Outwash Flow Behavior of Tandem Rotors near Ground Effect

In this study, multipurpose testing equipment with a sub-scale model of a specific tandem rotor helicopter constructed to conduct a number of experiments to accurate understanding from the tandem rotor's outwash in ground effect. The experiments conducted as measuring rakes were positioned at two distances equal to 1.5 R and 3R from the rotor(s). Unlike the experiments that have been performed in wind tunnels or in special hover chambers, these experiments were performed in an open environment with fewer side walls effects. The results show that when the single rotor operates in a fixed altitude, and the blades tip velocity of 0.2M, the outwash velocities reduces as the flow moving away and vice versa, but for tandem rotors, increasing the rack distance from the model does not have a noticeable effect on the average values of the flow velocity. A comparison of the results of these measurements with the CH-47D helicopter outwash patterns confirms the accuracy of the obtained patterns and showed that the overlap between the rotors increases the velocity values and causes to the occurrence of maximum outwash velocity at lower altitudes. No overlap between the tandem rotors makes the outwash flow pattern of each of them similar to a single rotor. Increase in ground effect as the height of the rotor(s) decreases to 1R, changes the flow pattern in the forward and aft of the model helicopter. In this altitude, unlike their operation in altitude of 2R, the outwash flow increases when moving away from the rotor(s).