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I wonder why most of those electric aircraft have so many small propellers. Aerodynamically, having fewer but much larger props would make more sense for effici

I wonder why most of those electric aircraft have so many small propellers. Aerodynamically, having fewer but much larger props would make more sense for efficiency, especially if they're only going 130 knots. But maybe they need the extra lift of the props blowing across almost the entire wing? I wonder if we'll ever see an electric tilt rotor aircraft with huge variable pitch props. Should be much more efficient, but probably also much more mechanically complex...

detritus 2y ago
To guess, I'd suggest it might be mostly due to the types of electric motor already produced and available in an appropriate weight and configuraton. Further it might be that redundancy in electric motors might be more of an issue (?) and here, I wonder if they've cracked some added efficiency with having a more consistent flow over the breadth of the wings?
- pbmonster 2y ago
  I was under the impression that compact 400 hp electric motors (Like in the Plaid Tesla) were available in large numbers. And hopefully, those would be higher reliably at much lower maintenance when compared to a turboprop or a turbojet. And even then, having 4 should be plenty of redundancy.
  - Retric 2y ago
    Tesla engines don’t fit inside a thin wing. Form factor is a huge deal for aerodynamics.
  - serf 2y ago
    it's unlikely that HP is the main thing at play here; an electric motor for a plane would need to be wound or gear-reduced to a point of kV efficiency that fits within a plane's necessities. a 400hp tesla motor is presumably built and wound to be efficient for a car. this isn't that, simply put.
jauntywundrkind 2y ago
> Aerodynamically, having fewer but much larger props would make more sense for efficiency, Source? I can think of a lot of flight reasons many small props could be better. You can spin faster while still keeping the tip subsonic. Typically fans have inefficiencies because outter blade moves so much faster than the inner blade & this effect is reduced with smaller blades. If you're looking for a blown wing effect you get much more surface area. Airflow is smoother because prop wash is more dispersed and even. For a while it seemed like many props was the sure way forward. It's definitely less seen now, but for example the Aurora XV-24A LightningStrike, NASA X-57 Maxwell, and GL-10 Greased Lightning seemed like they were setting the way forward. It seemed like technical difficulties that beset the Maxwell team that made them reduce Maxwell to a less ambitious more conventional electric offering. https://www.nasa.gov/stem-content/x-57-electric-airplane/ https://www.nasa.gov/stem-content/x-57-electric-airplane/ https://www.defensenews.com/digital-show-dailies/sas/2017/04/04/electric-drones-aurora-team-behind-vtol-x-plane-focuses-on-hybrid-propulsion/ https://www.defensenews.com/digital-show-dailies/sas/2017/04...
geor9e 2y ago
I can think of some possible reasons. Can't kill any ground crew if the prop ends are 8 feet up. Fuel engines get more efficient the bigger you make them, so big propeller aircraft being more efficient isn't necessarily because of the prop size, but rather a side effect of engine size. Electric motors don't gain much efficiency with increased size AFAIK, so they can get away with small motors. Lots of props close to the wing would guide the air well - you don't get that giant vortex of wasted energy. Redundancy - they could lose multiple propellers. That's especially needed since some of these electric aircraft are VTOL and can't glide or autorotate in an emergency. I'm thinking of https://wisk.aero https://wisk.aero . Market constraints - this is a new application so they're relying on other industries for the motors they can pick from.
serf 2y ago
interestingly almost all of the original aircraft conceptualizations had a lot of props -- they got simplified due to the realities of drivetrain complexity pre-electrification. redundancy is one reason, weight is another, cost is another. Lower amperage smaller motors require less copper and battery capacity, they're cheaper to produce, they can be placed opportunistically to maximize wing efficiency, and they're quite nearly COTS. bigger props necessitate more power in a singular spot, bigger heavier motor and cooling solutions, so on and so fourth. lots of motors at lower duty cycle can reduce these burdens, and if done right may even be able to get rid of the cooling systems and associated weight entirely.
darksaints 2y ago
Having multiple small propellers along the wing is done for lift augmentation. See the X-57 concept plane for an example. If this is anything like the X-57, most of those propellers will feather upon takeoff. Basically, most planes use wings that are much larger and less efficient than they need for cruising, because they need the extra lift for takeoff and landing. By introducing lift augmentation propellers which can augment lift for takeoff/landing, they can use a much smaller wing which has less parasitic drag and results in more efficient flight. https://en.wikipedia.org/wiki/NASA_X-57_Maxwell https://en.wikipedia.org/wiki/NASA_X-57_Maxwell

detritus 2y ago

To guess, I'd suggest it might be mostly due to the types of electric motor already produced and available in an appropriate weight and configuraton. Further it might be that redundancy in electric motors might be more of an issue (?) and here, I wonder if they've cracked some added efficiency with having a more consistent flow over the breadth of the wings?

pbmonster 2y ago
I was under the impression that compact 400 hp electric motors (Like in the Plaid Tesla) were available in large numbers. And hopefully, those would be higher reliably at much lower maintenance when compared to a turboprop or a turbojet. And even then, having 4 should be plenty of redundancy.
- Retric 2y ago
  Tesla engines don’t fit inside a thin wing. Form factor is a huge deal for aerodynamics.
- serf 2y ago
  it's unlikely that HP is the main thing at play here; an electric motor for a plane would need to be wound or gear-reduced to a point of kV efficiency that fits within a plane's necessities. a 400hp tesla motor is presumably built and wound to be efficient for a car. this isn't that, simply put.

jauntywundrkind 2y ago

> Aerodynamically, having fewer but much larger props would make more sense for efficiency, Source? I can think of a lot of flight reasons many small props could be better. You can spin faster while still keeping the tip subsonic. Typically fans have inefficiencies because outter blade moves so much faster than the inner blade & this effect is reduced with smaller blades. If you're looking for a blown wing effect you get much more surface area. Airflow is smoother because prop wash is more dispersed and even. For a while it seemed like many props was the sure way forward. It's definitely less seen now, but for example the Aurora XV-24A LightningStrike, NASA X-57 Maxwell, and GL-10 Greased Lightning seemed like they were setting the way forward. It seemed like technical difficulties that beset the Maxwell team that made them reduce Maxwell to a less ambitious more conventional electric offering. https://www.nasa.gov/stem-content/x-57-electric-airplane/ https://www.nasa.gov/stem-content/x-57-electric-airplane/ https://www.defensenews.com/digital-show-dailies/sas/2017/04/04/electric-drones-aurora-team-behind-vtol-x-plane-focuses-on-hybrid-propulsion/ https://www.defensenews.com/digital-show-dailies/sas/2017/04...

geor9e 2y ago

I can think of some possible reasons. Can't kill any ground crew if the prop ends are 8 feet up. Fuel engines get more efficient the bigger you make them, so big propeller aircraft being more efficient isn't necessarily because of the prop size, but rather a side effect of engine size. Electric motors don't gain much efficiency with increased size AFAIK, so they can get away with small motors. Lots of props close to the wing would guide the air well - you don't get that giant vortex of wasted energy. Redundancy - they could lose multiple propellers. That's especially needed since some of these electric aircraft are VTOL and can't glide or autorotate in an emergency. I'm thinking of https://wisk.aero https://wisk.aero . Market constraints - this is a new application so they're relying on other industries for the motors they can pick from.

serf 2y ago

interestingly almost all of the original aircraft conceptualizations had a lot of props -- they got simplified due to the realities of drivetrain complexity pre-electrification. redundancy is one reason, weight is another, cost is another. Lower amperage smaller motors require less copper and battery capacity, they're cheaper to produce, they can be placed opportunistically to maximize wing efficiency, and they're quite nearly COTS. bigger props necessitate more power in a singular spot, bigger heavier motor and cooling solutions, so on and so fourth. lots of motors at lower duty cycle can reduce these burdens, and if done right may even be able to get rid of the cooling systems and associated weight entirely.

darksaints 2y ago

Having multiple small propellers along the wing is done for lift augmentation. See the X-57 concept plane for an example. If this is anything like the X-57, most of those propellers will feather upon takeoff. Basically, most planes use wings that are much larger and less efficient than they need for cruising, because they need the extra lift for takeoff and landing. By introducing lift augmentation propellers which can augment lift for takeoff/landing, they can use a much smaller wing which has less parasitic drag and results in more efficient flight. https://en.wikipedia.org/wiki/NASA_X-57_Maxwell https://en.wikipedia.org/wiki/NASA_X-57_Maxwell