In order for a model of a UAV to be suitable for evaluating the performance of the real UAV, the two versions of the UAV must have dynamic similarity in the sense that there is a match between the groups of dimensionless variables (“pi groups”) of the two versions. Dynamic similarity is shown by using the Buckingham pi theorem to replace the dimensional physical parameters with dimensionless products and ratios. Heretofore, often it has been seen that once a scale model is developed, either (1) it is necessary to modify the fullsize vehicle to match the pi groups of the scale model, or (2) the pi groups of the scale model are such as to afford similarity to only a certain subset of UAVs. The concern in the present research is to overcome these limitations, enabling development of a model that can afford the full range of dynamic similarities for a general UAV.

Thus far, the research has included investigations of the scaling of flight and sensor dynamics between two surrogate vehicles and a basic UAV that could be used to search wide areas. One surrogate vehicle was a 1/20-scale model of a mammoth dump truck; the other was a Chinese-built jet airplane used for training pilots (see figure). The Buckingham pi theorem was applied to the governing equations of the UAV/sensor system under simplifying assumptions (ignoring wind, friction, and aerodynamic loads) flying over a rectangular ground area. The vehicle dynamics and sensor behavior were studied to develop seven pi groups. Computational simulations of the three vehicles were performed and the results of the simulations were compared to study the possibilities and effects of scaling.

This work was done by Jeevani I. Abeygoonewardene of the Air Force Institute of Technology for the Air Force Research Laboratory. For more information, download the Technical Support Package (free white paper) at www.defensetechbriefs.com/tsp under the Physical Sciences category. AFRL-0036

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