The purpose of this study was to obtain extensive experimental data on the effect of skin-friction drag by suspending solid particles in air. The investigation was divided into two parts: The first study consisted of obtaining extensive experimental data on the pressure gradients (which are proportional to skin friction) encountered in circular pipes when a mixture of air with solid particles suspended in it flows through the pipe. The second part deals with the skin friction as measured on a flat plate placed flush with the floor of a wind tunnel having a dusty sub-layer. These studies involved a large range of Reynolds numbers, particle sizes, and weight-flow ratios. A theory is also presented to try and explain some of the complex flow phenomena encountered in a two-phase flow system.