Here is a way a hybrid piston/electric configuration could make sense, increasing efficiency significantly but not in a revolutionary way: This works for very clean, well designed airframes with high aspect ratios. It could apply from a Lancair-like GA aircraft up to mid-sized commuter aircraft whose missions cannot take advantage of high-altitude turbine engine efficiencies. As an example, for the Lancair you might replace the 350 HP piston engine with, say, a coupled 200 HP piston+150 HP electric motor, and add a high power density battery sufficient to power the electric motor for, say , 20-25 minutes. You use the electric motor for pre-flight ground opps and taxiing, both for TO/Climb, the piston engine(ideally turbo-normalized) for cruise. The electric motor could probably be designed to operate as a generator as well, opening the option of slowly recharging the battery during cruise and/or recovering energy during the descent, possibly arriving with enough juice to refuel and repeat. The commuter could have the added advantage of swappable battery packs allowing arrival with a nearly discharged battery but still a quick turnaround.