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How Distributed Electric Propulsion Will Change the Way We Design Flying Vehicles
There is one huge flaw in the "lots of little motors" approach. The aerodynamic efficiency of a propulsion system increases as the specific thrust (thrust divided by area) decreases; the specific thrust decreases with the diameter of the rotor, even as the total thrust increases.
(What I mean when I talk about the efficiencies needed to power aviation is specifically the use of batteries and PV panels at large scales; PV systems and batteries have very low energy densities compared to fuel, so any unnecessary loss of efficiency worsens this disadvantage.)
Yes, that is exactly right. This is one of several reasons why the Moller Skycar never took off... (no pun intended). They can probably build flyable aircraft using many small motors, but they will still need at least an order of magnitude improvement in the specific energy (watts/kilogram) to become viable beyond proof of concept.
yes a larger prop does make for more efficient thrust generation, the issue is that the majority of the volume of air generated is not over the wing but rather to just generate thrust to move the aerofoil through the air to generate lift. What they are trying to achieve here is different, they are actively moving air over the entirety of the aerofoil, large props isnt going to help because beyond a certain size you are no longer moving air over the aerofoil, better to use an array of smaller props to create a blanket of moving air over the entirety of the aerofoil.