  For the human operator, it is often easier and faster to catch a small size
quadrotor right in the midair instead of landing it on a surface. However,
interaction strategies for such cases have not yet been considered properly,
especially when more than one drone has to be landed at the same time. In this
paper, we propose a novel interaction strategy to land multiple robots on the
human hands using vibrotactile feedback. We developed a wearable tactile
display that is activated by the intensity of light emitted from an LED ring on
the bottom of the quadcopter. We conducted experiments, where participants were
asked to adjust the position of the palm to land one or two
vertically-descending drones with different landing speeds, by having only
visual feedback, only tactile feedback or visual-tactile feedback. We conducted
statistical analysis of the drone landing positions, landing pad and human head
trajectories. Two-way ANOVA showed a statistically significant difference
between the feedback conditions. Experimental analysis proved that with an
increasing number of drones, tactile feedback plays a more important role in
accurate hand positioning and operator's convenience. The most precise landing
of one and two drones was achieved with the combination of tactile and visual
feedback.
