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Is many small propellers really more efficient than few big ones? Even if they didn't want to make a mini-helicopter, they could've also went with a 3-4 rotor d

Is many small propellers really more efficient than few big ones? Even if they didn't want to make a mini-helicopter, they could've also went with a 3-4 rotor design.

ralegh 3y ago
I'd assume it's more about redundancy - 1/18 failed motors vs 1/4 would be much safer to land. Also means the replacement motors would be cheaper (individually). Like the starship booster.
- wongarsu 3y ago
  And having more redundancy presumably means you can build each one cheaper and don't have to inspect them as much. Smallish helicopters aren't that expensive. Maintenance and fuel are though. Going electric already helps with the fuel, going for many motors helps with the maintenance.
  - onlyrealcuzzo 3y ago
    Helicopters are lucky to get 10 mpg. The range of an electric helicopter is going to make it impractical, and I doubt that's going to change any time soon. Gas stores energy in 100x less weight than lithium ion. Electric motors are 2x more efficient. You're still going to need space and weight for ~50x more battery, which is hard to come by...
0_____0 3y ago
I worked in this space for years. The answer is no. If you evolve a multirotor design to be more and more efficient you will end up with fewer, larger rotors, until the time comes that you require variable pitch to have sufficient control bandwidth. At which point you have a helo.
- ActorNightly 3y ago
  You can take it a step further and have a counterweighted single blade propeller, which is the most efficient. That being said, this is only aero efficiency. Once you start to add electric motor efficiency, things get interesting. To generate thrust with lower number of blades you need to have large torque torque to put into the prop , which means the motor has to be a low Kv (Kv = rpm/volt, and a low Kv means a high Kt where Kt = torque/amp), to keep the current lower to lessen electrical power losses. Or use a reduction gearbox. Both of the above necessitate a bigger package and more weight, which requires more thrust, and so on. There is a place in this design space where having multiple rotors is actually more efficient for the overall flight envelope.

Is many small propellers really more efficient than few big ones? Even if they didn't want to make a mini-helicopter, they could've also went with a 3-4 rotor design.

ralegh 3y ago

I'd assume it's more about redundancy - 1/18 failed motors vs 1/4 would be much safer to land. Also means the replacement motors would be cheaper (individually). Like the starship booster.

wongarsu 3y ago
And having more redundancy presumably means you can build each one cheaper and don't have to inspect them as much. Smallish helicopters aren't that expensive. Maintenance and fuel are though. Going electric already helps with the fuel, going for many motors helps with the maintenance.
- onlyrealcuzzo 3y ago
  Helicopters are lucky to get 10 mpg. The range of an electric helicopter is going to make it impractical, and I doubt that's going to change any time soon. Gas stores energy in 100x less weight than lithium ion. Electric motors are 2x more efficient. You're still going to need space and weight for ~50x more battery, which is hard to come by...

0_____0 3y ago

I worked in this space for years. The answer is no. If you evolve a multirotor design to be more and more efficient you will end up with fewer, larger rotors, until the time comes that you require variable pitch to have sufficient control bandwidth. At which point you have a helo.

ActorNightly 3y ago
You can take it a step further and have a counterweighted single blade propeller, which is the most efficient. That being said, this is only aero efficiency. Once you start to add electric motor efficiency, things get interesting. To generate thrust with lower number of blades you need to have large torque torque to put into the prop , which means the motor has to be a low Kv (Kv = rpm/volt, and a low Kv means a high Kt where Kt = torque/amp), to keep the current lower to lessen electrical power losses. Or use a reduction gearbox. Both of the above necessitate a bigger package and more weight, which requires more thrust, and so on. There is a place in this design space where having multiple rotors is actually more efficient for the overall flight envelope.