Is Distributed Electric Propulsion the Key to Efficient Private Planes?

[mheslep]

NASA has press release out today highlighting its electric aviation program via a new 4-seat X plane to explore the concept, the X-57.

Most interesting to me was this claim:

NASA’s aeronautical innovators hope to validate the idea that distributing electric power across a number of motors integrated with an aircraft in this way will result in a five-time reduction in the energy required for a private plane to cruise at 175 mph.

which I imagine comes about by having a much large propulsive air flow area across the 14 e-motors, allowing the prop flow velocity to be much lower.

Goal:

NASA researchers ultimately envision a nine-passenger aircraft with a 500-kilowatt power system in 2019. To put that in perspective, 500 kilowatts (nearly 700 horsepower) is about five times as powerful as an average modern passenger car engine

That goal is interesting but ill-considered IMO, as range is the issue in electric aviation, not power.

Video of fourteen motor wing under test

 

[Baluncore]

That goal is interesting but ill-considered IMO, as range is the issue in electric aviation, not power.

If you can do the same for 1/n of the power then you can do it for n times longer. That increases range by a factor n.

 

[FactChecker]

If you can do the same for 1/n of the power then you can do it for n times longer. That increases range by a factor n.

That is one optional benefit. It also could allow you to loiter in place and observe for a long time, even if the cruse speed is decreased. It might even allow sustained solar-powered flight where the plane can stay up indefinitely.

 

[mheslep]

If you can do the same for 1/n of the power then you can do it for n times longer. That increases range by a factor n.

Point taken.

Aside: I don't see efforts at many-motor designs among RC electric aircraft hobbyists, or in the UAV arena. Perhaps cost is an issue, though since the RC's with their LiPos are quite range (time aloft) limited, say, 15 mins, there should be ample motivation to try. Edit: here's a competition for time aloft (traditional design) with a maximum of 26 mins.

 

[mheslep]

.. It might even allow sustained solar-powered flight where the plane can stay up indefinitely.

Overnight unmanned electric flight was accomplished six years ago. http://tucson.com/business/local/tucson-firm-s-solar-batteries-power-record-flight-attempt/article_00ec5266-f412-5b3d-aaa5-04e9d87ec896.html made by QinetiQ with Sion batteries flew non-stop for 14 days using PV: dual prop, 70 ft wingspan, 110 lbs. The term of art going forward is "solar powered atmospheric satellites"
http://www.unmannedsystemstechnolog...for-solar-powered-atmospheric-satellite-uavs/

 

[CWatters]

Presumably the idea behind multiple motors and props is to increase the air flow over the wing meaning a smaller wing can be used?

 

[mheslep]

Presumably the idea behind multiple motors and props is to increase the air flow over the wing meaning a smaller wing can be used?

I don't think so. There's some work on a design indicating lower turbulence with aft fuselage mounted engines thus reducing drag by positioning motors in places not practical with combustion engines.

But as I understand it, the principal advantage of multiple motors is that they are theoretically more efficient than one at producing the same amount of total thrust. That is , thrust is proportional to both air velocity and the area through which the air is driven, but high velocity exit air is wasted kinetic energy in the atmosphere. Thus it is more efficient to achieve thrust by greater engine area when possible than by higher velocity air flow.

 

[FactChecker]

I don't think so. There's some work on a design indicating lower turbulence with aft fuselage mounted engines thus reducing drag by positioning motors in places not practical with combustion engines.

But as I understand it, the principal advantage of multiple motors is that they are theoretically more efficient than one at producing the same amount of total thrust. That is , thrust is proportional to both air velocity and the area through which the air is driven, but high velocity exit air is wasted kinetic energy in the atmosphere. Thus it is more efficient to achieve thrust by greater engine area when possible than by higher velocity air flow.

The arrangement shown has a very high aspect ratio with very even airflow from the props over the wing. That should give an exceptional lift to drag ratio.