>The smaller prop, the less efficient the thrust conversion.
Right, because the actuator disk loading[1] goes up, meaning that you're "throwing air downward" faster, with thrust scaling as v (momentum) while power scales as v^2 (kinetic energy). This is why propulsive efficiency decreases as exhaust velocity increases (for rockets this is simply exhaust velocity, while for air-breathing engines it's the change in velocity caused by the propeller/turbofan relative to the original airstream velocity).[2]
So bigger props are more efficient, because the actuator disk has more area. But the other way to add area is to add propellers. This has 12 props, so the disk loading is 1/12th as much.
The equation for hovering power is:
P = (mg)^3/2 / sqrt(2 A ρ)
So twelve rotors only needs 1/sqrt(12) = 29% as much power for the same thrust as a single rotor of that size. Or equivalently, having 12 rotors is like having a single rotor that's 3.5x as large in diameter.
>You also need laminar air for rear props to be efficient. The drag on the wings should create a ridiculous wake behind the aircraft. I would love to learn how KittyHawk are trying to mitigate these problems (which are huge constraints even for major players).
They showed CFD in the video, so apparently that combined with test flights and scale model flights (both shown on their website).
Notice that the propellers are rotated slightly inward, but the outer propeller (that rotates in the opposite direction) is angled outward. I suspect they're managing the vorticity of the propeller wakes, using it to lower induced drag (effectively creating a longer "virtual wing").
Also, it looks like they transition to horizontal flight and then shut down the rotors (or at least, dramatically lower the power). The video shows the hovering system shutting down and self-aligning the propeller blades into the airstream.
Right, because the actuator disk loading[1] goes up, meaning that you're "throwing air downward" faster, with thrust scaling as v (momentum) while power scales as v^2 (kinetic energy). This is why propulsive efficiency decreases as exhaust velocity increases (for rockets this is simply exhaust velocity, while for air-breathing engines it's the change in velocity caused by the propeller/turbofan relative to the original airstream velocity).[2]
So bigger props are more efficient, because the actuator disk has more area. But the other way to add area is to add propellers. This has 12 props, so the disk loading is 1/12th as much.
The equation for hovering power is:
So twelve rotors only needs 1/sqrt(12) = 29% as much power for the same thrust as a single rotor of that size. Or equivalently, having 12 rotors is like having a single rotor that's 3.5x as large in diameter.>You also need laminar air for rear props to be efficient. The drag on the wings should create a ridiculous wake behind the aircraft. I would love to learn how KittyHawk are trying to mitigate these problems (which are huge constraints even for major players).
They showed CFD in the video, so apparently that combined with test flights and scale model flights (both shown on their website).
Notice that the propellers are rotated slightly inward, but the outer propeller (that rotates in the opposite direction) is angled outward. I suspect they're managing the vorticity of the propeller wakes, using it to lower induced drag (effectively creating a longer "virtual wing").
Also, it looks like they transition to horizontal flight and then shut down the rotors (or at least, dramatically lower the power). The video shows the hovering system shutting down and self-aligning the propeller blades into the airstream.
[1] https://en.wikipedia.org/wiki/Disk_loading, https://en.wikipedia.org/wiki/Actuator_disk
[2] https://en.wikipedia.org/wiki/Propulsive_efficiency