Rotary drums, installed with longitudinal flights are often used to dry/cool granular materials in large quantities. Performance of such drums greatly depends on the uniform distribution of the particles over the drum cross section, which is attained by an optimal design and allocation of the flights. In this study, a mathematical model has been developed for the rectangular flight to optimize the total particle surface area which is a function of the cascading rate and falling time of the particles. The falling time in turn is a function of curtain height and can be estimated by geometrical analysis. Influence of the number of flights and the flight length ratios has been studied. It was observed that, as the flight length ratio increased the cascading rate decreased during the initial discharge, but increased rapidly at higher discharge points resulting in a bulk movement of the solids, which also determines the density of the curtains. Experiments were carried out to validate the developed model with a drum of 500 mm in diameter and 150 mm in length. The experiments were performed with different flight profiles and flight numbers (12 and 18). Good agreement was found between the experiments and the model predictions. (C) 2013 Elsevier Ltd. All rights reserved.