Making a good paper airplane is tough. Drop a simple sheet of paper and it will tumble and flip its way to the floor instead of gliding. The folds of a proper paper airplane add weight in just the right spots to stabilize its flight and let it glide smoothly through the air. To better understand what makes paper fly, researchers looked at how sheets of paper flew when weighted (with metallic tape) in different spots.
An unweighted sheet of paper tumbled end-over-end. Shifting the center-of-mass too far forward or backwards also resulted in tumbles and nosedives. But when the weighting placed the center of mass between these two extremes, there was a sweet spot where the paper glided smoothly. In this situation, the aerodynamic forces on the paper could correct for changes in flight angle; if the paper tilted too far upward, the forces pushed it back down — and vice versa. This ability of the thin wing to self-stabilize is different than most large-scale aircraft, which need tails and other structures to provide stability to the main wing. (Image credit: paper airplane – K. Eliason, paper trajectories – H. Li et al.; research credit: H. Li et al.; via Ars Technica; submitted by Kam-Yung Soh)