Agreed. Howver, as DonEstenan points out, the price (and even weight per HP) of these this little engines has a "knee" at about the 810cc level (about 28 stock HP). Above this the cost per HP jumps. In this case, one way to keep using the "cheap" engines, not hop them up or do anything heroic in exotic fuels, and still get good performance with one engine out is to use 3 of the things ( I can feel the eyes rolling!). Now we have even >more< spectacular performance with all engines operating, we aren't asking them to do anything they aren't designed to do, and the cost is probably about the same as modifying two engines with custom cams, billet conrods, a special carb (and then the dang things overheat anyway). If using three engines and sticking with a single place plane, maybe we can get away with the horizontal shaft 23HP 627-670cc engines (and not need to turn 810cc 28HP vertical-shaft engines on their sides). How does the cost/weight of an approx 28 HP (21KW) electric motor and the batteries needed to run it for 10 minutes compare to just having another gasoline engine? I'm not trying to be a wet blanket--I really don't know. A second gasoline engine would give us longer running time. How doe we charge those batteries (sounds like we need the weight of a generator, too, or we plug into the hangar). With lots of chemical energy already aboard the plane (in the form of gasoline), it seems inefficient to haul around more chemical energy (batteries) just for this emergency case. A little screaming, loud, 6000 RPM 28 HP "emergency" 2-stroke hooked to the PSRU with a centrifugal would probably be light, cheap, and would let us fly for hours with the fuel aboard if necessary. Meter some oil into the fuel on the way to the motor. If only we could count on it starting rapidly when we need it. Of course, any emergency powerplant (gas or electric) that uses the same prop and PSRU bearings/shaft/etc as the main engine is still subject to some single points of failure. Thanks. A side-by-side seating configuration might also work in solving the CG problem, possibly at the cost of putting the pilot's head behind the LE of the wing. Thanks to the prop tips just outside the canopy, the interior noise of the OV-10 made it the loudest cockpit in the USAF inventory at the time (which is saying a lot, since the T-37 "Dog Whistle" with it's centrifugal-compressor engines were also in competition). So you assumed the props are optimized for 110MPH (96 knots) and the plane climbs out at bout 68 knots. That's about the same climb airspeed I used. Does your sim/spreadsheet, etc take into account the drag from a stopped prop? Here's where my assumptions took me (3 engine airplane climbing on 2 engines with drag from one stopped 46" prop, from Post 46 ). You got better 1 engine out climb performance than I did, and I assumed 2 additional HP (but 70% prop efficiency). I don't know if a 25 HP engine with approx 42 lb/ft of torque will be able to swing a 54" prop at 3600 RPM. Maybe it can, but they will be wispy blades! Thanks again.