In theory the reduced thrust required for distributed propulsion which generates induced flow, combined with improved battery capacity you are getting your 20x jump in efficiency?
I have heard many times that prop planes have better runway performance (i.e. lower Vs) because the propwash generates some extra lift, but I don’t think this is a free lunch, because the generation of lift involves a redirection of the airflow (there are elegant ways of saying this e.g. the circulation theory but they are beyond me 
) and that will create a greater back pressure on the propeller, thus requiring extra power going into it.
Now that the German automotive powerhouses have woken up (either because of the success of Tesla or because they need to clean up their image/exhaust measurement methods, or both) I somehow think that we’ll see some battery advances quicker than we think. There is so much at stake here, so funding is there. You read of all kinds of new promising technologies which, admittedly, are on a rather far horizon. But, given the high stakes, could it not be that there is a lot of development going on secretly, the results of which suddenly pop up?
But even look at where we are right now, Pipistrel’s Taurus Electro as an example. Currently just an hour or so of endurance, so fine for circuit training, but even nice for people to do some local bimbling. Heck, I could even imagine myself buying one and have fun doing local flights over the island. Or even go to Menorca, where there would be a fully charged battery waiting for me to swap. My friends at the Aeroclub there would then charge the depleted one for me to use next time I go there. Or I go for a lunch while my battery gets partially charged. Or do a Spanish lunch which ends at 19:00 with some spirits and I would need to stay overnight, and then my battery would really be charged..
In spite of me being not being the youngest on this forum, my next aircraft is going to be electric. Like our cars.
“Hope to validate” when processed through my skeptical filter means some dude (probably smarter than me) came up with some equations, showed them to someone smarter than him who went on and got the funding on the back of that. Admittedly I am not privy to NASA’s procurement process, but if since 1972 they were able to build a couple space shuttles and do PR at the same time, this ought not be too difficult to assess (for them).
I am however privy with progress in carbon-free utility scale and distributed generation, and I can guarantee that it is nowhere near it was 50 years ago. As a matter of fact utility solar construction bids are now down to 3 US cents per kwh, which is cheaper than any carbon based solution (and have no variable costs once up and running). 50 years ago all US power generation was coal based. Now all the US coal miners are bankrupt (or close).
And finally if you read up about Crisper, you’ll see medical quantum leaps are knocking on our door… so things do change.
As I said earlier, the massive reduction in noise footprint could finally make airfields more acceptable near built-up areas…. And it is the loss of existing airfields and the hopelessness of building new ones that to me is the major issue for the future of private flying… Quiet airplanes may be the salvation!
Part of the advantage of multiple props is increased propulsive efficiency. That is thrust from a large mass flow of air with a small velocity change is more efficient than a small mass flow with a large velocity difference. That’s why airliners went from straight jets to high bypass turbo-fans. The noise reduction was another great benefit as jet noise scales with the 8th power of velocity.
Batteries are currently nudging 200WH/kg energy density and the consensus seems to be that 400WH/kg will be needed to lead to large scale adoption for aircraft use.
Current trends would put that about 10 years in the future although such extrapolation is highly speculative.
The Taurus G4 achieved the equivalent of 4 people and 200nm on 90KWH.
Mark_1 wrote:
Part of the advantage of multiple props is increased propulsive efficiency. That is thrust from a large mass flow of air with a small velocity change is more efficient than a small mass flow with a large velocity difference. That’s why airliners went from straight jets to high bypass turbo-fans. The noise reduction was another great benefit as jet noise scales with the 8th power of velocity.
The principle of mass flow being more efficient than velocity for achieving a given thrust is certainly true. However I don’t see that dividing up a given amount of propulsion power into more propellers automatically achieves the same end as increasing the disk area for a fixed number of propulsors and amount of power. I can see that it might work out that way if you’d run out of room for larger diameter propellers, as in underwing installations, but that is application specific. More generically I think the disk loading is not necessarily decreased by dividing up the power into more propellers, the blade area near the increased number of hubs is ineffective and the tip losses increase.
They claim that 12 of the motors are for take-off (and landing?) only. Certainly that would give you a big increase in static thrust for not too much extra power requirement.
I presume that in the cruise they would be stopped and feathered.
Presumably they could be used for extra drag and re-gen in the descent too.
They’re claiming a 5-fold reduction in cruise power – presume part of that will be due to not blasting the propeller slipstream past the fuselage and another part will be the very small wing that you can get away with if it only has to produce sufficient lift at cruise speed. The article states that you only use the 12 extra propellers for t/o and landing so they aren’t just using them to gain more mass flow.
Energy density of Gasoline = 46 MJ/Kg
Energy density of Lithium-ion battery = 1MJ
Efficiency of an electric motor – 90%
Efficiency of an Avgas engine – 30%
So if your motor is 3 x more efficient and your airframe 5 x more efficient (which seems ambitious to me… but good luck to them) then you should be approaching 1/3 the cruise endurance of the original aircraft – whatever that might be (it seems to me the fairest comparison would be with an unmodified Tecnam).
Obviously the gains during the climb phase will be less, because however aerodynamically efficient, it takes a certain amount of energy to lift something to 10,000 feet. However, if you take a 4 person aircraft and replace 2 people with batteries, you could potentially end up with a 2 person aircraft with quite an useful range.
The other gains are in potential STOL – if you can take off in your backyard and land in your friends’ backyard, then you’d use it for different trips where the limited range wouldn’t matter so much. You might not be able to fly to Greece without stopping, but for shorter journeys it might be more practical than a TB20 or Citation.
If they do achieve that x5 increase in efficiency then it really wouldn’t be that far from being a practical machine with an useful range.
