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How Distributed Electric Propulsion Will Change the Way We Design Flying Vehicles

Lots of little motors, rather than two big ones, means designers are no longer beholden to mechanical complexity

By Rain Noe - May 12, 2015

in Cars

NASA is not known for taking risks. But by partnering with smaller, pluckier transportation technology companies, they can bring their engineering might to bear on blue-sky technologies that would never have reached internal approval.

One such partner is Joby Aviation, a California-based company "founded to revolutionize how we commute." Joby's radical S2 aircraft is designed to increase commuting speed by a factor of five—while using five times less energy than an automobile traveling the same distance. They mean to achieve this via distributed electric propulsion; no less than twelve rotors, each with their own electric motor, launch the vehicle vertically, then tilt forward in concert to enable cruising. Take a look at this nuttiness:

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That CG animation was created last year. In the time since, NASA has been working on a remote-controlled scale model—in essence, a drone—that uses the same technology, to see if they can actually get it to work. Two weeks ago NASA's Langley Research Center released video of the test flight:

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The prototype you see in the video, dubbed "Greased Lightning," uses ten rotors rather than twelve, and tilts the entire wing assemblies rather than the individual rotors, but demonstrates that the crucial transition from hovering to flying is workable. It's true that the prototype doesn't look as sexy or feature the fancy folding propellers shown in the S2 renderings below, but it will presumably be Joby's task to realize those.

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NASA, Joby Aviation and third engineering partner ESAero are calling the technology LEAPTech, for Leading Edge Asynchronous Propellers Technology. Here NASA Aerospace Engineer Mark D. Moore explains the staggering efficiencies they're able to achieve with this configuration, and points out that distributed electric propulsion will change the way that we design vehicles:

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Austin Liu

9 years ago

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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.
For this reason, a single large rotor can produce enough thrust to hang the full weight of a helicopter on, but the thrust of a propeller is not enough to hover an aircraft oriented vertically. The problem with using a lot of little rotors or propellers is that the bulk of the air moved by a rotor or a propeller is moved by the part of the rotor near the tips; the amount of air moved is proportional to the area swept by each part of the rotor, and the tips sweep the largest path. It is far less efficient to move a given quantity of air using many rotors vs. using one large rotor. If electric drive systems cannot support large rotors, they cannot achieve the high efficiencies needed to make electric power systems cost-effective for powering aviation.

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- Austin Liu
  
  Austin Liu
  
  9 years ago
  
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  (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.)
  
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- Mathew Eis
  
  Austin Liu
  
  9 years ago
  
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  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.
  
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- Dreydon Le
  
  Austin Liu
  
  9 years ago
  
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  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.
  
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