Can quadcopters save energy by flying in formation?
Yes, drag can be reduced by formation flying.

 


Geese using a low drag "V" formation. The authors of a 2001 Nature article stated that pelicans that fly alone beat their wings more frequently and have higher heart rates than those that fly in formation. It follows that birds that fly in formation glide more often and reduce energy expenditure (Weimerskirch, 2001). 




Any flying object (plane, bird, helicopter, etc) that creates a down-wash to create lift must also produce an opposite and equal up-wash. A trailing aircraft positioned properly can ride this "wave".



Research aircraft flying a test point (low drag) for the Autonomous Formation Flight project over California's Mojave Desert. From the NASA Dryden Flight Research Center Web site. 



The goal of the Autonomous Formation Flight project was to demonstrate 10 percent fuel savings of the trailing aircraft. The project extended the symbiotic relationship of migrating formations of birds. In December 2001, an F/A-18 flying in the wingtip vortex behind another F/A-18 exhibited a 14-percent fuel savings. The project ended in late 2001