique for measuring the unsteady longitudinal derivatives of slender wings at low speeds. The technique used a stationary fluid medium, water, and a model wing was towed through the water with its flight path constrained to follow a fixed cam rail. The normal force and pitching moment on the wing were measured by strain gauges fixed to its sting support. The new technique had the following advantages over traditional oscillatory methods: (i) The ratio of aerodynamic forces to inertia forces was high because water was used as the fluid. (ii) It was able to discriminate the w, q and theta derivatives. (iii) Any non-linearity in the derivatives could be studied without interference of transient effects. The longitudinal derivatives zw, zq, ztheta , mw, mq, mtheta for the AGARD model G planform were measured but there was a good deal of experimental scatter, particularly in the z derivatives. The m derivatives compared well with results from oscillatory tests but the z derivative did not compare so well. A correlation was obtained between the theoretical virtual mass and the transient behaviour of the normal force at the start of the plunging and constant q manoeuvres. The derivatives which cannot normally be measured independently, zq and m q, were measured to a better degree of accuracy than the other derivatives. Photographs of the paths of the vortex cores were obtained using the hydrogen bubble technique. The photographs were not of sufficient accuracy to enable a correlation to be made with the strain gauge measurements but a difference in the position of the vortices for positive and negative pitching rates was observed. It was concluded that the new technique could be successfully applied to the measurement of the derivatives for a slender wing but that modifications would be necessary to improve the precision of the measurements.
|Date of Award||1975|