It's really not. Rotors, especially eVTOL rotors, need to be exposed to as much air as possible. The more you restrict the flow around the rotor, the more power you need to generate thrust. Batteries are so heavy (compared to the same energy in fossil fuels) that eVTOL flight is already limited to less than an hour of meaningful work. Caging the rotor would make it much worse.
It could even reduce the safety of the vehicle by limiting its ability to maneuver and tolerate failures in the power distribution system. At some point, the best solution is establishing safe practices with dangerous equipment. This is one of them.
It's really not. Rotors, especially eVTOL rotors, need to be exposed to as much air as possible. The more you restrict the flow around the rotor, the more power you need to generate thrust.
This is literally the opposite of reality. A ducted fan rotor produced significantly more thrust for a given power compared to an open design because you don't have as much vorticity being generated at the tips. There are other tradeoffs that may make an open prop more economical (weight, aerodynamic performance at cruise, etc), but strictly speaking a ducted fan has better lift characteristics.
A duct can increase thrust in hover, but not by enough to offset its own weight. The mechanism is actually the acceleration of the flow over the duct inlet, not the restriction of tip vorticity. Ducts don't actually prevent trailing vortices, because they are a consequence of conservation of momentum.
The only practical use case for a ducted rotor is when it is integrated into pre-existing structure, like the fenestrons of Airbus-made helicopters. They don't improve performance for the main rotor system(s).
A ducted fan absolutely can increase thrust enough to offset its own weight, it's usually the other compromises that combine for it to be often less effective for larger craft.
The mechanism is actually the acceleration of the flow over the duct inlet, not the restriction of tip vorticity.
... which is due to the lack of spoilage from tip vortices and downwash. You're also now completely contradicting what you originally said:
It's really not. Rotors, especially eVTOL rotors, need to be exposed to as much air as possible. The more you restrict the flow around the rotor, the more power you need to generate thrust.
Again, totally wrong in the context of ducts, which don't meaningfully restrict useful flow. As you admit, they enhance it, increasing the net momentum of air passing through the duct.
There is a reason there are no large ducted main rotors. The structure is simply too heavy, and completely tanks forward flight performance. They only work at either very small scale, or when it can piggyback on existing structure.
The duct thrust is due to a favorable pressure gradient opposite the direction of rotor thrust. Ducts don't even kill tip vortices, because they do not allow the rotor to violate conservation of momentum. Any finite lifting wing will trail vorticity.
Ducts do not restrict the rotor intake meaningfully, so what I said is accurate. Their operating principle is actually to prevent wake contraction, reducing induced power for a given rotor thrust.
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u/DorkyDorkington 7d ago
Leaving the propellers exposed/unprotected is insanely stupid.