The most dangerous place to have a failure is during initial take off (low hour student pilot here). The engine kicking on would give you a chance to actually do what is nearly impossible in this circumstance, get back to the airport safely without a stall.
As I understand usually the most dangerous circumstances are the one above described, stalling on base or final approach to runway and flying from VFR (clear of clouds with visibility) into IMC poor weather conditions.
Both of those circumstances wouldn't benefit from a backup engine. : (
As other people in the threads have posted, Fuel Starvation, water condensation in the fuel, some of those situations this would definitely help, but all of those conditions are 100% preventable with proper pre-flight planning and inspections.
The next step for this electric engine would be to replace that second heavy, expensive engine every large airplane carries around...
Additionally, the full power of the two engines is only needed at takeoff. At cruise they only run at 30-40%. An electric motor with a relativly small battery would be enough to provide enough power at takeoff.
As long as the electric engine can provide a sufficient backup to get to an airport after a failure in cruise, though, that would certainly be an interesting possibility.
Even helicopters don't fall that fast.
Lets say you're cruising 5000 feet above ground. That's a mile. You're in a plane with a roughly 10:1 glide ratio so you can glide 10 miles. At a best lift speed around 60 knots that's a mile a minute. Locally the only way to be more than 10 miles away from an airport or farm field would be over a great lake. During takeoff if you climb at 1000 FPM (maybe optimistic) that takes 5 minutes which puts you 5 miles away you can always turn around and glide back. All of my engineering estimates are wrong but wrong by far less than a factor of 2. Its a very long and stressful glide down if you have an engine failure.
Most engine failures don't make the news because nobody got killed or even landed off airport.
What tends to kill people is over confidence. Well, I have a big meeting tomorrow and I can handle a little rain storm, whoops. I only have a single engine and can't be bothered to IFR so I have to run scud under the overcast whoops I'm 5 miles from shore and only 1000 feet up when the engine dies (I think this most famously killed John Denver?)
Not exactly. John Denver NTSB report: http://www.ntsb.gov/news/press-releases/Pages/NTSB_Determine...
40 hp is 30kW. Assume 100% efficient gearing and motor. Stall speed is around 80 km/hr. For 20km of powered flight, that means 0.25 hours runtime. For high performance li-ion (250 Wh/kg) that is 30kg of batteries.
Add some weight for <100% efficiency, remove some weight for the part of the 20km where you glide.
Speculation: Given the difference in motor vs batteries, either they have an extremely light weight motor technology or they are using lower density li-ion for safety. A LiFePO4 pack (130 Wh/kg) would be 60kg. They could afford to double the battery size, since that is only a 25% increase in weight. Either they have a very lightweight motor technology, or there is some magic price or weight number they are trying to squeak under.
40hp 200kg brushless motors are extremely common by comparison. Dozens of places will sell you one for a few grand.
If I'm reading correctly, the AC-35 has a peak rating of 47.78 horsepower, and weighs 43.9kg.
Presumably the 200kg motors you're looking at are not intended to be part of a moving vehicle. Those are likely intended for use in factories or shops where the weight is of little concern but durability is at a premium.
That depends entirely on how close you are to the ground when the engine goes out. If its during takeoff, you might have less than 10 seconds to decide. If its during cruise, you probably have much more than 10 seconds to decide - and of course you can continue to adjust your decision as you get closer to the ground.