I don't get how it is physically possible that this plane is energy efficient? The smaller prop, the less efficient the thrust conversion. 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).
>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.
> I don't get how it is physically possible that this plane is energy efficient?
Energy efficiency is not all or none - it's a question of how efficient it is. Whichever way you dice it, the final result of the combination of energy density, thrust, weight lift, drag, is a plane is efficient enough to have a claimed range of 100km. If it were more efficient, it would have a longer range.
Efficiency is not relative especially when it comes to mechanical movement. It is quantifiable data and nature does not care whether the energy is "green" or "clean". Energy is energy regardless how it is produced. Though the best data they can provide is the lift to drag ratio at cruise speed. This will give us a pretty good idea on how this aircraft performs.
Just be careful about how you quantify, the energy needed to overcome that drag ratio will presumably be delivered far more efficiently via electric moters, since they'll not be throwing 2/3rds of the energy away as heat.
So an electric plane could be twice as inefficient in other ways (e.g. extra weight for batteries) and still come out ahead overall. And that's before taking carbon and pollution into account.
