The Effect of Rotor Tip Markings on Judgements of Rotor Sweep Extent

In this study we investigated the possibility of increasing the visibility of helicopter rotor tips when viewed from the underside. In an experimental study we used a model of a rotor blade spinning at a fast enough rate that it appeared as a transparent disk. The tips of the blades were painted fluorescent red-orange, and were observed under three different lighting conditions. In one condition the tips rotated against a bright background with no additional lighting. In a second condition the tips were lit from the underside by a spotlight to produce the same contrast that would be produced by a searchlight in daytime. In the control condition a darker blue background was used, so that the orange rotor tips were highly visible. Six observers used a computer-controlled pointer that they placed directly under the far edge of the visual disk produced by the spinning rotors. Overall, the results indicated that the ability to judge the distance of a spinning rotor is not strongly affected by lighting conditions. It therefore appears that the problem of judging the distance of rapidly spinning helicopter rotors is unlikely to be solved by enhancing their visibility.