If two or more quadcopters are aligned, would the effect of drag due to the lead quadcopter's slipstream lead to measurable energy savings? And if so, how would one determine the optimal distance between the quadcopters?
My intuition would be no, as the rotor wake will be choppy and downward flowing.
Unlike slipstreaming in ground vehicles which reduces body drag, bird and some aircraft formations rely on the effect of wing-tip vortices to gain lift, hence reducing lift induced drag. Not having wings†, 'copters lack wing-tip vortices, so would not experience this effect, but instead the flow from the rotors will probably cause a following 'copter to lose lift.
†there probably will be vortices shed from the rotors if not ducted, but they will process with the rotor tips so not be in a stable relative position that can be flown through.