Backup Electric Motor

[Fredbob711]

For small aircraft. This was designed for aircraft up to 1,650 lbs, but can't imagine it would be a stretch to extend it to heavier aircraft.

Won't get you to your destination but might get you to a safe place to land.

http://gizmodo.com/tiny-electric-backup-engine-for-planes-kicks-in-when-al-1710599149

 

[Jim Logajan]

I'd have just put an extra electric motor in the tail to form a push/pull center line thrust tandem system like a Cessna Skymaster. Electric motors generally don't need any special cooling arrangements when used for short periods during takeoff, so you'd get STOL benefit along with redundancy from a completely different prime mover from the IC engine up front and the advantage of two props vs one in case a prop decides to depart the plane.

During cruise you could continue to run the rear electric motor at a power level the front engine's alternator could supply to avoid windmill drag. Or feather or just lock the rear prop.

EDIT: Oh yeah, forgot to mention that on landing you could in theory run the rear electric motor backward for additional braking action. Not something you'd want to do during any other portion of flight, of course!